About eclipses
A solar eclipse happens when the Moon passes between the Sun and the Earth and its shadow falls on the Earth. This page explains the parts of that shadow, the four kinds of eclipse you can see, the moments astronomers use to describe one, and where the numbers in this app come from.
How a solar eclipse works
The Sun is about 400 times wider than the Moon and, by coincidence, about 400 times farther away. Seen from Earth the two discs are therefore almost the same size, and the Moon can just cover the Sun. Because the Sun is not a point of light, the Moon casts two shadows: a narrow dark core where the Sun is hidden completely, and a wide half-shadow around it where only part of the Sun is covered. The drawing is not to scale: the Earth is about 30 Moon-diameters away from the Moon, and the Sun is far larger and farther than shown.
The shadow in three parts
Umbra: the dark core of the shadow, where the Moon hides the whole Sun. On Earth it is a spot at most a few hundred kilometres wide that races across the surface at more than 1,700 km/h. Only inside it is the eclipse total. In the app the umbra is the orange ellipse.
Penumbra: the wide half-shadow around the umbra, where the Moon covers only part of the Sun. It is thousands of kilometres across, so far more people see a partial eclipse than a total one. In the app the outer edge of the penumbra is the 0% line, and the rings inside it show where 20, 40, 60 and 80 percent of the Sun is covered.
Antumbra: the shadow beyond the tip of the umbra. The Moon's orbit is not a circle, so sometimes the Moon is too far away to cover the Sun completely. Its dark core then ends in space before it reaches the Earth, and the region behind that tip is the antumbra. Anyone standing in it sees the Moon as a dark disc inside the Sun, with a ring of sunlight around it. In the app that ring-shaped zone is the light blue ellipse.
Four kinds of solar eclipse
Total: the umbra reaches the Earth and you stand inside it. For a few seconds to a little over seven minutes the Sun disappears completely, the sky darkens as at dusk, bright stars and planets appear and the corona, the Sun's outer atmosphere, becomes visible. This is the only phase during which eclipse viewers may be taken off.
While it plays, the Moon moves through the drawing and its shadow sweeps across the Earth. The panel beside it shows the sky as seen from the yellow dot. Around the maximum the playback slows down, or totality would be over in a blink. In the partial eclipse the Moon passes beside the Sun, out of the drawing; in the hybrid one the dot travels along with the centre of the shadow.
Partial: you stand in the penumbra, so the Moon takes a bite out of the Sun but never covers it completely. Most eclipses are seen this way by most people, and some eclipses are partial everywhere because the umbra misses the Earth altogether. Eye protection is needed from start to finish.
Annular: the Moon is too far away to cover the whole Sun and you stand in the antumbra. At maximum a bright ring of Sun remains around the dark Moon. The sky hardly darkens and the ring is as dangerous to look at as the uncovered Sun, so eclipse viewers stay on throughout.
Hybrid: the rarest kind. The tip of the umbra lies almost exactly at the Earth's surface, so along the same path the eclipse is annular where the surface curves away from the Moon and total where the bulge of the Earth reaches into the umbra. The eclipse of 14 November 2031 will be one: annular for its first eight minutes over the Pacific, total from there to Panama, and annular again in its last five minutes. The app shows where each phase applies.
Why not every new moon
A solar eclipse can only happen at new moon, when the Moon is between us and the Sun. But the Moon's orbit is tilted about five degrees against the Earth's orbit, so at most new moons the shadow passes above or below the Earth. The drawing of the partial eclipse above shows a near miss: the umbra passes by and only the penumbra still touches the Earth. Usually the Moon passes so far above or below that even the penumbra misses the Earth altogether. Only when new moon falls close to one of the two points where the orbits cross, the nodes, does the shadow hit. That happens two to five times a year somewhere on Earth, but a total eclipse returns to any given place only about once every 375 years on average.
Path, duration and speed
The strip the umbra or antumbra sweeps across the Earth is the central path; the dashed line in the app is its centre. It is typically 100 to 200 kilometres wide and thousands of kilometres long. The shadow moves at least about 1,700 km/h eastward because the Moon overtakes the Earth's rotation; near sunrise and sunset, where it strikes the Earth at a grazing angle, it moves much faster. The longest totality is possible near noon at the equator, when the shadow moves slowest relative to the ground and the Moon is near its closest point to the Earth.
Why the path curves
Seen from the Sun, the Moon's shadow crosses the Earth in an almost straight line, from west to east. In the drawings below that is the globe: the Earth as the Sun sees it, a full disc in daylight. The edge of that disc is where the Sun is just rising or setting, so every central path begins at sunrise and ends at sunset.
That straight line lands on a ball, though. Carried along the sunlight onto the Earth, it traces a circle on the surface. When the shadow passes close to the middle of the disc, that circle is as large as the Earth itself, a tilted equator, and on a flat world map a tilted circle like that becomes an S: up on one side of the equator, down on the other, like the track of a satellite. When the shadow passes close to the edge of the disc, the circle is small, and if that edge lies near a pole the small circle bends around it: on the map the path swings in a half circle towards the pole and back.
