Understanding solstices

At 22:23 Universal Time on the Dec. 21, that is, 17:23 EST and 14:23 PST, the Sun will reach the southernmost point in its yearly travels across our skies; the winter solstice.

We will have the day with the minimum length of daylight. After that the days start to lengthen. Even in our modern world, with artificial light and heat, we look forward to spring; imagine how our ancestors felt about it.

They were certainly very good at recording the patterns of things they saw in the sky: the movements of the Sun and the Moon, and the weird moving to and fro of the planets against the stars.

They identified the daily and seasonal movements of the Sun, including the solstices and equinoxes, predicted eclipses, made a workable calendar and many other things.

However, what they were seeing was made really hard to interpret because they were sitting on a spinning ball, hurtling around the Sun, while looking at the other planets as they do the same thing.

To understand the seasons, equinoxes and solstices, forget you are on the surface of a spinning, orbiting ball; imagine you are looking at the Earth and Sun from far out in space.

Our Earth and the other planets move in more or less circular concentric orbits around the Sun, all in the same plane. In addition, the Earth is spinning, like a top, with its axis tilted from upright by about 23 degrees.

The direction of that lean varies extremely slowly, over tens of thousands of years, so it is unchanging as far as most of us are concerned.

Completely fortuitously, for most of our history the Earth’s spin axis has been pointed at a star, which we call Polaris, the North or Pole Star.

Therefore, as the Earth moves around the Sun in its annual travels, there is a point where the Northern Hemisphere is leaning towards the Sun, and another point, six months later, on the other side of the Sun, when it is leaning away. When the lean is towards the Sun, we get the largest number of hours of daylight each day, and the Sun is highest in the sky at noon. It also rises and sets at its northernmost points on the horizon.

This happens around June 21 and is called the summer solstice. When the lean is away, we get the smallest number of hours of daylight and the Sun at noon is at its lowest in the sky. Sunrise and sunset are at their southernmost extremes on the horizon. This happens around Dec. 21 and is called the winter solstice.

There are two intermediate points, where the Earth is leaning sideways with respect to the direction of the Sun, neither leaning towards or away. These are called the equinoxes, because at that time we get equal hours of daylight and darkness. We get one around March 21, when the Sun is heading north, the spring equinox, and one around Sept. 21, when it is heading south, the autumn equinox.

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Here are two last-minute Christmas present suggestions. Your local science store will probably have them. All backyard astronomers need a planisphere. The ones worth getting consist of two plastic discs. Don’t buy a cardboard one. Remember it is likely to be dropped in mud or snow.

On the lower disc there is a starmap with a calendar around the edge. On the upper there is a window showing part of the starmap beneath, and time of day round the edge.

Match the local standard time on the upper disc with the date on the lower disc, and the window will show the constellations that are above the horizon. Make sure you get one for your latitude.

The second suggestion is the “Observer’s Handbook,” published annually by the Royal Astronomical Society of Canada. It is a gold mine, filled with astronomical information and listings of the coming year’s astronomical events.

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Mars lies in the south after sunset and Venus shines low in the dawn glow, with Mercury and Jupiter below it. The Moon will be Full on the 22nd.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.