For the last few weeks the western sky after sunset has been dominated by the planet Venus, which has been shining incredibly brightly, like an escaped aircraft landing light.
It has been so bright that it is hard to accept we are looking at a planet and not something more ìunidentifiedî. Now another object is moving closer to it, not quite as bright, but like Venus shining steadily. You can usually tell planets from stars in the sky because stars twinkle, planets donít. This other body is the planet Jupiter. The two planets are not really getting physically close together; they just lie in the same direction when viewed from the Earth. Around July 1 they will be at their closest.
Both these planets are interesting. Venus is the second planet out from the Sun; we live on the third. It is about the same size as Earth, but covered in a thick, permanent layer of cloud. This cloud layer reflects most of the sunlight back into space. If you have ever taken off in a plane from a cloudy airport, like Vancouver, or London, UK, you will remember climbing through the gloom, then through the cloud layer, and suddenly finding yourself blinded by the sunlight scattering off the top of the clouds. Assuming that we could survive long enough on the surface of Venus to notice, we would find it very gloomy. The temperature on that planetís surface is high enough to melt lead.
Jupiter is very different. It is the fifth planet out from the Sun, and its diameter is nearly 12 times that of the Earth and Venus. It is mostly atmosphere, a big ball of gas, mainly hydrogen, methane and ammonia, with a relatively small rocky body somewhere in the middle. Venus regularly passes between us and the Sun; since Jupiter’s orbit lies outside the Earth’s, it can never do that.
Planets produce no light of their own. We only see them because they are being illuminated by the Sun. That means half of a planet is lit up and the other half is dark. That is why we have day and night. If you look at a planet with the sunlight coming from behind you, the lit up side is facing you and you see the full disc. Then, as you view from different directions you see only part of the lit side. This is what gives us the phases of the Moon. Compared with Jupiter’s distance, our world lies relatively close to the Sun, so we always have the sunlight coming from behind us, and the planet always appears as a fully lit disc. That is not the case with Venus. As it approaches for a pass between us and the Sun, we see the planet as a full disc. However, as it gets closer we see more of the unlit side. Although the lit side appears brighter and brighter, we are seeing less and less of it. As it passes between us and the Sun, we will see mostly the unlit side, and Venus will become invisible.
Even though in the sky Venus looks enormously brighter than Jupiter, we have to remember that at the moment Venus is very close to us, only about 80 million kilometres, compared with Jupiter’s 910 million kilometres, and of course it is closer to the Sun. The sunlight illuminating the planet is stronger, and the light scattered in our direction does not have as far to travel. If we moved Venus to where Jupiter is, the sunlight hitting it would be about 50 times weaker, and what gets scattered in our direction would about 130 times weaker. So Venus would be several thousand times fainter than Jupiter. If we moved Jupiter to the distance of Venus, we would have a glare problem.
Binoculars or a small telescope will show Jupiter as a tan disc, crossed with cloud belts, and accompanied by its four largest moons. Venus will appear as a searingly bright crescent.
It is worth having a regular look into the western sky over the next month or two as the two planets perform their slow dance. Saturn lies in the southern sky. The Moon will be Full on the 2nd.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton