Meteor Activity
Upcoming meteor showers, their peak dates, and expected visibility at your location -- not just when a shower peaks worldwide, but how many meteors an hour you'd realistically see from where you are, once moonlight and the radiant's position in your sky are factored in.
Page help ⌄
How to use this page: the highlighted card is the next major shower to peak from your location. A shower that's already past its peak but still producing meteors shows as "Still active" instead, with tonight's actual rate rather than its (now past) peak rate. Click any card for details, including its parent comet or asteroid. Rates account for the radiant's height above your horizon and the current moon brightness -- the shower's official peak rate (ZHR) assumes perfectly dark skies with the radiant directly overhead, which is rarely the real-world case.
How to read the map: color shows how many meteors per hour an observer could expect right now at that location, from Gold (low) to Blue (high), relative to the best rate any currently-active shower could theoretically produce. It fades to nothing in daylight -- meteors aren't visible until the sky is dark, even where a radiant is technically above the horizon -- and combines every shower active on the selected date at once, the same way the "Meteors Tonight" card's total does for your own location. The Night toggle shades everywhere past the terminator, so you can still see which side of the world is dark even where nothing is currently active; Twilight (only available with Night on) splits that shading into its civil/nautical/astronomical stages. Use the time slider to see how the bright band shifts as different radiants rise and set through the night; hover (or touch and drag) anywhere on the map for the exact rate at that point.
How to read the map: color shows how many meteors per hour an observer could expect right now at that location, from Gold (low) to Blue (high), relative to the best rate any currently-active shower could theoretically produce. It fades to nothing in daylight -- meteors aren't visible until the sky is dark, even where a radiant is technically above the horizon -- and combines every shower active on the selected date at once, the same way the "Meteors Tonight" card's total does for your own location. The Night toggle shades everywhere past the terminator, so you can still see which side of the world is dark even where nothing is currently active; Twilight (only available with Night on) splits that shading into its civil/nautical/astronomical stages. Use the time slider to see how the bright band shifts as different radiants rise and set through the night; hover (or touch and drag) anywhere on the map for the exact rate at that point.
Key Definitions
- Meteor activity
- This map's combined rate from every currently-active shower at once, scaled from Gold (low) to Blue (high) relative to the best rate any of them could theoretically reach that night -- a relative comparison across tonight's sky, not an absolute meteors-per-hour scale.
- Meteor shower
- A period when Earth passes through a comet or asteroid's debris trail, producing an increased number of meteors that all appear to streak outward from one point in the sky.
- Parent body
- The comet or asteroid whose shed debris, burning up in Earth's atmosphere, produces a given shower's meteors.
- Radiant
- The point in the sky a shower's meteors appear to originate from -- named after the constellation it sits in, like the Perseids radiating from Perseus.
- Relative moonlight
- How bright the Moon is compared to an average full moon (100%), combining its phase and distance -- higher values wash out fainter meteors.
- ZHR (Zenithal Hourly Rate)
- The number of meteors a single observer would see per hour under perfectly dark skies with the radiant directly overhead -- a theoretical best case, not a typical real-world count.
Aurora
Aurora forecast and visibility odds for your location, based on current geomagnetic activity. The aurora borealis (northern lights) and aurora australis (southern lights) happen when charged particles from the sun collide with Earth's upper atmosphere near the magnetic poles -- how far that glow reaches toward the equator, and whether it's worth watching for tonight, depends on how disturbed Earth's magnetic field currently is, how dark your sky is, and how bright the moon is.
Page help ⌄
How to use this page: if NOAA has an active geomagnetic storm watch, warning, or alert in effect, it appears as a clickable banner at the top of the page -- otherwise nothing shows there. The highlighted card below it shows whether the aurora is likely visible from your location right now, combining the current geomagnetic activity (Kp index) with your geomagnetic latitude, how dark it currently is, and moon brightness. Click any card for more detail. The graph icon on the Current Kp Index, Solar Wind Speed, IMF Bz, and IMF Bt cards plots that value over the last 7 days (plus, for Kp, NOAA's own forecast a couple of days ahead as a dashed line) -- select up to two at once to compare them on one chart, hover the chart for exact readings at a given time. The two Hemispheric Power cards aren't graphable, since NOAA only publishes the current day's reading for that product, too little history for a meaningful trend line. When Kp is one of the two plotted metrics, a dashed reference line marks the Kp your own location needs to see the aurora, so you can see at a glance when the line crosses into "visible for you" territory.
How to read the map: color shows NOAA's live aurora probability at that location, from green (low) to red (high), on a real, pannable and zoomable map, with true night and twilight shown underneath it -- switch between 2D and 3D (globe) with the toggle, toggle Nighttime off to hide it (and Twilight along with it, since twilight only means anything relative to true darkness -- turning Nighttime back on restores Twilight to whatever it was last set to), and hover (or touch and drag) anywhere for the exact probability and light conditions at that location. Your own location is marked, along with the magnetic north and south poles the auroral oval is centered on. Space weather data updates every few minutes from NOAA.
How to read the map: color shows NOAA's live aurora probability at that location, from green (low) to red (high), on a real, pannable and zoomable map, with true night and twilight shown underneath it -- switch between 2D and 3D (globe) with the toggle, toggle Nighttime off to hide it (and Twilight along with it, since twilight only means anything relative to true darkness -- turning Nighttime back on restores Twilight to whatever it was last set to), and hover (or touch and drag) anywhere for the exact probability and light conditions at that location. Your own location is marked, along with the magnetic north and south poles the auroral oval is centered on. Space weather data updates every few minutes from NOAA.
