Dear Kristian,

There are two ways to know. First of all, since planets are closer to us than stars, they don't look like pinpoints, and thus do not twinkle like stars. (This is true for the planets you can easily see with your naked eye, for example Jupter, Mars, Saturn and Venus). Mercury is difficult to see because it is always so close to the Sun. Another way to tell whether what you are seeing is a star or a planet actually requires that you go out and observe many nights in a row. For example, take Venus, which is now visible in the evening sky right after sunset. If you go out at the same time of the night every day for several days, you will see that the position of the planet changes slightly each night, especially if you start to see other stars in the background behind Venus. Notice how the position of Venus moves against the fixed pattern of background stars - this tells you it is a planet you are looking at. The word 'planet' means wandering star in Greek, because the planets, being closer to us than the stars, are seen to move against the fixed pattern of background stars as they move along their orbits around the Sun. The stars move too, but in fixed patterns or constellations, rising in the east and setting in the west, but that is due to the Earth's own rotation on its axis. The planets also show this apparent east to west rise/set motion caused by the Earth's spin, but also show the 'wandering' motion in addition.

Isabel