Why giant Jupiter looks smaller than dwarf Venus

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

07 December 2008

There was an amazing spectacle in the sky on Monday evening. The crescent moon was sandwiched between two bright “stars” to form a “smiling face” in the sky. The two shiny objects were planets Venus (the brighter one) and Jupiter (dimmer).

The two planets are special in different ways. Venus is the closest one to us and it is almost equal in size to the Earth. It is about 12,000km in diameter compared to the Earth’s 12,800km.

With a 140,000km diameter, Jupiter is the largest planet in the solar system (that is, our Sun and its planets). Even though that is almost 12 times as large as Venus, Jupiter appears to be the smaller of the two. Could this be due to the difference in the distances of the planets from Earth?

To find out, we have to work out the apparent sizes of the planets. That is; the ratio of the diameter of an object to its distance from the observer. The larger the ratio, the larger the object “appears” to be.

For example, even though the moon is a small spec compared to the sun, the two appear to be same size when viewed from Earth. This is because their apparent sizes are almost equal.

At their present positions, Venus is “only” 100 million kilometres away while Jupiter is over 900 million km from Earth. Thus the apparent size of the former is 120 millionths while that of the latter is 150 millionths.

Note that these are millionths and NOT millions! A millionth is the number one divided by 1,000,000.

Clearly, the apparent size of Venus is slightly smaller than that of Jupiter. Therefore, the former should appear larger. But that is not what we see when we look in the sky.

The reason for that discrepancy is that, as a rocky planet, Venus is brighter than the gaseous Jupiter. But how can a planet be made of gas? If Jupiter is a blob of gases surrounded by empty space, why don’t the gases escape?

The answer is that the gas molecules (mainly hydrogen and helium in Jupiter) attract one another by mutual gravity and keep each other in a common place….the place that we call Jupiter.

In fact, since the quantity of gas is so large, the gravitational attraction near the centre of the planet is big enough to force the molecules to coalesce to form liquid and even solid. Other elements are also present on the planet but they account for only about five percent of the mass.

Since its atmosphere is very large compared to the liquid and solid part, the planet does not reflect a lot of sunlight thus it is dimmer than the smaller Venus. Indeed, if Jupiter was not so big in size, it would not be visible from Earth with the unaided eye.

As far as the “smiling” face is concerned, don’t look out for it again next year; it is expected to return in some fort-odd years! Still, you can look out for other planets “dancing” in the sky. It is easy to recognise them since they don’t twinkle like stars.