On Satellite Dishes Looking in the Same Direction

Posted by jns on 23 May 2008

I recently finished reading Richard Dawkins’ Climbing Mount Improbable (New York : W.W.Norton & Company, 1996, 340 pages). It wasn’t bad, but it wasn’t his best by any means. All of the little things that irritate me about Dawkins’ writing seemed emphasized in this book. There’s more in my book note, of course.

Dawkins is usually such a careful writer so I was surprised by the brief lapse of analytical perspicacity he exhibits in this passage. He is describing the fascinating compass termites that build tall and surprisingly flat mounds, like thin gravestones.

They are called compass termites because their mounds are always lined up north-south–they can be used as compasses by lost travellers (as can satellite dishes, by the way: in Britain they seem all to face south). [p. 17]

Well, of course they seem to face south–the satellite dishes, I mean–and there’s a very good reason. I can’t believe Dawkins would say something this…well, I can’t think of just the right word to combine unthinking lapses with scientific naiveté, specially since he’s the Charles Simony Professor of the Public Understanding of Science at Oxford. Tsk.

Satellite dishes are reflectors for radio waves transmitted by satellites; the dishes are curved the way they are so that they focus the radio signal at the point in front of the dish where the actual receiver electronics reside, usually at the top of a tripod arrangement of struts. In order to do this effectively the satellite dish must point very precisely towards the satellite whose radio transmitter it is listening to.

If the satellite-radio dish is stationary, as most are, that means that the satellite itself appears stationary. In other words, the satellite of interest always appears at the same, unmoving point in the sky relative to the satellite dish, fixed angle up, fixed angle on the compass.

Such satellites are called “geostationary” for the obvious reason that they appear at stationary spots above the Earth. In order to appear stationary, the satellites must rotate at the same angular velocity as the Earth, and they must appear not to move in northerly or southerly directions.

In order not to appear to move north or south, and to have a stable orbit, the satellites must be positioned directly above the Earth’s equator (i.e., in the plane that passes through the Earth’s equator). In order to have the necessary angular velocity they must be at an altitude of about 35,786 km, but that detail isn’t terribly important for this purpose.

Armed with these facts, we may now consider two simple questions, the answers to which apparently eluded Mr. Dawkins:

1. For an observer in Great Britain, in what direction is the equator?
2. If a satellite dish in Great Britain wishes to listen to a geostationary satellite, in which direction will it point?

The answers: 1) south; and 2) southerly.^* Now it’s no surprise that (virtually) all satellite dishes in Britain do point south.
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^* Yes, there are slight complications having to do with the longitude of the particular satellite, but most of interest to Great Britain will be parked near enough to 0° longitude not to affect the general conclusion.