Key Definitions
- Auroral oval
- The ring-shaped zone around each magnetic pole where the aurora is most active -- it expands toward the equator as the Kp index rises.
- CME (coronal mass ejection)
- A huge burst of plasma and magnetic field ejected from the Sun's outer atmosphere. When one reaches Earth, it typically drives a sharp jump in solar wind speed and IMF strength, and often a sudden swing in Bz -- the classic trigger for a major geomagnetic storm.
- G-scale
- NOAA's G1 (minor) through G5 (extreme) geomagnetic storm scale, corresponding to Kp 5 through 9.
- Geomagnetic latitude
- Your position relative to Earth's magnetic (not geographic) pole -- this can differ from your map latitude by several degrees, and it's what actually determines how close you are to the auroral oval.
- Geomagnetic storm watch/warning/alert
- NOAA's own advisories for expected or already-occurring geomagnetic storms, issued separately from the routine 3-day Kp outlook -- a watch predicts a storm days ahead, a warning means one is expected within hours, and an alert means a threshold has already been reached. Shown here as a banner whenever one is currently active.
- Hemispheric Power
- NOAA's estimate, in gigawatts, of the total energy currently being deposited into one hemisphere's upper atmosphere by the aurora -- updated every 5 minutes, faster than the 3-hourly Kp index, from the same OVATION model behind the probability map.
- IMF Bt
- The total strength of the interplanetary magnetic field carried by the solar wind, in nanotesla (nT), regardless of direction. Bt sets the ceiling on how strong geomagnetic coupling could be; Bz's direction determines whether that potential is actually used.
- IMF Bz
- The north-south component of the interplanetary magnetic field carried by the solar wind, measured in nanotesla (nT, a unit of magnetic field strength). A southward (negative) Bz is the strongest single driver of geomagnetic activity; a northward (positive) Bz largely deflects the solar wind's energy around Earth instead.
- Kp index
- A 0-9 scale of global geomagnetic activity, updated every 3 hours by NOAA -- higher values mean a stronger disturbance and an auroral oval that expands toward the equator.
- Solar wind speed
- How fast the stream of charged particles from the Sun is moving, measured by NOAA's DSCOVR satellite about 1 million miles upstream of Earth. Typical background speed runs 300-400 km/s; sustained speeds above 500 km/s often accompany stronger geomagnetic activity, though speed alone doesn't guarantee it.
- Terminator
- The moving boundary line between the day side and night side of the Earth.
- Twilight
- The period between full daylight and full darkness, split into civil, nautical, and astronomical stages as the sun sinks progressively lower -- the sky isn't dark enough for the aurora to stand out until well into it.
How this works: aurora probability comes from NOAA's OVATION model, which forecasts the auroral oval worldwide from real-time solar wind and geomagnetic measurements and refreshes every few minutes. That probability is combined with your geomagnetic latitude, whether it's currently dark enough at your location (the aurora is essentially invisible against a bright sky), and how much moonlight is washing out the sky, to estimate real visibility odds rather than just raw geomagnetic activity. Meteor shower dates, rates, and radiant positions come from the International Meteor Organization's published shower calendar; expected rates then adjust that shower's official peak rate for your radiant's height above the horizon and current moonlight, the same way the Aurora tab adjusts for local sky conditions.
Dark Skies
How good stargazing actually is tonight at your location -- combining how light-polluted your sky is to begin with, how long it stays truly dark, how much the moon interferes, and the cloud forecast, into one plain-language rating instead of leaving you to weigh four separate numbers yourself.
Page help ⌄
How to use this page: the featured card is tonight's overall Dark Sky Quality at your location -- click it (or any card below) for more detail and a rotating tip. The Light Pollution card is essentially a fixed property of where you are (it won't change from night to night, though it does ease off through the small hours as lighting switches off -- see that card for why); the other three change nightly with the moon, weather, and season. It reports three numbers together -- Bortle class, ALR, and SQM -- since Bortle's class boundaries aren't perfectly standardized across sources, and the underlying numbers let you compare against a real Sky Quality Meter reading or another site's own tools directly. Use the map to compare your own sky against elsewhere, or to plan a trip somewhere darker -- switch between 2D and 3D (globe) with the toggle, and hover (or touch and drag) anywhere for that point's estimated Bortle class, ALR, and SQM. The "Recent Satellite Imagery" toggle overlays a real NASA satellite pass instead of this app's own computed estimate -- useful for a sanity check, but it's a single night's raw imagery (so moonlight or cloud from that specific night can show through it), not a stable baseline the way the computed layer is.
How to read the map: color shows modeled sky brightness on the real, named Bortle Dark-Sky Scale astronomers use, from Class 1 (darkest) to Class 9 (brightest) -- real NASA satellite light measurements, propagated through an atmospheric-scattering model into the same ALR (All-sky Light Pollution Ratio) and SQM (Sky Quality Meter reading) the cards compute, not raw satellite brightness. That's why the glow extends smoothly around cities and even out over open water near a lit coast, instead of stopping wherever the satellite itself measured no light -- see the Light Pollution card's own definition for why those are different things. Color blends continuously between the 9 class colors rather than stepping at each class boundary, because sky glow itself falls off smoothly -- a hard edge would draw a line where the sky has none. Hover anywhere on the map to read that point's exact class, SQM and ALR below it, and to light up the matching class in the legend.
