For some time The Dog and I believed that, when the sun was where it would be around midday, it would be at its zenith. Now, we find that the zenith is directly above your head and not where the sun is at all. Through some convoluted and tortuous semantics, almost too complex to be believable, medieval Latin scholars produced our word zenith from an Arabic phrase meaning ‘directly above the head’. So, while it isn’t correct to talk of the sun reaching the zenith – because it certainly doesn’t, unless you live within the tropics – which we certainly don’t, the sun still reaches a high point above the horizon in its daily peregrinations. When it arrives at this height, it is all right to call this ‘the sun’s zenith’ because that’s what it would be if we lived somewhere where it was.
All of this is interesting of course, but not really the sort of, handy for day to day use information you’re used to finding here. Unless, you happen to be one of the many telescope mounting makers that we meet wandering pensively along the country byways. Knowing all about zeniths, for them, is crucial if they want to avoid the keyhole problem.
Imagine you are following the sun through your telescope. Just a note here – it is not safe to do this during the day, please only indulge in this type of experiment after sunset. As the sun rises your telescope points higher and higher until it is aimed directly overhead. Now, the sun is going to go down the other side, so the whole telescope must turn round. This leaves you with a slot of sky that doesn’t make it into your observation. The dreaded keyhole!
I’m happy to say that currently, the sun does not rise high enough to give us this type of problem.
I’m sure you’re all very relieved.