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 1,500 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.
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.
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.
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.
Safety
Never look at the Sun without certified eclipse viewers. Only remove them during locally visible totality; eye protection is always required for annular eclipses.
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.