How to read the map: color shows modeled sky brightness on the real, named Bortle Dark-Sky Scale astronomers use, from Class 1 (darkest) to Class 9 (brightest) -- real NASA satellite light measurements, propagated through an atmospheric-scattering model into the same ALR (All-sky Light Pollution Ratio) and SQM (Sky Quality Meter reading) the cards compute, not raw satellite brightness. That's why the glow extends smoothly around cities and even out over open water near a lit coast, instead of stopping wherever the satellite itself measured no light -- see the Light Pollution card's own definition for why those are different things. Color blends continuously between the 9 class colors rather than stepping at each class boundary, because sky glow itself falls off smoothly -- a hard edge would draw a line where the sky has none. Hover anywhere on the map to read that point's exact class, SQM and ALR below it, and to light up the matching class in the legend.
Key Definitions
- Airglow
- Faint light given off by the upper atmosphere itself, from a layer around 90 km up: sunlight breaks molecules apart during the day, and they release that stored energy as light when they recombine at night. It is the largest natural contributor to the brightness of a dark sky -- more than starlight or zodiacal light -- and it covers the whole Earth, so no site anywhere is ever completely dark. At a pristine site it accounts for about half of everything a meter sees at the zenith — roughly 105 of the 205 S10 in a natural sky, the single largest contributor, ahead of both starlight and zodiacal light. That is also its ceiling: because it is a fixed natural quantity, the only thing that changes its share of your sky is how much artificial light is added alongside it, which is why the Light Pollution card notes whether your sky is still mostly natural. This app assumes a fixed natural sky of about 22 mag/arcsec², and the real sky moves around that figure by an amount measured directly from the 656 National Park Service readings this app is calibrated against. Two readings at the same site, taken at any time apart, differ by 0.28 mag typically and 0.56 at the 90th percentile -- the figure on the Airglow card is that spread halved, as a plus-or-minus either side of the middle. Narrowed to two readings in the same month, the difference falls to about 0.10 mag, which is close to what a Sky Quality Meter can itself resolve. Those are readings taken straight up, and the low sky moves around about 1.7 times more, because airglow brightens toward the horizon while starlight and zodiacal light are dimmed by the thicker air -- so airglow accounts for roughly 45% of the natural sky overhead but about 75% at 20 degrees up. Only a small part of that is predictable -- comparing those same readings near solar minimum against solar maximum, the sky is about 0.12 mag brighter at maximum, which is real and in the expected direction but still smaller than the night-to-night scatter, so this app does not attempt to forecast it. The variability is the main reason a Sky Quality Meter reading taken on the night rarely matches a predicted figure exactly, and why the same dark site can look noticeably different on two apparently identical nights. Because it comes from a thin shell rather than the whole atmosphere, it also brightens toward the horizon -- the van Rhijn effect -- so the sky low down is always brighter than the sky overhead.
- ALR (All-sky Light Pollution Ratio)
- How much brighter the whole sky is than its natural background, as a ratio -- ALR 0 is pristine, ALR 1 means artificial light doubles the sky's natural brightness. This is the actual number this app computes at each location (Bortle class and SQM are both derived from it) and the same metric the US National Park Service uses for its own night-sky assessments.
- Astronomical darkness
- The period when the sun is more than 18 degrees below the horizon -- astronomical twilight has fully ended and no residual sunlight is scattering in the atmosphere at all.
- Bortle scale
- The standard 1-9 scale (Bortle, 2001) astronomers use to describe how dark a sky actually is -- Class 1 is the best a sky can naturally get, Class 9 is a bright inner-city sky where only the Moon, planets, and the brightest stars are visible. Its class boundaries aren't perfectly standardized across sources -- different tools draw the line between classes slightly differently -- which is why this app shows the underlying ALR and SQM numbers alongside the class, not just the class alone.
- Light pollution
- Artificial light scattered by the atmosphere back down toward the ground, brightening the sky and washing out fainter stars -- this app models it by taking real NASA/NOAA satellite measurements of night-time light emissions and propagating them through an atmospheric-scattering model, the same way a real ground observer's sky actually works (satellite measurements alone only show light escaping upward into space, not how much of it scatters back down to an eye looking up).
- Moon-free window
- The longest stretch of tonight's darkness window during which the moon is below the horizon and so isn't washing out the sky, regardless of its phase.
- SQM (Sky Quality Meter reading)
- Sky brightness in magnitudes per square arcsecond (mag/arcsec²), the unit a real handheld Sky Quality Meter reports and the unit most amateur astronomers actually compare sites in -- lower numbers mean a brighter (more light-polluted) sky, since magnitudes are an inverted log scale. A pristine natural sky reads about 22; a bright city center can read below 18.
- Terrain shielding
- Hills, mountains, canyon walls and tree lines that block part of the sky, or block the light traveling toward you from a distant town. Real observers notice it -- a ridge between you and a city genuinely darkens that side of the sky -- but this app's model, like the published atlases it is built on (Duriscoe et al. 2018; Falchi et al. 2016), treats the ground as smooth and does not include it. Checked against 656 real US National Park Service measurements spanning sea level to 6,800 m, an observer's elevation shifts the modeled value by only about 10% per 1,000 m of height, far less than night-to-night changes in atmospheric haze -- so the omission is small on average. It is still why a sheltered canyon floor or a valley ringed by peaks can look darker in person than the number here suggests, and why a Bortle class you judge by eye may differ by a step from the one shown.
