The solar system consists of a central star, our Sun, which has a family of planets, moons, meteors and comets orbiting around it. In the Planetarium we are mainly concerned with what can be seen with the naked eye. As far as the solar system is concerned that means the Sun, the moon and the planets, Mercury, Venus, Mars, Jupiter, and Saturn.
As the planets, including Earth, orbit the Sun in the same plane or level, we see the planets in the sky only in the same plane. This is the plane in which the Sun appears to move also and is known as the Ecliptic. Students can remember the name ecliptic if it is explained that the Sun in its apparent path eclipses the stars as it moves in front of them. Of course it is not the sun that is moving but the Earth in its annual journey around the Sun. So the planets can only be found in the ecliptic.
When talking about distances we explained how incredibly far away the stars are. So far away in fact that although they appear to move in space they do not appear to move relative to us, just as a jet plane very high up appears to move very slowly when we know it is travelling at least 400km/h. Stars are so far away that their movement cannot be discerned with the naked eye in our lifetime, or even several lifetimes.
So stars can be considered fixed in space, not so for some objects:
- The Earth spins on its own axis each day; actually with respect to the stars in 23 hours and 56 minutes.
- The Earth orbiting the Sun each year (in 365.25 days.) The only effect this has on our view of space is that the Sun appears to circle the Earth in an easterly direction each year, moving at about 1 degree each day.
- The planets as they circle the Sun do move and do appear to move from night to night against the starry background.
- The Moon moves through the sky about 12 degrees a day easterly as it circles the Earth each month.
- The Earth's rotation is of course the most, 15 degrees per hour, on top of any proper motion each individual body may have its own.