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By Degrees

“Look over there!” shouts an excited astronomer.

“Where?” you reply.

“There! Due south, about forty degrees above the horizon. It’s the International Space Station”.

Panicked that you’ll miss this amazing sight, you spin around in a flap. South you can do: just find the north star and turn around. But how high up is forty degrees?

There’s a useful rule of thumb – quite literally – that you can use you measure angular distance in the sky.

Hold your thumb out at arm’s length. The width of your thumb at its widest point is around 2°. As you can see in this picture, the top of the University of Glasgow’s tower is just short of 2° above the surrounding rooftops, when seen from the Millenium Bridge about 1.5km (1 mile) away.

University of Glasgow Tower

University of Glasgow Tower

This is all very well for small angular distances, or objects close to the horizon, but what about forty degrees up? Counting twenty thumb-heights up above the horizon is quite tricky, so there’s an easier way of measuring larger angular distances in the sky, using your fist. A clenched fist, again held at arm’s length, is about ten degrees top to bottom.

So to find something that’s forty degrees above the horizon, just count one-potato, two-potato, three-potato, four, and you’ll be looking in the right area!

Here’s a picture of Glasgow Science Centre’s tower, taken from the end of Millenium Bridge. It’s about 20° high from here, I’d guess.

Glasgow Science Centre Tower

Glasgow Science Centre Tower

Let’s check that’s right. Here’s a view of Glasgow Science Centre from above.

The tower is about 250m from the end of Millenium Bridge, and is about 125m tall. As you’ll recall from school trigonometry, tan A = h / d = 125/250 = 0.5, and so A = 26.5°, making my fist (which is about half the height of the tower) around 13° across, top to bottom.

[I was holding my fist a bit closer them normal, awkwardly taking the picture with the other hand, so this discrepancy can be forgiven!]

Heavens Above

Visit heavens-above.com to find out when the next bright satellite is passing over your head, and they’ll give you the info in this format:

So for tonight (29 March 2010) at 2048 there’s a very bright (mag -9, or 100 times brighter than Venus at its brightest) Iridium Flare satellite 66° above the ESE horizon. Find ESE (using a compass or the North Star), and count about 6 or 7 fist heights above the horizon, and that’s where it’ll be!


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