Dance of the planets

Ken Tapping, 18th November, 2015

In the sky this week…

  • Venus, Jupiter and Mars lie close together in the sky before dawn.
  • The Moon will reach First Quarter on the 18th.

It is really hard to miss the current spectacle in the eastern sky before dawn. There is Venus, shining brilliantly, Jupiter nearby, almost as bright and more yellowish, and the planet Mars, which is much fainter and easy to miss. At the moment Venus is the lower one, Jupiter the highest in the sky, with Mars lying between. Over the coming days Venus will be slowly slipping back into the sunset glow, while Jupiter will be rising earlier and earlier until eventually it will be in the sky overnight. The “Dance of the Planets” in the night sky is just one example of the cosmic gymnastics that have fascinated and puzzled sky watchers since our first ancestors looked at the sky.

The explanation is simple, although the calculations can be complicated. We live on the third planet out from the Sun, and it takes a year for us to complete a lap around the Sun. Starting out from the Sun there is Mercury ( taking 88 days to circle the Sun), Venus (225 days), Earth (365 days), Mars (687 days), Jupiter (5.2 years) and Saturn (29 years). Uranus, Neptune and Pluto lie further out and are very faint.

From our moving viewpoint, the apparent motions of the other planets against the star background can look very complicated, with their tracks looping and reversing. Of course all they are really doing is proceeding steadily around the Sun in their almost circular orbits. Mercury and Venus appear to loop back and forth, east and west of the Sun, so we see them in the dawn sky when they are west of the Sun and in the sunset sky when they are east. They never lie very far from the Sun in the sky. Mars and the other outer planets are different in that the Earth can pass between them and the Sun. At that point we can see those planets in the southern sky at midnight, while the Sun is in the southern sky at noon. With the knowledge we have today, getting our minds round these ideas is fairly easy. For our ancestors it was a lot more difficult.

The starting point of their thinking was not unreasonable. They assumed the Earth was sitting still and everything in the sky moved around it. They associated speed with a blast of wind and vibration. Since we experience neither as we go through our lives on Earth, it was reasonable to assume we are standing still and everything else must be moving around us. This certainly worked for the Sun and Moon, but the other planets posed problems. What simple explanation could there be for the planets looping and reversing their paths in the sky? All sorts of improbable ideas were tried out. Galileo and Copernicus pointed out that the picture simplified a lot if we put the Sun at the centre and had the other planets and us, orbiting around it. The Church took exception to the idea of us no longer being at the centre of everything. However, this did not stop this new, Sun-centred picture of the universe spreading.

With a stationary Sun in the middle and the planets orbiting it in concentric orbits, what we saw in the sky started to make sense. Unfortunately, as observations improved and calculations got more accurate, errors between the predicted and observed positions of the planets appeared. Lots of very strange ideas were proposed to correct the calculations. However, it was not until Johannes Kepler suggested the planets move in ellipses, not circles, that ideas and observations really started to fit each other. We could see what was going on in the sky, and finally, when Isaac Newton produced his theory of gravity, together with the mathematical tools to use it, we could properly explain what we were seeing.

Making the dance of the planets predictable does not subtract from the spectacle. Instead it tells us when we can enjoy it, so that we only have to get up at unpleasant times where we are pretty sure there will be something to see.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
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