Astronomy Online

II.3 Three motions for the Earth

This exercise is connected with another, I.1 Rotation of the Earth, but is at a somewhat more demanding level.

The Earth moves in several ways. First, it turns around its polar axis; one turn takes 24 hours. Then it moves along its orbit around the Sun; one full revolution takes 1 year. And third, its polar axis changes direction very slowly, just like a spinning top. This effect is called precession and one full turn lasts almost 26,000 years.

When you live on the Earth, these motions are not obvious. This is why most ancient Greek astronomers and many others after them thought that it is the sky that moves around a motionless Earth, not the Earth that turns under the sky.

The next pages do not tell you which one of Earth and Sky is moving. They only show you how you can see the effect of these motions in the stars.

It's easy to detect the rotation (I.1 Rotation of the Earth), it's more difficult to detect the revolution (here an activity for AOL middle level) and much more to detect the precession (that is an exercise for 16 years and more, Astronomy On-Line advanced level; still to be inserted!).

The Earth goes along its orbit :

Figure 1: We expect to see the yearly polar motion and we can't see it!

However, the celestial pole should move in the sky, as and when the polar axis goes around the Sun.

You could imagine that the polar axis leans while the Earth goes on, aiming always at the same place. But, doing so, it could not aim at both poles, and there would be no more seasons on the Earth.

We must look for another way of explaining why the celestial pole seems motionless. The Solar system incredible isolation produces this fact. Yearly polar motion among the stars is imperceptible because Sun-Earth distance is far much shorter than Sun-Stars distances.

As a result, we can't see yearly Earth motion by observing the pole, but we can see it by observing the Sun. If we could see the Sun moving among the stars, we could imagine how the Earth really moves.

Exercise 1 : What are the constellations behind "wandering stars" ?

These "wandering stars" are forgotten on permanent skymaps because they don't stay for a long time in front of one constellation. It's the translation of their ancient Greek name : "planets" !

Equipment required:

Observations (when the sky is not cloudy and not enlightened):

At the beginning or at the end of the night, we can often see the Moon and several planets. Because of their brightness, Venus, Jupiter and sometimes Mars can be easily identified. It's a bit more difficult to identify Saturn ; Mercury is rarely visible ; and a telescope is required to see Uranus or Neptune.

If the Moon and 2 or 3 planets can be seen in several directions, stretch your arm successively towards them. You'll be surprised to see that your arm lies on a plane.

What does it mean ?

The Earth is at one end of your arm, the Moon and several planets are at the other end when you strech your arm toward them. So we may say that all these objects are laying on a plane. You see this plane from its inside because you are on the small Earth (as you see the equator plane).

Constellations behind planets are well-known : they are zodiacal constellations.

Let's sum up : Zodiac marks out the tracks of Moon and "wandering stars ". The planes of the planets go through the zodiacal constellations.

Notice that the cylinder which holds zodiacal constellations is not always at the same place, because its orientation changesalong the year.

Exercise 2: Does Zodiac mark out the track of the Sun ?

Equipment required:

What and when will you observe ?


During one year, the Earth moves on a plane orbit around the Sun. Zodiacal constellations are the celestial landscape near this orbit. From the Earth, we cann't see the stars behind the Sun because of the brightness of the blue sky. But as the Earth moves on, the brightness hides new stars.

To locate the annual track of the Sun with more precision, use eclipses:

During a total Sun eclipse, the sky becomes dark enough, and you can see the black Sun among a few bright stars.

During a Moon eclipse, the Moon is not completely dark and it shows the opposite point of the Sun among the stars.

Data gathered up with several eclipses alllow you to draw a large circle in the sky, around the Earth; this circle is the apparent track of the Sun in the middle of Zodiac. We call it "ecliptic" because the Moon must be exactly on this circle to produce an eclipse.

During an ordinary night you can imagine ecliptic when you see together several planets: they also are near the ecliptic, among zodiacal constellations.

Author:D. Toussaint, CLEA
20, rue Renaudot
F-10 160 Aix-en-Othe
Contact: Josée SERT

Astronomy On-line | EAAE | ESO | Help | Search