The page opens on the sky over you right now, and on your own location if the browser offers it. The details panel opens on Your location for the same reason, since what the sky is doing where you are standing is the question most readers arrive with. Use my location on that panel asks the browser again at any time, which is the way back after the pin has been moved, or after a first refusal has been reconsidered in the browser's settings; it brings the view along only if the pin would otherwise be off the screen. The Eclipse tab beside it holds the circumstances of the event itself. The countdown at the top belongs to the next solar eclipse; Show this eclipse puts it on the map, and the Show menu reaches every solar and lunar eclipse besides.
Scroll or double-click to zoom, drag to pan, pinch on a touch screen, click to drop a pin. The pin gets a bearing line to the Sun and another to the Moon, for lining up a photograph, and a line to each planet and deep sky object once those are switched on. Its label carries the local date and time where it stands. The Date and time box and the Now button work in every mode, and the −1 day and +1 day buttons step the date without opening the calendar. Setting any of them leaves the eclipse behind and shows the Sun and the Moon over the Earth at that moment instead. The date box reads in whichever clock Show times in is set to, so switching that menu to the pin puts the whole page on local time where you will be standing.
Base map chooses what lies under the chart. Plain is the quiet gray wash the eclipse lines were drawn for, Streets colors the land and water and draws the roads, Terrain adds contour lines and hill shading, and Satellite is aerial photography. Map labels paints street and place names into the base map itself; with them on the page stops drawing its own names, so the two sets never collide. Terrain already carries names in the imagery, so the switch stays on there. Every base map works in every projection, the globe included.
Night sky adds two overlays, both off until you ask for them, since a map carrying everything at once is a map nobody can read. Planets marks the seven planets besides the Earth where each one stands directly overhead, drawn as itself: Mars rust, Jupiter with its belts, Saturn with its rings open by the angle they really present this year, and Mercury and Venus showing the phase they are in. Deep sky marks five targets worth a camera, each drawn roughly as it looks, a tilted ellipse for Andromeda, a lumpy cloud for the Orion and Eagle nebulae, a scatter of stars for the Pleiades.
Every mark is sized by how large that body actually appears from the Earth, on a compressed scale so that Neptune stays findable, which is why the Sun and the Moon dominate: nothing else in the sky comes near half a degree across. The marks keep their size as you zoom, since they say where a thing is rather than how big it looks on the ground.
Drop a pin and a line runs to everything that can be seen from there, tagged with the object's name and how high it stands. A solid line means the object is up now and the camera can point along it tonight. A dashed one means it is under the horizon at this moment, and its tag gives a negative height saying how far down, because a night is planned around what will rise later as much as around what is up already. Zoomed in on a shooting spot the marks themselves are far off the screen, so the line and its tag are what remain.
An object that never rises at that latitude gets no line at all. Something forty-one degrees north like Andromeda is permanently below the horizon from the far south, and no amount of waiting brings it up, so drawing a bearing to it would be an invitation to point a camera at the ground. The table under Your location repeats the same figures, highest first, and names anything it had to leave out for that reason.
The Moon mark on the map shows the real phase for the moment on the slider, lit limb facing the Sun mark. Tracks works like a satellite ground track. Each mark rides a line showing where it stands overhead through the surrounding day, solid behind it and dashed ahead. The thin wave is the Moon's overhead point day by day for half a month either side of now, one swing north and south per lunar month. The dotted parallels mark how far north and south each body ever stands overhead in the displayed year; between a body's pair of dotted lines lies every point on Earth where it can be overhead that year. The Sun's pair is the two tropics, and the Moon's pair drifts through an eighteen and a half year cycle that can carry it five degrees beyond them.
The penumbra is the outer cone, thousands of kilometres across, where part of the Sun is hidden. Its edge is the thin closed curve sweeping the map.
The umbra is the inner cone, rarely more than 250 km wide, where the Moon covers the Sun completely. The shaded band it leaves behind is the path of totality, and the dashed line down its middle is where totality lasts longest.
When the Moon is near the far end of its orbit the umbral cone runs out before it reaches the ground. What lands instead is the antumbra, and the eclipse is annular, leaving a ring of Sun around the Moon. The same geometry covers both cases, since the only difference is whether the cone has passed its apex.
