Eclipses explained

Are there eclipses on other planets?

There are over 180 moons casting shadows in the Solar System, but none feature the magical moment of Totality

Three cheers for The Moon! There are around 180 natural moons orbiting planets in the Solar System, and while they come in all shapes and sizes, precisely none of them can cause what over 100 million people in the US will witness on August 21 2017 – a Total Solar Eclipse.

The Moon is in very special position right now, and will be for the next 550 million years (or so). Being around 400 times smaller than the Sun, but also about 400 times closer to Earth, the Moon appears to be exactly the same ‘angular size’ in the sky. It’s because of this celestial coincidence that a Total Solar Eclipse can occur when a New Moon passes precisely across the Sun, along a line known – rather fittingly – as the ecliptic.

Since the Moon’s coverage of the Sun is total, but no more, observers on Earth standing within the Path of Totality get to see the Sun’s mighty corona around the Moon’s edge, which is known as the ‘lunar limb’. Nowhere else in the Solar System does this happen precisely enough for Totality to exist as a visible phenomenon.

It doesn’t even always happen on Earth. Occasionally the New Moon is further away than usual along its slightly elliptical orbit, so when it crosses the Sun, it doesn’t block out all of the Sun’s disk. This is called an Ring of Fire, or annular eclipse. You could also call it a transit, which is what often happens elsewhere in the Solar System.

Moon-shadows do often creep across the planets in the Solar System, but Totality never occurs. Giant planet Jupiter is around 1300 times bigger than Earth and has 67 moons, so it’s hardly surprising that moon-shadows are thrown across its surface relatively regularly. If you have an eight-inch telescope you can watch one of the big ‘Galilean’ moons – Io, Europa, Ganymede or Callisto (and sometimes three of them simultaneously) – cast a shadow on Jupiter’s surface every few weeks or so, and for hours at a time.

Meanwhile, Saturn, which is about 750 times bigger than Earth, has over 60 moons, but again, no moons are positioned so that their angular size is precisely the same an the Sun. For example, Titan appears to be bigger in the sky than the Sun as seen from Saturn’s surface, so while Titan can block out the Sun and cause an eclipse – and darkness in the day – it lacks Totality.

Photo credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute