Eclipses explained

Why a Total Solar Eclipse is so rare

Since there’s a New Moon every 28 days, why doesn’t it eclipse the Sun every month?

The Moon orbits the Earth every 27.55 days (to be precise), so why doesn’t it block the Sun once every month? After all, during a New Moon the Moon is invisible, with only its back-side illuminated by the Sun, so it must be between the Earth and Sun, right?

Wrong, sadly. The Moon’s journey around Earth each moon-th is a bumpy ride, and the New Moon usually just misses the Sun.

That’s unfortunate for eclipse-chasers, because everything in the Solar System is generally flat. The planets – including Earth – all orbit the Sun on the same plane. If you viewed the Solar System from the outside, you would see the planets as pin-pricks of light in a line through the Sun. The Moon also orbits the Earth roughly on that same plane.

That plane is visible to us as the ecliptic (doesn’t that sound like a promising word for eclipse-chasers?), an imaginary line from the eastern horizon to the western horizon that represents the Sun’s apparent path through the sky. When the Moon crosses that line, you get an eclipse. That’s why it’s called the ecliptic.

If the Moon followed the ecliptic perfectly, we would expect to see both a solar and a lunar eclipse every month. It’s only when a New Moon crosses the ecliptic that a solar eclipse can occur, and a lunar eclipse only happens when a Full Moon crosses the ecliptic.

Sadly, the Moon’s orbit is tilted as well as wobbly, so it passes through the ecliptic only occasionally. It’s elliptical orbit also means that the Moon varies in distance from Earth, from about 363,000 km to 405,000 km. If a new Moon crosses the ecliptic while its furthest away, an Annular Total Eclipse – a so-called Ring of Fire – occurs. Pretty cool, but it does not get dark, and nor can you take-off your solar eclipse safety glasses.

But a Total Solar Eclipse can only occur when the apparent size of the Moon is exactly the same as the apparent size of the Sun, AND it crosses the ecliptic. Despite the Sun being 400 times larger, it’s also 400 times further from Earth – and that’s the celestial coincidence eclipse-chasers treasure.

And that is why a Total Solar Eclipse is so rare. Luckily, there’s a big one coming; on Monday, August 21, 2017, the Moon’s shadow will take 90 minutes to race across the USA from coast-to-coast – Oregon to South Carolina – for the first time since 1918, giving a glimpse to anyone standing on or near the Path of Totality.

Main image credit NASA/NOAA