Shading always marks where the event cannot be seen. On the solar map that is the night side, and on the lunar map it is the half of the Earth where the Moon is below the horizon.
The orange mark is the point where the Sun is exactly overhead at the time on the slider, and the gray mark is the same point for the Moon. Drop a pin and two lines run from it, orange to the Sun mark and gray to the Moon mark. The direction a line leaves the pin is that body’s compass bearing from where you stand, so zoomed in on a shooting spot it points straight at the Sun or the Moon, and the table under the map gives the same bearing and altitude in degrees, updating as you scrub the time. During a solar eclipse the two marks nearly coincide, since the Moon then stands directly in front of the Sun. The Date and time box always shows the moment currently on the map, in the clock named beside it, and typing a different one into it gives the same marks, lines, and bearings for any date you like, with sunrise, sunset, moonrise, and moonset at the pin. Now does the same for the present moment. Both leave the eclipse behind and switch the Show menu to the Sun and Moon locator, where the time slider then covers twelve hours either side of the chosen instant.
Night sky puts the rest of the sky on the same footing. Tick Planets and each of the seven planets besides the Earth appears where it stands overhead, drawn as itself rather than as a token. Mars is rust, Jupiter carries its belts, Saturn its rings, and the ring opening is computed from the angle Saturn’s pole makes with the line of sight, so the near edge-on rings of the middle 2020s draw as a line and widen again through the following decade. Mercury and Venus show their phase, which they have for the same reason the Moon does, and a thin crescent Venus is also the largest Venus, since a crescent means it is on our side of the Sun and close.
Tick Deep sky and five targets appear, each drawn roughly as it looks. Andromeda is a tilted ellipse with a bright core, the Orion and Eagle nebulae are lumpy clouds, the Pleiades are a scatter of stars, and the Large Magellanic Cloud is a lopsided galaxy for southern skies. Four of the five are within reach of a camera on a tripod. The fifth is the Eagle Nebula, which is easy enough itself, but the Pillars of Creation standing inside it are a few arcseconds of dust that no amateur instrument resolves. Five is the whole list on purpose. A short list of things worth photographing beats a long one nobody reads.
A planet moves against the stars from night to night, while a galaxy or a nebula is fixed, so its overhead point simply circles the Earth once a day along its own parallel of latitude.
Every mark is sized by how large the body actually appears from the Earth. Those sizes span a factor of eight hundred, from half a degree for the Sun and the Moon down to two arcseconds for Neptune, so drawing them to scale would leave everything but the first two invisible. The scale is logarithmic instead, which keeps the ranking a telescope would show while leaving the smallest disc large enough to find. Marks do not grow as you zoom, since what they mark is a position rather than an extent.
From a dropped pin a line runs to everything visible from there, tagged with the object’s name and its height in degrees, and the direction it leaves the pin is the bearing to aim a camera along. Zoomed in on a shooting spot the marks are far off the screen and the tagged lines are all that is left, which is the point of tagging them. A solid line is an object that is up now. A dashed one is an object under the horizon at this moment, and its height reads as a negative number, which is how far it still has to climb. The Sun and the Moon are tagged the same way, so a Sun sitting twenty degrees under the horizon says exactly that rather than only that it has set.
What never rises at all gets no line. An object culminates at ninety degrees minus the gap between the observer’s latitude and the object’s declination, so once that gap exceeds ninety the object stays under the horizon on every night of every year. Andromeda at forty-one degrees north is out of reach from below roughly forty-nine degrees south, and the Large Magellanic Cloud, seventy degrees south, is out of reach from most of Europe. Those cases are dropped from the lines and from the table, and the table names them so that a missing object reads as a fact about the latitude rather than as a fault. The half degree of slack in the test is refraction, which lifts a body sitting just under the horizon into view.
The overhead marks do not sit on the path of totality, and that is not an error. The path is where the Moon covers the Sun, which happens wherever the Moon’s shadow lands, and the shadow usually strikes the Earth off center. The marks are where the Sun or the Moon is at the zenith. The two places coincide only for an eclipse whose shadow passes through the center of the day side, and from the path itself the Sun stands high in the sky but short of overhead, exactly as the pin readout reports.
