Put simply, a solar eclipse is a shadow play between three actors: the Sun, the Moon, and the Earth. Illuminated by the Sun, the Moon casts a shadow out into space: a long, thin cone, stretching about 374,000 km out into space, called the umbra. Anyone inside that umbral cone will see the Sun completely blocked out by the Moon.
Surrounding the umbra, there is a zone where the Sun is only partly hidden: this is known as the penumbra. Extending out from the tip of the umbral cone is a part of the penumbra known as the antumbra, which comes into play during annular eclipses.
Solar eclipses may only happen at New Moon, when the Sun, Moon, and Earth are lined up in that order. This condition must be met so that the Moon’s umbra or penumbra can reach the surface of the Earth.
The Earth also casts a shadow. Lunar eclipses necessarily happen at Full Moon, when the Sun, Earth, and Moon are lined up in that order, and the Moon passes in Earth’s umbra or penumbra.
No eclipse every month
Since most months have at least one new moon, why don’t we get a solar eclipse on each occasion? Because the orbit of the Moon is tilted slightly more than 5 degrees with respect to Earth’s orbit around the Sun. At most new moons, the Moon’s shadow passes well above or below our planet.
For an eclipse to happen, the new moon or full moon must take place when the Moon is near one of the two points, called “nodes”, where its orbit intersects the plane of Earth’s orbit. The so-called “line of nodes” of the Moon’s orbit then point in the general direction of the Sun. These favourable periods occur twice a year for a few weeks: they’re known as eclipse seasons, and they shift about 19 days from one calendar year to the next.
The Sun is really 400 times wider than the Moon. But it’s also 400 times farther from the Earth, on average! That’s why these two celestial bodies have nearly the same apparent size in Earth’s sky: about half a degree.
This remarkable coincidence is at the origin of solar eclipses. If the Moon was slightly smaller (or further away), it could never completely hide the Sun, and we’d only get partial or annular eclipses. If the Moon was larger (or closer to Earth), solar eclipses would be longer and more frequent, but arguably less spectacular because a larger part of the Sun’s corona would also be blocked out.
The distance of the Moon and the Sun at the time of an eclipse, thus the relative size of the Moon and the Sun, determines the type of solar eclipse, total or annular, that will occur.
No two identical eclipses?
The aspect, duration and area of visibility of an eclipse depend on a number of factors (precise celestial coordinates of the Sun and Moon, their respective distances, etc.) It shouldn’t come as a surprise that there are no two perfectly identical eclipses! And yet, there are series of eclipses that share similar general circumstances.
Indeed, 6585.3 days after a given eclipse (18 years, 10 days and 8 hours later), three of the Moon’s orbital cycles fall back in almost perfect sync. The moon is then at the same phase, it is again near one of the nodes of its orbit, and it is almost at the same distance from Earth. And since we’re near the same calendar date, the Earth-Sun distance will also be comparable. Another eclipse will therefore occur, whose duration and geographical track are very similar to the previous one, except with a 120 degree shift in longitude toward the west, and a slight shift north or south. That 6585.3-day cycle, called the saros, has been known since Antiquity.