Three planets

Ken Tapping, May 14th, 2014

In the sky this week…

  • Venus rises about 4 am, in the predawn twilight.
  • The Moon will be Full on the 14th.

These evenings, if it is clear, we can enjoy an astronomical spectacle. There are three bright planets to see. If you have a small telescope or a pair of binoculars, you are in a position to enjoy them even more. In the west lies Jupiter. This planet shines so brightly there should be no problem in finding it. High in the south is Mars, not as bright as Jupiter, but still bright enough to be obvious, shining orange-red. Low in the southeast lies Saturn. This planet is a moderately-bright, golden object. Currently it lies in the constellation of Libra, which does not contain any bright stars, and on the 13th the Moon is nearby. Saturn should not be difficult to find. An aid in identifying planets is that they twinkle much less than stars do.

Almost any sort of optical aid, whether binoculars, small telescope or backyard monster telescope will show Jupiter to be a spectacular sight. Binoculars will show it as a small tan-coloured disc, definitely not a star, with up to four faint “stars” close by. These “stars” are really Jupiter’s four largest moons: Io, Europa, Ganymede and Callisto. If you take a look on consecutive evenings, you will see them changing position as they circle the planet. These moons were discovered by Galileo, and his deduction that they orbited Jupiter gave him the idea for a Solar System with the Sun at the centre and all the planets, including the Earth, circling it.

A small telescope will show Jupiter’s disc to be crossed by darker and lighter belts. A larger telescope will show white spots and one large red blotch. These are not features lying on the planet’s surface. They are structures in an immensely deep atmosphere. Jupiter’s diameter is more than ten times that of the Earth, and it rotates once every 0.4 of an Earth day, so at the equator Jupiter’s atmosphere is moving more than 25,000 km/h. This draws the clouds out into belts and provides the energy to drive huge, violent storms. These show up as white spots and bites out of the belts. The largest storm visible is the Great Red Spot, which is big enough to drop the Earth into.

Saturn, in the southeast, is a planet very similar to Jupiter. It is another gas giant – a planet made mainly of gas. It too rotates very quickly, and its clouds are drawn out into belts. There are storms here too, but harder to see than the ones on Jupiter. However, the main difference is immediately apparent. It is surrounded by a spectacular system of rings. These rings are material that was prevented from coming together to form a moon or two by tidal forces from Saturn’s gravity and perturbations by moons lying further out. These rings surprised and impressed Galileo, whose small, rather primitive telescope made it hard to see exactly what they are. A starlike object lying close to Saturn and changing position from night to night is Titan, Saturn’s largest moon.

In the south lies Mars. Of the three this world is most like ours. Its diameter is a little over half that of Earth. Its atmosphere is very thin compared with ours, and today its surface is a cold desert. It has polar ice caps that get larger and smaller with the seasons. There are clouds, dust storms and we have even seen dust devils moving over the desert. However, all over the place there is evidence that in the distant past the planet had rivers, lakes and perhaps seas. Robot explorers have scraped the surface dust and found ice beneath. We have known for centuries that Mars is the planet in the Solar System most like ours. This fact triggered a multitude of science fiction stories in which alien beings living on a dying planet Mars cast covetous eyes on our warm, watery Earth. Obviously our writers were putting down what we would have contemplated doing under those circumstances. However, although Mars is a hostile place, with appropriate technologies we would be able to live there, which is something that is very unlikely in the cases of Jupiter and Saturn.

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

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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