A lunar eclipse has no path. Everyone who can see the Moon sees the same thing at the same moment, which is why a total lunar eclipse is visible from roughly half the planet while totality in a solar eclipse falls on well under one per cent of the surface.
Mercator keeps rhumb lines straight, the courses a navigator holds at a fixed compass bearing. It cannot show the poles and it inflates high latitudes, so a polar track looks far larger than it is.
Robinson is the atlas compromise. Nothing is exactly right, but the whole world looks close enough to right, which is why it is the familiar wall-map shape.
Equirectangular maps longitude and latitude straight onto x and y. Distorted almost everywhere, but the only flat option that shows the poles honestly as full-width edges.
Globe has no distortion at the center and is the only one that shows the shadow as the round patch it actually is.
Positions come from Astronomy Engine, implementing VSOP87 and ELP2000. The shadow geometry is computed directly from the Sun and Moon vectors rather than from published Besselian elements, and the axis intersection with the Earth reproduces the library’s own greatest-eclipse coordinates to better than one meter.
Planet positions, apparent diameters, phases and Saturn’s ring angle all come from the same library. The deep sky objects are carried at their J2000 right ascension and declination and precessed to the displayed date, so the marks stay honest in any era. Their overhead points are geometric, taking no account of whether the object is bright enough to be worth pointing a camera at from where you stand, which depends on the Moon, the weather, and how dark your sky is.
The colors, the sizes and the glyphs live in astro-assets/astro-core.js rather than in this page, which is why Mars is the same rust here as on the planetarium and one edit changes both. The shapes are caricatures. A real photograph of Andromeda shows dust lanes and a disc four times wider than the ellipse drawn here, and the point of the drawing is only that you recognize which object the mark belongs to.
The Earth is modeled as the WGS84 ellipsoid at sea level. Contact times for a given site will differ by a second or two from tables that account for local elevation and for the lunar limb profile, since mountains on the Moon’s edge shift second and third contact. For chasing the exact edge of the path, use the NASA eclipse bulletins.
Place names come from GeoNames, coastlines, borders, lakes and rivers from Natural Earth. Every city above fifty thousand people ships with the page, plus every national capital, and zooming further in fetches a finer layer of towns down to five thousand people, so small places label themselves once you are close enough.
The map ships with world-scale outlines so the page opens instantly, and they take over as the base map whenever tiles have not arrived yet.
Plain and Streets are not fetched as pictures at all. Their styles are held with the page, and the browser draws them from vector tiles into an offscreen canvas, which the chart then samples exactly as it samples a grid of downloaded images. That is why those two are sharp at any magnification, why their names can genuinely be switched off, and why light and dark are choices this page makes rather than a supplier’s product line. Terrain and Satellite are still ordinary image tiles, since no open equivalent of aerial photography exists at that resolution.
Beyond that, bundled data runs out and the base map takes over, which is the one feature here that talks to the network. Four are offered, all free to use and all asking only for attribution, which is why the credit line changes with the choice. Plain is a quiet gray wash meant to sit under the chart, and Streets colors the land and water and paints the road network. Both are drawn from OpenFreeMap vector tiles, which carry OpenStreetMap data through the OpenMapTiles schema. Terrain is OpenTopoMap, OpenStreetMap drawn with contour lines over SRTM elevation. Satellite is Esri World Imagery, which carries photography from Maxar and Earthstar Geographics.
On Plain, Streets and Satellite, names start off. The page already draws its own place names from GeoNames, and two sets of names on one map is one too many. Turning them on keeps the page’s own names quiet instead. Terrain paints its names into the imagery, and the switch cannot remove them.
Imagery is published in Web Mercator, and under every other projection, the globe included, it is warped onto the view on the fly. Coverage stops at 85 degrees of latitude, so the polar caps keep the plain chart background. How far the view goes depends on how deep the chosen source reaches. Plain, Streets and Satellite run to about fifty thousand times, deep enough to show individual buildings, and Terrain stops at a quarter of that, its deepest level being shallower. Each ceiling is where the deepest tiles available are still being drawn at their own resolution, so magnifying further would only stretch the same pixels. Close in, the path of totality is drawn as its two limit lines rather than a solid band, because a filled band at that scale would cover everything underneath it.
Weather is not modeled, and in practice it decides more eclipse trips than geometry does.