The Zodiac!

A few people have contacted me to ask for more details about how the Moon can 'pass through' a sign of the zodiac as it did last night: I hope the birders among you won't mind me giving a brief explanation!

As I'm sure you all know, the Earth and Moon are twin planets that revolve around a shared axis: because the Earth is the more massive partner, this is actually just inside our planet. The Moon takes just over 28 days to revolve around this axis (hence the 'lunar month') The plane of the Moon's rotation is tilted slightly, so that it is not always directly above the equator, but can wander above and below it. The result of all this is that we see the Moon against a background of stars that changes as the Moon travels around its orbit.

Now: the Earth-Moon system is also revolving around the Sun, taking just over 365 days to do so. What's more, the Earth rotates on its own axis (like a classroom globe) at the surprising speed of around 1,600 k/h at the Equator! The result of all these motions is that, from our point of view in the northern hemisphere, it appears that the Sun is traveling across the sky from East to West. Its path is through a narrow band of sky called the ecliptic. In ancient times people found patterns in the stars that lay behind the ecliptic and gave them names. Because the Earth takes 12 months to orbit the Sun, it seems that each 30 days or so the Sun passes in front of one of these star groups: these are the 'Signs of the Zodiac'. Some are quite bright (Taurus and Gemini for example) others are not!

Now the Moon (in its tilted orbit) is, of course, rotating roughly in the plane of the ecliptic, so that it passes through or near each of the zodiac constellations every month....