- VIIRS Day/Night Band
- A NASA/NOAA satellite instrument that measures night-time light emissions from space, aboard the Suomi NPP satellite. Its night-time pass crosses the equator at around 1:30 in the morning local time, so what it records is a world with much of its commercial and municipal lighting already switched off -- worth knowing when comparing the class shown here against what your own sky looks like earlier in the evening, which is usually somewhat brighter. This app uses its 2025 annual composite (VNP46A4) -- a full year of cloud-free, moonlight-corrected observations averaged together -- so every town, industrial site, and highway that emits real light shows up, not just large cities. It does have one blind spot worth knowing about: the instrument measures any light leaving the ground at night, and cannot tell what produced it. Lava and gas flares are recorded the same way streetlights are, and because they burn far redder than lighting -- red light both scatters much less in the atmosphere and is nearly invisible to dark-adapted eyes -- this app can over-estimate the sky glow near them. The clearest case is the Big Island of Hawaii, where the lava at Kilauea registers over twenty times brighter than the whole town of Hilo, and the model consequently reads several classes too bright at Mauna Kea. Active volcanoes and major oil and gas flaring fields are the places to treat the number here with suspicion; everywhere else it is calibrated against 656 real measurements.
Zodiacal Light
Whether the zodiacal light -- the broad cone of sunlight scattered off interplanetary dust that leans up from where the sun set, or from where it is about to rise -- is actually visible tonight from your location, combining how steeply the ecliptic stands against your horizon on this date with the real measured brightness of the dust cone, the moon, and how dark your own sky is.
Page help ⌄
How to use this page: the featured card is tonight's zodiacal light at your location -- click it (or any card below) for more detail and a rotating tip. Use the Evening/Morning toggle first, because this object is strictly one or the other and the two peak in opposite seasons at the same place: if the evening view looks hopeless right now, the morning one may not be. The Ecliptic Angle card is the one worth understanding -- it is pure geometry, depends only on your latitude and the date, and it is the reason this is a seasonal object at all. Sky Darkness is essentially a fixed property of where you are and won't change from night to night, though it does drop through the course of a night as lighting switches off; Moonlight and the viewing window change nightly. Best Nights Ahead scans the next 60 nights and ranks them, which is genuinely useful here because good geometry and an absent moon drift in and out of step with each other.
How to read the graph: the graph under the map plots the same score over time at your own location -- switch between Week, Month and Year with the toggle at its top left. The Year view is the one to look at first: it shows the whole seasonal shape at a glance, including the fact that the good months come in one broad block rather than being scattered. The rapid up-and-down inside that block is the moon, cycling roughly every 29.5 days, which is why a good season still only contains a handful of genuinely usable nights. Pick a second line from the dropdowns (or click the small dot on any card) to plot it against the score: Ecliptic Angle shows the pure seasonal geometry with no moon in it at all, so where the score dips but the angle does not, the moon is the culprit. Gaps in the line are dates with no astronomical darkness at all, which is a different thing from a bad night and is drawn as a break rather than as a zero. One thing that can look wrong on the Ecliptic Angle line but is not: it is drawn against a 0 to 90 degree axis, but your latitude caps it well below 90 (about 90 minus your latitude, plus 23.4), so at mid-latitudes it peaks partway up the axis and comes back down. Only in the tropics does it reach 90 at all, and there it can touch that ceiling and fold back -- see the Ecliptic angle definition below for why.
How to read the map: color shows how visible the zodiacal light would be at each place on Earth on the selected date, scored against its own local dusk (or dawn) rather than one shared instant -- so the whole globe is colored at once, each point answering "is tonight a good night here". Brighter, warmer color means a better-placed, higher-contrast cone. Two things drive it: how steeply the ecliptic stands at that latitude on this date, and whether the moon is above the horizon at that point's own twilight. Importantly, the color assumes you are at a genuinely dark site (around Bortle 2-3) -- it deliberately does not include local light pollution, both because that is already mapped in its own right by the Dark Skies tab and the Light Pollution overlay in NovelEarth, and because folding it in would mean downloading a 50 MB dataset just to open this tab. Hover (or touch and drag) anywhere for that point's score plus the real Bortle class there, which is what tells you whether that particular spot could actually deliver what the geometry allows. The coral band is a second, separate thing from the color, and answers a different question: it marks everywhere that is inside its viewing window at the moment shown on the slider -- astronomically dark, with the bright inner cone still above the horizon. Drag the slider and it sweeps around the globe. It is narrow for a real reason rather than a drawing choice. Measured across latitudes and seasons, the stretch when the sky is dark and the bright inner cone is still above the horizon runs a median of about twenty-five minutes, so at any instant only a few percent of the Earth is inside one. That is a stricter window than the whole cone's, since the fainter outer part lingers longer after the base has set -- the band marks the part actually worth going outside for. Note the slider moves the band and nothing else -- the colors behind it are the whole night's best at each place, so they do not change as you scrub. For the same reason the band's edges are not the edges of the color: the color covers everywhere that gets a good window at some point tonight, while the band is one instant's slice through it, so it is always far narrower east to west. It also stops a few degrees short of the color at the poleward end, and that is deliberate: the band tracks the bright base of the cone, the part actually worth going out for, while the color scores the whole cone including its fainter outer reaches. Near the limit the base skims the horizon while the outer cone is still high, which is a real night worth a look but not one with a sharp window to point at. The band can be switched off with the "Viewing window" checkbox, which takes the slider away with it, since driving the band is the only thing that slider does.
How to read the graph: the graph under the map plots the same score over time at your own location -- switch between Week, Month and Year with the toggle at its top left. The Year view is the one to look at first: it shows the whole seasonal shape at a glance, including the fact that the good months come in one broad block rather than being scattered. The rapid up-and-down inside that block is the moon, cycling roughly every 29.5 days, which is why a good season still only contains a handful of genuinely usable nights. Pick a second line from the dropdowns (or click the small dot on any card) to plot it against the score: Ecliptic Angle shows the pure seasonal geometry with no moon in it at all, so where the score dips but the angle does not, the moon is the culprit. Gaps in the line are dates with no astronomical darkness at all, which is a different thing from a bad night and is drawn as a break rather than as a zero. One thing that can look wrong on the Ecliptic Angle line but is not: it is drawn against a 0 to 90 degree axis, but your latitude caps it well below 90 (about 90 minus your latitude, plus 23.4), so at mid-latitudes it peaks partway up the axis and comes back down. Only in the tropics does it reach 90 at all, and there it can touch that ceiling and fold back -- see the Ecliptic angle definition below for why.
How to read the map: color shows how visible the zodiacal light would be at each place on Earth on the selected date, scored against its own local dusk (or dawn) rather than one shared instant -- so the whole globe is colored at once, each point answering "is tonight a good night here". Brighter, warmer color means a better-placed, higher-contrast cone. Two things drive it: how steeply the ecliptic stands at that latitude on this date, and whether the moon is above the horizon at that point's own twilight. Importantly, the color assumes you are at a genuinely dark site (around Bortle 2-3) -- it deliberately does not include local light pollution, both because that is already mapped in its own right by the Dark Skies tab and the Light Pollution overlay in NovelEarth, and because folding it in would mean downloading a 50 MB dataset just to open this tab. Hover (or touch and drag) anywhere for that point's score plus the real Bortle class there, which is what tells you whether that particular spot could actually deliver what the geometry allows. The coral band is a second, separate thing from the color, and answers a different question: it marks everywhere that is inside its viewing window at the moment shown on the slider -- astronomically dark, with the bright inner cone still above the horizon. Drag the slider and it sweeps around the globe. It is narrow for a real reason rather than a drawing choice. Measured across latitudes and seasons, the stretch when the sky is dark and the bright inner cone is still above the horizon runs a median of about twenty-five minutes, so at any instant only a few percent of the Earth is inside one. That is a stricter window than the whole cone's, since the fainter outer part lingers longer after the base has set -- the band marks the part actually worth going outside for. Note the slider moves the band and nothing else -- the colors behind it are the whole night's best at each place, so they do not change as you scrub. For the same reason the band's edges are not the edges of the color: the color covers everywhere that gets a good window at some point tonight, while the band is one instant's slice through it, so it is always far narrower east to west. It also stops a few degrees short of the color at the poleward end, and that is deliberate: the band tracks the bright base of the cone, the part actually worth going out for, while the color scores the whole cone including its fainter outer reaches. Near the limit the base skims the horizon while the outer cone is still high, which is a real night worth a look but not one with a sharp window to point at. The band can be switched off with the "Viewing window" checkbox, which takes the slider away with it, since driving the band is the only thing that slider does.
Key Definitions
- Cone height
- How far above the horizon the zodiacal light stays bright enough to pick out, in degrees. Bortle used the same idea as a yardstick for describing skies: reaching halfway to the zenith, about 45 degrees, marks the difference between a properly dark site and a merely decent one.
- Contrast
- How much brighter the cone is than the sky immediately around it. This, rather than absolute brightness, is what decides whether the eye can pick it out -- which is why the cone is a relatively easy object while the much flatter zodiacal band, barely fainter in absolute terms, needs a superb site.
- Dark adaptation
- The slow shift of the eye to night vision -- the pupil opens within seconds, but the chemical change in the retina takes 20 to 30 minutes to complete and is undone by a few seconds of white light, including a phone screen. It matters more for the zodiacal light than for stars, because a fully dark-adapted eye is far better at broad, low-contrast glows than at points. It is also why the pre-dawn cone often looks better than the evening one at an identical predicted score: before dawn you may have been in darkness for hours, where in the evening you have had minutes.
- Detection threshold
- The least contrast a human eye can pick out against a given sky, which is not a fixed number -- the darker the background, the smaller the difference the eye can detect. The map's hover readout quotes the cone's contrast as a multiple of this threshold, so 2.5x means comfortably visible, 1.0x means right at the edge of detection, and anything below 1 means the cone is there but too faint for the eye to separate from the sky around it. This is what the visibility score is built from.
- Ecliptic
- The plane of the solar system projected onto the sky -- the line the sun, moon and planets all appear to follow through the year. The zodiacal light lies along it, because that is where the dust is.
