Angular size, as I describe on the Blogstronomy post, is a way that astronomers use to talk about how big things appear in the sky. That post talks about how big Jupiter and its moons Io and Ganymede would appear in the sky above Europa. This post explains how to work it out.
In order to work it out we need to be fairly comfortable with trigonometry - that's that GCSE topic with SOH CAH TOA and right-angled triangles. We also need to know two things about the object we want to look at: its actual size, and its distance from us.
Here's the general maths that we need:
The grey lines are imaginary lines from our eyeball to the extents of the object in the sky. r stands for the radius of the object we're looking at, and the yellow line (d) represents the distance between us and the centre of the object. Luckily for us, it also cuts the (grey) isosceles triangle in half and gives us a right-angled triangle that we can get to work on with trig.
If we're trying to find the angle that's marked (that's half the angular size), we can label the radius as the 'opposite' side, the yellow line as the 'adjacent' side, and the grey line is the hypotenuse. Let's call the angle "x", to make things easy. We know the radius and distance, so from our school-days "SOH CAH TOA", we know that we have to use the Tangent function:
Tan(x) = Opposite / Adjacent
Or, using more sensible notation from the diagram:
Tan(x) = r / d
We want to find x, though, so we can rearrange:
x = Tan-1(r / d)
But if we're being honest with ourselves, we want to find the entire angle between the two grey lines, so we need to double whatever result we get (calling the angular size "a"):
a = 2x = 2 x Tan-1(r / d)
Now all we have to do is substitute the values we know for each of the objects. Here's the data you need:
- Io: r = 1830 km ; d = 249,300 km.
- Ganymede: r = 2631.2 km ; d = 399,300 km.
- Jupiter: r = 69,911 km ; d = 671,100 km.
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