Section 2.1: What stars can I see from my house?
In the night sky, we can see a wide variety of stars with just our own eyes – stars of different ages, sizes and colours (therefore temperatures).
In ancient times, people grouped together random collections of stars into constellations, and gave the patterns names and made stories about them (not too dissimilar to children playing join-the-patterns in books today). Those names and stories helped them to recognise the random patterns of stars, which was very useful for navigation around the stars – if a comet appears, for instance, we can tell each other where to look by stating which constellation it has appeared in.
Screen snap from the Sky in Google Earth depicting boundaries around major constellations. (credit: NASA, ESA, Digitized Sky Survey Consortium, and the STScI-Google Partnership)
ACTIVITY: Take a look at Google Sky and the constellations there are.
At night time, we are on the side of the Earth facing away from the Sun, and as the Earth orbits around the Sun we look out in different directions. In August, for instance, we are on the side of the Sun that faces towards the centre of our galaxy, called the Milky Way. In January, the Earth has moved to the other side of the Sun and we look out in a completely different direction, with the heart of our galaxy completely hidden behind the Sun.
ACTIVITY: To keep track of what stars we can see at different times of year, you can use either a planisphere (a rotating disk that shows a map of the stars), or some free planetarium software such as Stellarium. Try this virtual planisphere to see what stars are visible where you are at the moment.
Note that the planets behave quite differently. The planets we can see with our eyes are all quite close and orbit the Sun, so move relative to the Sun and therefore are independent of background stars. Planets orbit the Sun on the same plane. This is marked out on the sky using an imaginary line known as the ecliptic. As the planets orbit around the Sun, their movements can appear to be in quite different directions in the sky, but in reality will always lie close to that imaginary line.