- Ecliptic angle
- The angle the ecliptic makes with your horizon at the moment the sky becomes fully dark. It is the single number that decides whether the cone stands up out of the horizon haze or lies flat along it, and it depends only on your latitude and the date. This is why the zodiacal light is a spring-evening and autumn-morning object at northern mid-latitudes, a year-round object in the tropics, and a genuinely difficult one at high latitudes. The number has a hard ceiling. It is the angle between two planes, so it can never exceed 90 degrees, which would mean the ecliptic standing perfectly upright, straight through the point overhead. Your own ceiling is usually lower than that: roughly 90 degrees, minus your latitude, plus 23.4. An observer at 40 degrees north can never see the ecliptic steeper than about 73 degrees, and one at 34 degrees tops out near 79, however good the date. Only inside the tropics does that ceiling reach the full 90, and only there can the yearly curve touch it and bend back down -- which looks like a graph clipping but is not. Past upright the ecliptic simply leans the other way, and this angle records only how far it is from flat, never which way it tilts.
- Elongation
- How far around the sky something is from the sun, measured in degrees. The zodiacal light is brightest at small elongations -- close to the sun, low in the twilight sky -- and fades steadily outward along the ecliptic.
- Gegenschein
- A faint oval counter-glow directly opposite the sun -- the same interplanetary dust seen at full phase, where the grains backscatter sunlight most efficiently, the same effect that makes a full moon disproportionately brighter than a half moon. Far harder to see than the cone: it needs a pristine sky, complete dark adaptation, and is lost entirely whenever the antisolar point happens to lie against the Milky Way.
- S10 unit
- The surface-brightness unit used in the published measurements this model is built on: the brightness of one 10th-magnitude star spread evenly over one square degree of sky. Convenient for diffuse glows that have no edges to measure.
- Sky brightness (mag/arcsec²)
- How bright the background sky is at the place in it where the cone shows best, in the same magnitudes-per-square-arcsecond unit a Sky Quality Meter reports -- lower numbers mean a brighter sky. Note this is deliberately not the zenith figure the Dark Skies tab quotes: the brightest part of the cone sits low, where airglow is already brighter than it is overhead, so the sky the cone actually has to compete against is a few tenths of a magnitude brighter than a meter pointed straight up would read.
- Viewing window
- The stretch of a night when the zodiacal light can actually be seen: the sky is fully astronomically dark and the bright inner part of the cone is still above the horizon. It is short. Measured across latitudes and seasons it runs a median of about twenty-five minutes, which is why arriving already dark-adapted matters so much, and why the band drawn on the map covers only a few percent of the Earth at any instant. The fainter outer reaches of the cone linger longer, so a looser definition would give a longer window, but this one marks the part worth going outside for.
- Visibility score
- The 0-100 number this tab quotes for a given night, and the vertical axis of the graph. It combines how much brighter the cone is than the sky around it with how far up the sky it stays detectable, so a bright cone low on the horizon and a taller but washed-out one can land in the same place. Zero means the sky is genuinely dark and there is still nothing to see; a gap in the graph means something different, that the sky never gets astronomically dark on that date at all.
- Zodiacal band
- The very faint bridge of light joining the evening and morning cones right around the ecliptic. It is only marginally fainter than the outer cone in absolute terms, but because it is a near-flat gradient spread across the whole sky rather than a localized wedge, it is dramatically harder to see.
- Zodiacal light
- A broad, softly tapering cone of light leaning up from where the sun set, or from where it is about to rise -- sunlight scattered off a disc of interplanetary dust orbiting the sun in roughly the same plane as the planets. From a properly dark site the base of the cone outshines the Milky Way, and it is often mistaken for light pollution from a distant town. It needs full astronomical darkness, a dark site, and no moon.
SkyView
The whole night sky at once, as it looks from where you are standing: zenith at the center, horizon at the rim. Everything this app models about sky brightness -- airglow, the zodiacal light, moonlight and light pollution -- is summed into a single picture rather than shown one map at a time, with the meteor radiants, the gegenschein and the auroral oval marked on top of it.
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Page help ⌄
How to read this: the disc is the whole sky over your head. The center is straight up, the outer rim is the horizon, and north, east, south and west are marked around the edge -- so it reads like a map held above you rather than one on the ground, which is why east appears on the right of north rather than the left. The two dashed rings mark 30° and 60° above the horizon. Color shows what is making the sky bright in each direction, not just how bright it is: warm orange low down is a town, warm gold leaning up from the sunset is the zodiacal light, cool blue-white around the moon is scattered moonlight, and the faint green wash that strengthens toward the rim everywhere is airglow. Hover anywhere for the sky brightness in that direction and which of them dominates it. Click any star for its name, constellation and catalogued color, where this catalogue has a match. Scroll, the +/− buttons, or the keyboard zoom the whole sky in around wherever you are pointed -- drag or the arrow keys pan once zoomed, and Reset (or Home) returns to the full dome.
Two ways to look: Whole sky flattens the entire dome onto one disc — good for seeing where everything is at once. Look around points a camera at one part of it instead, the way you would actually stand and face something: drag to turn, scroll or +/− to zoom, arrow keys to move without a mouse. On a phone, Use device lets the view follow wherever you point it, so you can hold it up and match what is in front of you. Both modes draw the same sky from the same model and share the date, time, exposure and every overlay below.
