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What is a Solar Eclipse?


Eclipses: a shadow play

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 (about 374,000 km!), thin, pointed cone 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.

Umbra, penumbra and antumbra (diagram)

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.

Eclipse nodes diagram

An eclipse is possible only when the Moon is near one of the two points, called “nodes”, where its orbit intersects the plane of Earth’s orbit. 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.

Amazing coincidence

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.

Earth, Moon and Sun to scale (diagram)

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.

It’s all about distance

The Earth-Moon distance is always changing. Over the course of the month, it varies between 357,000 and 406,000 kilometres, because of the elliptical shape of the Moon’s orbit around Earth. And Earth’s orbit around the Sun isn’t a perfect circle either: over the course of the year, it varies between 147.1 million km (early January) and 152.1 million km (early July). Combining the two factors, the apparent size of the Moon with respect to the Sun varies considerably from one solar eclipse to the next.

Total vs annular solar eclipse

At the time of a solar eclipse, if the Moon is farther away from Earth (Moon smaller) or the Sun is closer (Sun larger), the Moon’s umbral cone falls short of reaching the surface of the Earth. But beyond the tip of the cone extends what’s called the antumbra, which does reach Earth’s surface: viewed from anywhere within the strip of land swept by the antumbra, a ring of sunlight remains visible around the dark silhouette of the Moon. This is an annular eclipse of the Sun.

When the Moon is closer to Earth (Moon larger) or the Sun is farther away (Sun smaller), the Moon’s umbra may touch the surface of the Earth. If we’re within the ground track of the umbra, the Moon completely covers the dazzling surface of our star. We witness a total eclipse of the Sun.

Who sees what?

A total or annular solar eclipse is therefore visible to only a small fraction of Earth’s inhabitants: those who are within the narrow path swept by the umbra or antumbra of the Moon. That’s why they’re so rare!

As the Moon advances in front of the Sun, it blocks out an increasingly larger portion of our star, and the ambient luminosity decreases accordingly. These are the partial phases of the eclipse, and the Sun appears like a thinner and thinner crescent.

If we’re inside the path of annularity, an annular eclipse culminates with the annular phase, or annularity, when the silhouette of the Moon detaches completely from the rim of the Sun. For a few minutes (theoretical maximum is 12 min 30 sec), the Sun looks like a ring of fire.

In the case of a total solar eclipse, if we’re within the path of totality, the Moon eventually blocks out the bright surface of the Sun completely. This is totality: for up to 7 min 31 sec (theoretical maximum), the sky becomes dark, except for the horizon which takes on the colours of twilight; the brightest stars and planets appear; and the delicate solar corona reveals itself around the black silhouette of the Moon. A stunning spectacle!

On either side of the path of totality or annularity, we find two, much larger zones swept by the Moon’s penumbra. From there, one only gets to see a partial eclipse to some degree: it doesn’t become total or annular.

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’s 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 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 similar to the previous one, except with a 120 degree shift in longitude toward the west. That 6585.3-day cycle, called the saros, has been known since Antiquity.

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