While it plays, the Earth turns underneath. The yellow line is the route of the shadow as the Sun sees it, and it stays straight. The orange trail is where the shadow fell on the ground: it turns along with the Earth and bends.
Meanwhile the Earth turns underneath the shadow, by fifteen degrees an hour; the globe shows it as it stands halfway through the eclipse. The turning does not make the bend, but it does twist the circle. The start of the path lands on ground that by halfway has turned on further east, the end on ground that halfway was still further west. Both ends therefore shift along the parallels towards each other, and because the circle is tilted, that turns it into a gentle S. That is the S you also see on the globe in the app with a path near the equator, and on a flat map it makes the S sharper. How strongly depends on the latitude: at the equator the ground turns at about 1,670 km/h, at 45 degrees at about 1,180 km/h and near the pole hardly at all. A path near the equator therefore gets the clearest S, while the arc of 2026, close to the pole, keeps its shape; there the shift also runs mostly along the path itself.
The contacts: the scientific moments
Astronomers describe an eclipse at a given place by the moments when the edges of the Moon and the Sun touch. The app shows these in the watch guide for your chosen point.
- First contact (C1): the Moon's edge first touches the Sun. The partial phase begins.
- Second contact (C2): the Moon covers the Sun completely, or fits entirely inside it during an annular eclipse. Totality or annularity begins. Just before it, the last sunlight shining through the valleys on the Moon's edge breaks up into Baily's beads, and in a total eclipse the diamond ring flashes.
- Maximum: the moment of greatest obscuration at your place, halfway between the second and third contact. For a partial eclipse it is the moment the Moon covers the largest part of the Sun.
- Third contact (C3): the Sun's edge reappears. Totality or annularity ends and the partial phase resumes.
- Fourth contact (C4): the Moon leaves the Sun. The eclipse is over at your place.
For the Earth as a whole the same idea gives P1 to P4: the moments the penumbra first and last touches the Earth (P1, P4) and the moments it lies entirely on the Earth (P2, P3). The time slider in the app runs from P1 to P4. The moment the umbra's axis passes closest to the Earth's centre is called greatest eclipse.
The day itself
A total eclipse takes a few hours, but what matters happens in the last minutes before totality and in totality itself. This is what you see, in order, and what to keep in mind.
At first contact the Moon takes a first bite out of the Sun. For an hour or more you notice nothing without glasses; with them the bite slowly grows. Under trees the gaps between the leaves project small crescents onto the ground.
In the last quarter of an hour the light changes: shadows become sharp, colours flatten and the landscape looks as if a lamp lights it. It cools down noticeably, sometimes by several degrees, and the wind can change. Birds fall silent or fly to roost.
In the last minute the Moon's shadow comes in from the west; from a high spot it is a dark wall on the horizon. Just before totality faint shadow bands can ripple across a light surface, and the last sunlight breaks up into Baily's beads between the mountains of the Moon. The last bright point is the diamond ring.
Then the Sun is covered, and the glasses come off. The corona appears, the pale outer atmosphere of the Sun, sometimes with red prominences at the edge. It is as dark as deep twilight, the brightest planets and stars come out, and all round the horizon glows like a sunset. After totality everything repeats in reverse: the diamond ring on the other side, and the glasses go back on.
A few practical points. Be in place well before first contact: roads near the path get busy. Know the times of second and third contact for your spot; the viewing guide gives them, with the duration. In a hot place take water, a hat and sunscreen, and something warm for the cooling elsewhere. And watch, rather than spending totality behind a screen.
Taking photos
During the partial phases a camera or a phone needs a solar filter in front of the lens, just like your eyes. In totality the filter comes off, and it goes back on when the light returns. A phone does best with a wide view: the dark sky, the glowing horizon and the people around you. For the corona itself a telephoto lens on a tripod works better than a phone's zoom. Set everything up and try it out in the days before, and keep it simple: totality often lasts only two or three minutes.
Safety
Never look at the Sun without protection, not even when most of it is covered: the remaining crescent is bright enough to damage the retina. Use eclipse glasses or a handheld viewer that meets the ISO 12312-2 standard, buy them from a known seller, because fakes are sold, and do not use them if they are scratched or damaged. Ordinary sunglasses do not protect, however dark.
Only during totality, when the Sun is completely covered, may the glasses come off: after the diamond ring at the start, second contact. Put them back on as soon as the first bright point returns at the end, third contact. The viewing guide gives both times for your spot. Outside the path of totality, even at 99%, there is no moment without glasses, and at an annular eclipse neither: the ring of the Sun stays far too bright.
A camera, a phone with a zoom lens, binoculars or a telescope needs its own solar filter in front of the lens. Eclipse glasses behind binoculars or a telescope do not protect you: the concentrated light burns through them. Never look through the eyepiece of an instrument without a filter at the front.
Without glasses you can still watch safely, indirectly: a pinhole in a piece of card, a colander or the gaps between leaves project the crescent onto the ground or a sheet of paper. Keep an eye on children. The retina feels no pain, so damage is not noticed at once; if your sight is blurred or has a spot after an eclipse, see a doctor.
Sources and further reading
The geometry on this page and the calculations in the app rest on the same body of work. These are the sources this page draws on and the places to read further.