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Cosmic mechanics explain motion of heavenly bodies

Ken Tapping's weekly astrophysics column from the Herzberg Institute

When I arrived home from the gym this evening the full Moon shone in the eastern sky, quite close to the planet Mars, shining bright and orange-red. In the west were Venus and Jupiter, Venus brilliant and white, Jupiter a bit fainter and yellowish. The sight was fascinating for two reasons. Firstly, against the clear, blue-black sky, it was a beautiful spectacle. Secondly, that precise arrangement could be predicted, even years ago, using cheap or free software. Our ancestors did it by hand.

Our ancestors thought things out pretty well the same way as we do today. They saw things happening around them and tried to find explanations that seemed logical, at least at the time. One thing that was very obvious is that things on the ground were hard to predict. The weather still cannot be forecast accurately, and there were always floods, plagues and crop failures that seemed to turn up out of nowhere. The rhythm of the heavens was obvious. Planetary arrangements and eclipses could be routinely predicted. It was widely believed that things were chaotic down here and perfect “up there.”

For us at ground level, it was patently obvious that things like barrels, carts, logs and so on moved because we or draught animals pulled or pushed them, and when we stopped pulling or pushing, the things stopped moving. Carts kept moving as long as the animals kept pulling, and sailing ships sailed as long as there was a wind or someone rowed.

Those ideas encountered problems in the sky, because we could not see anything pulling the celestial bodies, but they maintained their precise rhythm, punctuated occasionally by a comet. This suggested either the heavens were fundamentally different, or that things were being moved by some sort of metaphysical agency. We have Isaac Newton to thank for getting us out of the hole.

He proposed something completely different. Objects sit still or continue in motion unless a force acts on them, like friction. He also introduced the concept of gravity. He then expressed these ideas mathematically, which made possible a physical foundation for astronomy. Things stop moving on the ground because of friction and the need to push stuff out of the way as they move. In space there is almost nothing to push out of the way, so things continue in their orbits for billions of years.

Once we got the calculations down pat, we could search for small deviations in the paths of objects and deduce their causes. The planet Neptune was discovered by calculating the position and mass of the body needed to explain perturbations in the orbit of Uranus. More recently, Dark Matter was postulated to explain the motion of stars in galaxies, and Dark Energy to explain the accelerating expansion of the universe. All this is thanks to Isaac Newton, who, although not the nicest man in history, must certainly be one of the smartest.

Mercury, Venus and Jupiter lie in the west after sunset. Mercury is close to the horizon and hard to see. The other two are easy to spot. Venus is the bright one. Mars is in the east, and Saturn rises about 9 p.m. The Moon will reach last quarter on the 14th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.