Combining overlays: the first four are physical contributions to one sky and are added together, so switching one off shows you the sky you would have without it -- that is the point of the controls. The gegenschein, meteor radiants and aurora are markers drawn on top; they tell you where something is rather than adding light. The All box turns everything on at once. Stars is drawn on top like the markers, but it is the one overlay that responds to the others: each star is only plotted if the sky where it actually sits is dark enough to show it, so switching Light Pollution on visibly erases most of them — which is the clearest statement this page can make about what a bright sky costs you. Planets behaves the same way, gated on sky brightness like the stars rather than always shown like the markers — click one, the same as a star, for its current brightness and distance. Two overlays behave differently on purpose: Sun & twilight is brightest before darkness rather than during it, and Aurora grays out on any date or time more than about an hour and a half from now, because the forecast behind it does not reach that far.
Two ways to look: Whole sky flattens the entire dome onto one disc — good for seeing where everything is at once. Look around points a camera at one part of it instead, the way you would actually stand and face something: drag to turn, scroll or +/− to zoom, arrow keys to move without a mouse. On a phone, Use device lets the view follow wherever you point it, so you can hold it up and match what is in front of you. Both modes draw the same sky from the same model and share the date, time, exposure and every overlay below.
Combining overlays: the first four are physical contributions to one sky and are added together, so switching one off shows you the sky you would have without it -- that is the point of the controls. The gegenschein, meteor radiants and aurora are markers drawn on top; they tell you where something is rather than adding light. The All box turns everything on at once. Stars is drawn on top like the markers, but it is the one overlay that responds to the others: each star is only plotted if the sky where it actually sits is dark enough to show it, so switching Light Pollution on visibly erases most of them — which is the clearest statement this page can make about what a bright sky costs you. Planets behaves the same way, gated on sky brightness like the stars rather than always shown like the markers — click one, the same as a star, for its current brightness and distance. Two overlays behave differently on purpose: Sun & twilight is brightest before darkness rather than during it, and Aurora grays out on any date or time more than about an hour and a half from now, because the forecast behind it does not reach that far.
Key Definitions
- Altitude and azimuth
- The two numbers that fix a direction in the sky as seen from the ground. Altitude is how far up from the horizon, from 0° at the rim to 90° straight overhead; azimuth is the compass bearing you face, measured clockwise from north. Every position on this chart is one pair of them, which is why the same star sits somewhere different at a different hour or a different latitude.
- B-V color index
- How a star's brightness compares through a blue filter against a yellow-green one, which is the standard astronomical stand-in for its surface temperature: a negative value is a hot blue-white star, near zero is white, and the scale runs up through yellow and orange to red for the coolest stars. Click any star for its value and the plain-language color it corresponds to -- Rigel is about -0.03 (blue-white, hot), the Sun would read about 0.65 (yellow-white), and Betelgeuse is about 1.5 (red, cool).
- City lights
- The individual towns behind your light pollution, from the very same satellite measurements the Dark Skies tab sums into one number — the difference between “your sky is five times its natural brightness” and “that is the town doing it, over there”. A light only appears if the land actually lets you see it, so from a valley floor you may see almost none while the sky above you still glows. Each point is the combined output of a patch about a kilometer across rather than a single lamp, and its position within that patch is not known, so the lights are spread across it rather than pinned to its center.
- Constellation figures
- The stick figures joining a constellation’s brighter stars. They are a convention, not a standard — different cultures and different atlases join different stars, and the only official definition is the set of IAU boundaries, which are regions of sky rather than figures. Unlike the stars themselves the lines are not hidden by a bright sky, deliberately: from a city they are most useful exactly where the stars in them have been erased.
- Distance (light-years)
- How far away a clicked star actually is, from its Hipparcos satellite parallax -- the tiny yearly wobble in a star's apparent position caused by Earth's own orbit, the only direct way to measure a star's distance without assuming anything about how bright it "really" is. A small number of the catalogue's stars have no reliable measurement and show no distance at all rather than a guessed one.
- Equidistant projection
- The rule this chart uses to flatten a dome onto a disc: distance from the center is directly proportional to angle down from the zenith, so the ring halfway out is exactly 45° up. It keeps altitudes evenly spaced and gives the horizon -- where the light domes, the airglow and the base of the zodiacal cone all live -- more room than the alternatives, at the cost of stretching shapes near the rim.
- Field of view
- How much sky the Look around mode fits across the frame, in degrees. For scale, your fist at arm’s length covers about 10°, and the bowl of the Plough is roughly 25° across. Zooming in shows fainter detail more clearly but makes it easier to lose track of where you are pointing — the compass marks sit on the horizon line for that reason.
- Integrated starlight
- The combined glow of all the stars too faint to see individually, which is what makes the Milky Way a band of light rather than a collection of points. It is part of the sky brightness this chart computes rather than something drawn on top of it, and it is why the sky is measurably brighter along the galactic plane than toward the galactic poles — the stars plotted as points are the ones bright enough to be excluded from it.
- Limiting magnitude
- The faintest star a sky will let you see, on the magnitude scale where lower is brighter. It is set by how bright the sky behind the star is, not by the star: a pristine sky reaches about 6.8, a suburban one about 5, and an inner city about 4, which is the difference between a few thousand visible stars and a few dozen. This chart applies it separately in every direction, so stars thin out toward a town on the horizon and hold on in the dark half of the sky.
- Magnitude
- How bright a star appears, on a backwards scale inherited from antiquity: each step of 1 is about 2.5 times fainter, and the brightest stars have negative values. Sirius is -1.5, the stars of the Plough are around 2, and roughly 6 is as faint as an unaided eye can reach under a perfect sky.
