THE SUN, THE MOON AND US

On this page I have examined some of the ways in which the sun and moon interact with us here on earth. The relationships between the motions of the three bodies are very complex, a fact that stands out to anyone who observes the sky frequently and has noticed how the moon's position in the sky varies.

Shadows on the Quarter Days

Imagine that you stand all day in one spot, preferably during a time of year when the sun is not at its highest in the sky. As the sun moves across the sky, how would you expect the path traced throughout the day by the shadow of your head to look? Well the sun moves across the sky in an arc, so surely the shadow of the sun was also move in a curve, wouldn't it? 

After conducting some equinoctal activities at a nearby ancient monument with a friend who is very knowledgeable about the relationships between the alignments of the component standing stones of the monument and certain relationships between the position of the earth and various astronomical bodies, I have learned that there are two days in the year when your shadow would trace out a straight line on the ground, as the sun moves throughout the day! 

This is rather counter-intuitive, don't you think? The two days are the days on which the two equinoxes occur. So, why is this?

The diagram below shows the relationships between the earth and the sun on each of the quarter days. 

Notice that in mid-summer, at the Summer Solstice, our northern hemisphere is tilted directly towards the sun. Transferring our imaginary point of reference to that northern hemisphere, now, we can visualise the fact that the sun will be at its annual highest point in the sky at noon on the day of the Summer Solstice. Throughout the day, it will transcribe an arc that runs to the south from a north easterly point to a north westerly point.

Correspondingly, we can visualise the fact that the sun will be at its annual lowest point in the sky at noon on the day of the Winter Solstice, because the northern hemisphere is pointing directly away from the sun at this time. Throughout the day, it will transcribe a smaller arc than that of summer - an arc that runs to the south from a south easterly point to a south westerly point.

Now consider the Spring (Vernal) Equinox, or the Autumnal Equinox. At these times, the tilt of the earth is `side-on' to the sun, and the sun rises due east and sets due west. Therefore, the rotation of the earth throughout the day will cause the sun's apparent motion in the sky to follow an arc which lays concentric with the arcs of the lines of latitude of the earth. This is the key point!

Of course any shadow movement will mirror the apparent motion of the sun in the sky, so that shadow will follow  a course which is also concentric with lines of latitude, which, at the very short shadow scale of distance will be a straight line, running due east/due west.

On other days of the year, the shadows will trace out a curve, because the sun's motion in the sky will NOT follow a path concentric to the earth's lines of latitude.

The next diagram, see below, summarises how shadows are cast upon the ground throughout the year, as discussed above. It can be seen that the curvature of the shadows changes polarity either side of the equinox. To measure for curvature is one method of telling precisely when the Equinox is. It is truly a time of change, the changeover period for the polarity of the curve direction.

At the time of the two equinoxes, we can see that the sun rises due east and sets due west by observing its alignment with the stones of this neolithic monument near where I live. There are many such monuments in the world. It seems that the `ancients' had a good knowledge of the apparent motions of the sun in the sky.

The Nineteen year Lunar (Metonic) Cycle

The sun, earth, and moon come back into the same relative position every 18.6 years. This cycle is known as the  Metonic cycle, named after the Greek mathematician/astronomer Meton (4th century BC).  It shows the relationship of the phase cycles of the moon to the solar cycles.  

Observing the moon from Earth, there is an interval of nineteen years (as the earth has orbited the sun 19 times in this period) between seeing the moon in the sky at a particular phase and position against the star background and seeing it in that same phase and position again. 

This information can be used to predict lunar eclipses. It appears that neolithic man may have been aware of this cycle, suggested by various stone monuments which have circular  arrangements of 19 stones, one of these being the Torhouse Stone Circle near where I live.

The Torhouse Stone Circle is shown in the picture below. The diameter of the circle is slightly variable, between 61 and 66 feet, and it is noticeable that the stones on the north-west side are smaller and are more closely grouped than those on the south-east side.

In addition to the nineteen stones in the circle, there are three central stones at the centre of the circle. These comprise one relatively small stone in the centre and two larger stones, arranged in a line that is northeast to southwest.

On a low ridge nearby is a row of three stones, probably associated with the major monument. I intend to do some investigation of the alignments of these stones. 

There are other monuments in the UK that have 19 stones arranged in a circular fashion, notably in Cornwall. These include:

Merry Maidens Stone Circle, near Penzance.

Boscawen-ûn Stone Circle, between Penzance and Land's End.

The Boyne Valley complex, in Ireland. 

The Bluestone horseshoe, at the centre of the Stonehenge monument.