Jupiter and Venus: so far, yet so near
The Sunday Nation
Nairobi,
01 April 2012
Ignatius Kimani is wondering how two objects that are far apart can appear very close together. He writes: “Astronomers have discovered that Venus and Jupiter are close to each other – just a few degrees apart – and are currently visible from earth.” So he asks how this is possible.
Let’s start with some basic facts. Venus is the second planet in the solar system. It is about 109 million kilometres from the Sun. Jupiter is number five and lies about 778m km. The Earth lies somewhere between these two planets at 150m km.
With those basic facts, the first question we ask is how far apart are Venus and Jupiter when viewed from Earth? The answer is not straightforward and to understand why, one needs to draw a diagram of the orbits – I suppose this is within the scope of this column, after all, a diagram is a figure!
So, please dear reader, take a piece of paper and draw three concentric circles – that is, they have the same centre. Let these represent the orbits of the three planets Venus (the innermost circle), Earth (the middle one) and Jupiter (the outermost one). The sun will be at the common centre of the three circles.
At any one time, each planet can be located at a particular position on its own circle. Now suppose that the four bodies were lying in a straight line in the order Jupiter – Venus – Sun – Earth. What would be the distance from Venus to Jupiter in this arrangement? Looking at the diagram, it easy to see that the answer is 669m km (that is, 778m minus 109m).
But is this the only possible separation between the planets? Well, no! The reason is that the planets move around the Sun at different rates. The Earth takes 8,760 hours (one year) to complete one revolution, Venus about 5,400h and Jupiter about 104,000hours (almost 12 Earth years). Therefore, their arrangement in the solar system is constantly changing.
Thus eventually, a time will come when the order will be Jupiter – Earth – Sun – Venus. In this arrangement, the distance between these planets is 887m km (778m plus 109m).
Now if you look at your diagram, you will notice that, in the first arrangement, both Jupiter and Venus are “behind” the Sun and are therefore invisible from Earth. In the second figure, Jupiter is visible but Venus is not.
The only way that the two planets can be visible from Earth at the same time is when the Sun gets out of the picture. That is, the planets lining up alone with the Sun on the side.
This is the sort of arrangement that we have at the moment: Jupiter and Venus are sitting side-by-side in the evening sky – the pair can be seen in the western horizon from sunset to about 8pm. In this arrangement, the two are several hundred million kilometres apart but when viewed from Earth, they appear very close to one another.
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