- Observer elevation
- How high you are standing, taken from the same elevation data. It matters because there is less air above you: the light of every star is dimmed as it comes down through the atmosphere, and higher up there is less of it to come through, so more stars cross the threshold of visibility. From sea level to 2,500 m is worth roughly 0.13 magnitudes straight overhead and more near the horizon — on this chart, about 14% more stars at a dark site. This is a large part of why observatories are built on mountains.
- Planets
- Mercury through Uranus, computed from published orbital elements rather than looked up from a star catalogue -- they move, so their position and brightness are worked out fresh for whatever date and time this page is showing. Uranus is the faintest, right at the edge of naked-eye visibility even under a genuinely dark sky; the other five are easy naked-eye objects whenever they are above the horizon and the sky is dark enough. Neptune is left out entirely -- it is never bright enough to see without a telescope.
- Point brightness vs an SQM reading
- The number under this chart and the SQM on the Dark Skies tab are in the same unit and will not agree, on purpose. This one is the brightness in one direction at one moment, and it includes the moon. The Dark Skies figure describes your location: it never includes the moon, it is an all-sky average rather than a single direction, and it carries a correction for the fact that a real Sky Quality Meter averages over a 20–40° patch of sky rather than reading a true point — so it is what a handheld meter would actually show you. When the two disagree badly, it is almost always the moon. At a bright suburban site with a full moon well up, this chart reads about 18.9 overhead against the card’s 20.7, and the moon alone accounts for 2.4 of those magnitudes. The other two differences pull in opposite directions and mostly cancel: with the moon down the same site reads 21.2 here against 20.7 there, because straight up is the darkest part of a light-polluted sky while the card averages the whole hemisphere.
- Radiance field
- The set of sky contributions that genuinely add together as light -- airglow, zodiacal light, light pollution and moonlight. They are summed before anything is drawn and then converted to color once, which is why switching one off changes how the rest look. Markers such as the meteor radiants are not part of it; they sit on top and carry no brightness of their own.
- Scattered moonlight
- Moonlight bounced back down by the atmosphere. It does not light the sky evenly: it is far brighter near the moon than away from it, and brighter still toward the horizon where there is more air in the way. This is why a moonlit night can be perfectly usable in the half of the sky away from the moon, which a single all-sky number can never tell you.
- Sea horizon
- Over open water the horizon is not level with your eye — it sits below it, by an amount set purely by how high you are standing: about a quarter of a degree from a 70 m clifftop, and 1.2° from a mountain at 1,700 m. That is why the sea appears to curve away, and why a taller vantage point lets you see further. Inland the horizon stays at eye level or above, because the land simply continues.
- Sky brightness (mag/arcsec²)
- How much light the sky itself gives off in one direction, in the unit a Sky Quality Meter reports -- lower numbers mean a brighter sky, since magnitudes run backwards. A pristine sky overhead reads about 22; under a full moon it can reach 18, which is roughly fifty times brighter.
- Terrain horizon
- The skyline your surroundings actually cut out of the sky, computed from elevation data within about 58 km, drawn as shaded relief that hides whatever sits behind it. The ground is lit the way it really is at night: by the sky itself, which is why unlit terrain under a moonless sky is almost black and a light-polluted one is not, and by the Moon when it is up, which is what makes slopes and gullies visible at all on most nights. Distant ridges fade toward the color of the sky behind them, because you are seeing them through kilometers of the same glowing air. On a plain it is almost nothing; on a valley floor it can remove more than half the sky. It is bare earth — no trees and no buildings — so a real horizon in a forest or a town is higher and closer than this shows, and anything beyond that radius (a distant range on a very clear day) is not included.
- Twilight
- The sky after sunset but before true night, and the brightest thing this chart shows apart from the Moon — at the end of nautical twilight the sky still outshines a full-moon night. It is warm and low toward where the sun set, because that light takes a long slanting path through the atmosphere and arrives reddened, and cool higher up where the path is short. Nothing faint is visible until it has faded, which is why scrubbing the time slider forward through dusk is the quickest way to see when the zodiacal light stops competing with it.
- Water and the moonglade
- Sea is detected from the elevation data itself — anything below sea level — and rendered as water rather than land: a poor scatterer, so nearly black, but an excellent mirror. When the Moon is up over the sea that mirror produces the moonglade, the shimmering path running toward you, stretched along your line of sight because waves tilt through a range of angles. Lakes are found a different way, since most sit well above sea level and Lake Superior at 179 m would otherwise read as dry land: a lake surface is exactly level and terrain never is, so a patch of ground that does not change height at all across 300 m is taken to be water. The limits worth knowing: genuine dry land below sea level, such as Death Valley, is wrongly taken for water, and a small pond is below the resolution of the elevation data entirely.
- Zenith
- The point directly overhead, at the center of this chart. It is the darkest part of any sky, because you are looking through the least air and the least of everything suspended in it.
- Zodiacal cone and zodiacal glow
- Two things the same dust does, and the reason switching this overlay on can brighten the whole sky rather than lighting up one wedge of it. The cone is the localized tapering wedge leaning up from where the sun set or is about to rise, and it is only above the horizon for a stretch around twilight in the right season. The glow is the same dust seen in every other direction at once — far fainter per patch of sky, but spread over all of it, and worth roughly 30% of the brightness of a natural dark sky even on a night when the cone has already set. The readout under the chart says which of the two you are currently looking at.