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Established 16/3/2000
MoonThe Moon's path through the starsMoon

Most people seem to know that the Sun changes its rising and setting positions during the course of a year, and that its noon height in relation to the horizon varies with the seasons. In winter, the Sun rises in the southeast and sets in the southwest, while in summer it rises in the northeast and sets in the northwest, spending much longer in the sky than during winter. At the equinoxes, the sun rises at due east and sets and due west and the length of day and night are the same.

The imaginary line which the sun follows through the heavens is called by astronomers the "ecliptic". It takes the sun one year (365.25 days) to complete one journey around the sky along the Ecliptic. The graphic below shows part of the Ecliptic through the winter night sky, stretching from west to east through the Zodiac constellations Pisces, Aries, Taurus, Gemini, Cancer and Leo.

The Ecliptic line

The 12 Zodiac constellations are those through which the Sun, and the planets, pass on their course through the stars. However, while the planets follow the general path of the Ecliptic, not all of them stick rigidly to it. The Moon is one of these planets.

SEEING THE SEPARATION
The line formed by the Moon's course on its 29-day journey around the sky is tilted slightly to the Ecliptic. This means that, while the Moon's pathline intersects the Sun's pathline (Ecliptic) twice during one lunation, its path is tilted slightly so that it separates from the Sun's path by 5.15 degrees at maximum separation - a distance of roughly 10 moonwidths as seen by the observer.

A good way to visualise this 'separation' of the Moon's path from the Ecliptic is shown in the animation on the right (filesize 72k). You should see the Moon moving through the constellations from west to east - the grey line is the Ecliptic. Note how the Moon's path crosses the Ecliptic twice. Keep watching if you didn't notice it the first time. If you would like to see a larger version of this animation (162k) just click here.

During one 29-day lunation, the Moon seems to spend half its time north of the ecliptic and half its time south. You should be able to see this clearly in the animation.

The Moon's path relative to the ecliptic

THE NODES

The points where the path of the Moon crosses the Ecliptic are called the "Nodes" - specifically the "ascending node", or the point where the Moon crosses the Ecliptic moving south to north, and the "descending node", the point where it crosses heading from north to south. The two nodes are always located 180 degrees apart, or at opposite sides of the sky to each other. It takes the Moon just 14 days to make the journey from one node to the other, but it takes the Sun six months to make a similar 180-degree trip through the sky.

Ascending node
Descending node

The Moon at ascending node taken from the above animation.

The Moon at descending node from the animation shown above.

The nodes themselves slowly move through the sky, so that the Moon's intersection with, and maximum separation from, the Ecliptic move through the constellations slowly over time. This movement is westerly, taking the rotation of the nodes in the opposite direction of the Moon's movement through the stars, which is from west to east.

The ancient Stone Age builders would have been able to see the rotation of the nodes by doing two simple things: (1) keeping an eye on the Moon's position relative to the background stars and (2) watching the Moon's rising and setting positions on the horizon.

RULES OF THUMB

Remember, the Moon crudely follows the path of the Sun and a few simple rules of thumb can be learned from watching it. Firstly, New Moon is the phase of the Moon which is invisible to us, because it occurs when the Moon is in the same position in the sky as the Sun. Full Moon is ALWAYS located 180 degrees from the sun on an east-west plane, so if the Full Moon is rising, the Sun is setting. Now divide this 180-degree distance into two using the Moon.

A week after New Moon is the First Quarter (which should probably be called First Half, because you can see half the face of the Moon), which is located 90 degrees east of the Sun, or in terms of Solar tropical time, three months. In other words, when you see the First Quarter Moon you know the Sun will be in that east-west position in three months' time. When you see Full Moon , that's where the Sun will be in six months' time. A good method of remembering this is the Sun-Moon positions on the solstices. If you see a Full Moon rising around the time of Winter Solstice, mark or remember its rising position - that's roughly where the Sun will rise in six months time - at the Summer Solstice. Notice how in Summer time the Sun is high in the sky while the Full Moon is always low, and in Winter when the Sun is low the Full Moon is always high.

Finally, there's the last quarter (or last half!) which marks out the position the Sun will be in nine months' time.

MOON DIAGRAM
This diagram by Martin Brennan should help show you where the Moon is in relation to the Sun in its various phases. Click on the diagram for a larger version.

Diagram 2 shows the first quarter , which is at its highest point in the sky when the Sun is setting. This phase marks where the Sun will be three months later. Diagram 3 shows the Full Moon , rising in the east while the Sun sets in the west. The Moon's full phase lasts for about three nights. Diagram 4 shows Full Moon at midnight, 180 degrees opposite the Sun. Six months later the Sun will be in this part of the sky.

In diagram 7, the last quarter phase is reached, showing where the Sun will be located in nine months' time. Diagram eight shows the last crescent, which disappears to become "New Moon" before the next lunation begins. It has to be said that Diagram 6 is a bit misleading, showing the last quarter rising but at a slightly odd angle.

CLICK FOR LARGER VERSION

The rotation of the nodes, and knowledge of the Moon's separation from the Ecliptic is crucial to understanding how the ancients predicted eclipses, and crucial to understanding why they marked the so-called "standstills" of the Moon using standing stones, stone circles and other sites.

Some facts to remember:

New Moon

New Moon: In conjunction with the Sun; position of Sun at the present time.

First Quarter

First Quarter: 90 degrees from the Sun; shows position of Sun in three months.

Full Moon

Full Moon: 180 degrees from the Sun; shows Sun's position in six months' time.

Last Quarter

Last quarter: 270 degrees from the Sun; shows where sun will be in nine months.

ACKNOWLEDGEMENTS:
My thanks to astronomer Charlie Scribner who has watched the Moon and planets for twenty years without a telescope and whose authoritative guidance has been a constant inspiration.

All information and photos, except where otherwise stated, copyright, © Anthony Murphy, 1999-2015
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