Some remarks shall be made on coordinates before getting into the movements of moon, earth, and sun. These coordinates are used to describe positions and orbits of heavenly bodies.

Since there is no real fix point in the universe it becomes necessary to define certain points as fixed. Such a point can be chosen as the origin of a coordinate system in which distance and speed can be measured. It is possible to define a coordinate system to have its origin in the centre of our galaxy. Tracking the sun one would find that the sun moves on an elliptical orbit (though very close to a circle) around the galaxy centre. The earth's orbit would be a bit complicated to describe. It would have many loops, the earth wobbling around the sun. Yet, for an observer on earth these coordinates are not of great interest.

Another coordinate system can be defined using the sun's centre as origin. It is used when stating that the earth moves around the sun. The orbits of the planets, including earth, have comparatively simple shapes. With the origin in the sun this system is called heliocentric. Heliocentrically the moon's orbit is waving around the earth's. It is greatly disturbed by the extremely close earth and therefore has no exact elliptical form.

Another coordinate system can be used to describe the moon's movement around earth. Its origin is the centre of gravity of earth and moon. Although this point is not in the earth's centre it still lies deep beneath the earth's surface. For considerations with less required accuracy a coordinate system with the earth's center as origin might even be used. Such a coordinate systen is called geocentric.

Compared to cosmic distances all observers on earth are relatively close to the origin of such a geocentric coordinate system. Hence we experience cosmic events geocentrically.

The earth is revolving around the sun on an elliptical orbit only slightly different from a circle (Kepler's first law). Due to the ellkiptical form of the orbit, the earth's distance from the sun is not constant, ranging from 147.1 million kilometres in the point nearest to the sun (the perihelion) to a maximum of 152.1 million kilometres when the earth is in the most distant point of its orbit (the aphelion). The perihelion is passed in January, the aphelion in July. The medium distance between earth and sun is thus 149.6 million kilometres. The orbit of the earth around the sun lies in a plane called the ecliptical plane. Perihelion and aphelion are called the apsides, connected by the line of apsides. The small difference between maximum and minimum distance may illustrate that this is not the reason for the seasons on earth.

The form of the earth's orbit is illustrated in Fig. 1. The figure, as all figures on this site, must not be scaled. The sizes of sun, moon, and planets do not correspond with the real sizes compared to the orbit. The real orbit is much more closer to a circle than in the figure.

The position of the apsides is not constant over long periods. Besides the annual revolution of the earth itself the line of apsides rotates in the same direction. The earth thus "loops" around the sun. The loops although are very close because the line of apsides rotates at a very low speed. The following Fig. 2 shows the resulting form of the orbit. However, the speed of the apsides is very much slower than in the figure.