In the sky we see rhythm

At 10:02 Universal Time (2:02 a.m. PST) on Dec. 21 the sun will reach its southernmost point in its yearly travels. The sunrise and sunset points on the horizon will be at their furthest south and at noon, the sun will be at its lowest in the sky.

For those living at latitude 67 degrees north, the sun won’t come up at all. This point in the year is known as the winter solstice. Our ability, even millennia ago, to predict celestial events contributed greatly to the idea that things in the sky are in a different category from things on Earth.

On Earth, unpredictability seems to be the rule. In the sky we see precise rhythms, which led to special attention being paid to events in the sky, particularly to events that were not predicted, such as comets. This is the foundation of the science of astronomy and the non-science of astrology.

This year’s winter solstice brings with it another one of these predicted and special events, an unusually close pass by the planets Jupiter and Saturn, an event known as a conjunction.

You can see these planets low in the southwest after dark. Jupiter is the bright one; Saturn is around 10 times fainter. They are close together now, and every night until the solstice they will appear a bit closer together, until on the 21st they will be about a tenth of a degree – a fifth the diameter of the full moon – apart.

We get conjunctions between these planets every 20 years or so, but the last time we had an encounter this close was in the year 1226. The two planets are not really passing close by one another. It just means that on the 21st, the Earth, Jupiter and Saturn will be in line.

Imagine a circular racetrack marked with concentric lanes for the competitors to run in. A planet is allocated to each lane. The Earth is running in the third lane out from the centre; Jupiter and Saturn run in lanes five and six respectively. It is not actually much of a race because the inner planets, in addition to having smaller distances to travel each lap, also travel faster.

The Earth manages one lap a year. Jupiter takes 12 years to complete a lap and Saturn 29. With all the planets orbiting the sun at their particular distances and speeds, and all moving in the same plane, like marbles rolling around a plate, we are occasionally going to see two, or more rarely more, planets lying close together in the sky. It is just a consequence of how they move. Can such arrangements affect us?

All the planets are tied to the sun by gravity. Since the it is by far the most massive object in the Solar System, the sun plays the main role in controlling the way the Earth and other planets move.

However, the story is a bit more complicated than that. All the planets exert their gravitational attraction on each other too. It is much smaller than the sun’s but it is there. Jupiter, being the largest planet has the greatest effect. In fact, it stopped a planet forming between it and Mars, leaving a load of unused construction material: the asteroid belt.

These little tugs cause small perturbations in the orbits of the planets. Small changes in the Earth’s orbit is one of the possible explanations of our having occasional ice ages.

Will having Jupiter and another planet or two in line and all tugging us from the same direction for a month or two change the Earth’s orbit to a worrying extent?

The answer lies in our history. Our civilizations date back several thousand years or more, planetary conjunctions have been coming and going every few years, and have had no noticeable effect. When it really gets down to it, the main disturbances in human history have one way or another been due to our own activities.

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Saturn and Jupiter lie very close together, low in the southwest after dark. They are a bit low in the sky for telescope use, but it is worth a try.

Mars is high in the southeast, and is good for telescope observations.

Venus lies low in the dawn glow.

The moon will reach first quarter on the 21st.