The Earth in Orbit
recall that the ecliptic is the plane of the Earth’s orbit
around the Sun
demonstrate an understanding that one astronomical unit (AU) is the mean
distance between the Earth and Sun.
recall that planets move in elliptical orbits, slightly inclined to the ecliptic
The Sun and all the planets lie approximately on the same plane. This suggests that the solar system formed from a disc of material that was in orbit around the Sun. By definition the ecliptic plane contains the Earth's orbit. The angle of other planet's orbits relative to this is called their inclination, e.g. Mars has an inclination of 1.8 degrees. Mercury's inclination is greatest of all the planets at 7 degrees.
The Astronomical Unit (AU)
What units we use to measure a distance depends on the distance involved.
To measure really BIG distances we need really BIG units.
We have seen that the distance between the Sun and the Earth actual varies between about 147 and 152 million km. The average value is about 150 million km (actually 149597870.691 kilometres).
We call this distance 1 astronomical unit or AU. It is a very useful measure of distance within the Solar System.
Consider this table.
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto | |
| Orbit (AU) | 0.39 | 0.72 | 1 | 1.52 | 5.20 | 9.54 | 19.2 | 30.1 | 39.5 |
| Inclination (degrees) | 7.0 | 3.4 | 0 | 1.9 | 1.3 | 2.5 | 0.8 | 1.8 | 17.1 |
The Ecliptic
Imagine, for a moment that the Sun is actually in orbit around the Earth. This is after all what people believed for a long long time. The Sun takes about 365 days to go round the Earth. As the Earth spins around daily the Sun rises in the east and sets in the west.
Imagine also for a moment that the Earth's axis isn't tilted. The Sun appears to go round the Earth from our point of view. Its path in the sky is called the ecliptic. Notice that on this diagram the equator is on the ecliptic plane. The Sun, on this diagram, takes a year to orbit the Earth.
1. Imagine that the Sun
moved round the Earth as shown on the diagram above.
If you lived in a country on the equator describe the apparent motion of the Sun
every day.
Because the Earth's axis of rotation is tilted we must use a different diagram.
2. How would the apparent motion of the Sun change because the Earth is tilted?
This is a star chart (without any stars). The celestial equator is the straight blue line. The ecliptic, the path of the Sun, is the red line. It takes a year for the Sun to travel this path from right to left on this chart.
3. The summer solstice is the day when the Sun is at its highest in the sky at midday. Where would that be on each of the diagrams above?