Spring equinox 2012 and the basics of the seasons

Stonehenge 1975

By observing the position of the Sun at sunrise or set the giant stones of Stonehenge on Salisbury Plain in England could be used as a seasonal calendar. Photo Nick Lomb

Spring equinox is coming up on Sunday 23 September 2012. At 12:49 am Australian Eastern Standard Time the Sun crosses from the northern to the southern part of the sky. From then on daytime is becoming longer than night time and we are heading rapidly towards summer.

In some countries spring officially starts on the day of the spring equinox. This is an arbitrary choice and in Australia it has already started three weeks ago on 1 September, as having spring extend throughout September, October and November better suits out weather conditions.

We normally refer to the Sun rising in the east and setting in the west. In reality, it only rises due east and sets due west at the time of either the spring equinox in September or the autumn equinox in March. Have a look at, or take a photo of, sunset on Sunday evening and subsequent evenings. You will see that the position of sunset on the horizon is shifting towards the south. The motion of the Sun’s motion along the horizon was well known in ancient times and places like Stonehenge were built to use its motion like a calendar.

diagram of the seasons

Diagram of the seasons. In this diagram we are looking the Earth’s circular path around the Sun from side on and hence the path appears oval. The seasons take place because of the tilt of the Earth’s axis. If there was no tilt life would be boring with no seasons. Note that “NCP” on the diagram stands for north celestial pole, which is the point directly above you if you were standing at the north pole. Drawing Nick Lomb

Strangely, the builders of Stonehenge four or five thousand years ago would have had a better understanding of the motion of the Sun during the year than most people today. This is because living close to Nature out on Salisbury Plain with nothing blocking their view they would have been constantly aware of happenings in the sky. In contrast, most modern city dwellers only look at the sky once or twice a day to check the weather, while others even manage to avoid looking up altogether and just check the weather with an app on their smart phones.

Not surprisingly, one reader, Jim, is confused about the seasons and asks:

I have trouble understanding some of the basic concepts of how the Earth, Sun and stars move throughout the year, for example why is it that we see Orion in the summer but during the winter it disappears or why is it that during Summer the sun is high in the sky but during the Winter it stays low.

Let’s try to answer….

The Earth circles the Sun once a year. When the Sun is in the sky it is day time. During any month the part of the sky in the direction of the Sun is not visible to us as the Sun is too bright. For most of the Australian winter that applies to the stars of the constellation of Orion. On the other hand, in winter Scorpius is in the part of the sky away from the Sun and so that constellation is visible.

In summer the Earth has moved to the other side of the Sun and hence those stars, like those of Orion, previously in the direction the Sun appear in the night sky. Stars, like those of Scorpius, now are in the direction of the Sun and hence are not visible in the night sky.

The path of the Sun across the sky in different seasons.

The path of the Sun across the sky in different seasons. Drawing Nick Lomb

Why does the Sun make a large arc in the sky in summer? At that season the southern hemisphere is tilted towards the Sun putting it higher in the sky. As mentioned above, it then is rising south of east and setting south of west, leading to a large arc in the sky. Similarly in winter it is low in the sky as the Earth is tilted away from the Sun. It then is rising north of east and setting north of west, leading to a low arc in the sky. At the equinoxes it is in between.

For those who can visit Sydney Observatory, a three-dimensional model of the motions of the Sun, Earth and the Moon is on display and for many people that leads to the understanding of these basic motions for the first time.

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