The Moon and its Eclipses

The moon orbits the earth, but this is not the reason that we see the moon rise and set - that is because the earth is spinning. When the earth is directly between the moon and the sun we can see the whole of the illuminated side of the moon.

We call this Full Moon.

When the moon is directly between the earth and the sun we cannot see any illuminated part of the moon; this we call New Moon. Between these times we see a varying segment of the moon illuminated.

The moon takes 27.3 days to complete one orbit of the earth. But this is not the period between full moons.

In the diagram it is full moon at position 1.

27.3 days later the moon has made one orbit of the earth which has moved to position 2.

The diagram shows that it is not yet full moon, which does not occur until position 3.

The time period between full moons is 29¹/₂ days, or more precisely 29 days 12 hours and 44 minutes. This period is known as one Synodic Month.

Eclipses

The moon is eclipsed when it is obscured by the earth’s shadow. So are there eclipses at positions 1 and 3 in the diagram? The answer is yes - there would be an eclipse if the moon’s orbit were in the same plane as the earth’s orbit. It is not, of course, otherwise we would have twelve eclipses every year.

In fact the plane of the moon’s orbit is at 5 degrees to that of the earth’s. The moon spends most of its time either above or below the plane of the earth’s orbit.

There are only two occasions in each orbit of the moon when it plunges through the earth’s plane and these are the only times when it is possible for a lunar eclipse to occur. These occasions are called nodes.

In the left diagram the moon is above the earth’s orbital plane, so no eclipse occurs. In the right diagram the moon is at a node, so an eclipse does occur.

It would be quite straightforward to calculate the time period between eclipses if the nodes kept the same orientation. But they don’t; the plane of the moon’s orbit, and consequently the position of the nodes, very slowly rotates. Because of this it takes the moon one Draconic month (27.212 days) to return to a node.

Another eclipse will occur when whole numbers of both Synodic months and Draconic months have elapsed. This will be after 6585.357 days, or a few days over 18 years. This period is known as a Saros.

Because the earth’s shadow is much larger than the moon there are other opportunities for an eclipse to occur, with the result that there are a number of separate Saros series running concurrently. This gives rise to an irregular pattern of eclipses, particularly when partial eclipses are included.

In order to predict full and partial eclipses it is necessary to know the position of the moon’s nodes. If a line joining the two nodes is extended it points to two locations on opposite sides of the sky. These locations mark the position of the Dragon’s head and tail, and they travel around the sky at the rate of one revolution per 18.6 years in the opposite direction to the earth’s orbit. The position of these markers for any specific date is given in various tables.

Whenever the moon is observed to line up with one of the two markers, it means that the moon and earth are correctly aligned for an eclipse subject only to the other ingredient - the position of the sun.

For a good explanation of the moon’s orbit and eclipses visit http://www.hermit.org/Eclipse/why_cycles.html