The stars in the sky

The night sky. Millions of twinkling white lights. Each light, a star, is billions of kilometres away. Because the stars are so unbelievably far away, we don’t notice changes in their movement in relation to each other. It’s as if the stars were attached to the background.

As if a giant sphere were surrounding Earth. Although this isn’t real, it'™s a useful model for mapping out where the stars are. We call this model the celestial sphere. Looking up at the sky, Philip only sees the stars of half of the celestial sphere. Kim is in a different location on Earth.

They both see some of the same stars, but also some stars that the other cannot see. As the Earth rotates, it appears as if the celestial sphere were rotating, as other parts of it become visible. If you extend the Earth'™s equator out to the celestial sphere, you find the celestial equator. And just like Earth has a North Pole and a South Pole, the celestial sphere has poles too, just above Earth’s poles. There happens to be a star right at the celestial North Pole.

We call it the Northern star, or Polaris. If you face Polaris, you know you are looking North. Then spread out your arms, and your right arm points East and your left arm points West. It's like a compass in the sky. Because Polaris is at the celestial North Pole, you can only see it if you are North of the equator.

If you look at the sky every night for some time, you'll notice that there are some stars that seem to wander across the celestial sphere, gradually changing position in relation to the stars in the background. Those aren't stars at all. They are the planets of our solar system. And that'™s where the word planet comes from - it means wandering star in Greek. In Greek and Roman mythology, the planets represented Gods, and their movements across the sky inspired stories about their adventures.

The reason planets appear to move in relation to the celestial sphere is that they are much closer to us than the stars. The sun is also a star, but is close to us compared to the other stars, so it also moves in relation to the celestial sphere. Of course, you know that it's Earth that rotates around the sun, and not the other way around. But in the celestial sphere model, we view the sun from Earth, as if it were moving. We can mark the line that the sun appears to follow on the celestial sphere.

Now, if you locate the planets and the moon, you'll notice that all appear to follow roughly the same path. That path is the ecliptic. On the celestial sphere, along the ecliptic, we find twelve notable star constellations - referred to as the zodiac. Throughout the year, the sun blocks out these constellations one at a time, as Earth slowly rotates around the sun. In traditional astrology, people believe that these constellations are important for what happens to us, on Earth Especially important is which of the constellations in the zodiac was covered by the sun on the day you were born.

But that's astrology. And astrology is full of entertaining stories. But don't confuse it with astronomy, which is a science.