Under Desert Stars   Elen síla lúmenn’ omentielvo

Amateur astronomers often use the phrase “Clear skies!” when closing a letter or an online post. The meaning of the sentiment is obvious to anyone who knows anything at all about stargazing. Without clear skies, you can’t really do much in the way of astronomical observing. Short of setting up a radio telescope, I mean. (And that has been done by amateurs.) But it takes more than a clear sky for astronomy to happen. Other things need to line up just so.

For one thing, the “seeing conditions” need to be pretty good as well. When amateur astronomers talk about “seeing,” they’re concerned with the steadiness of the atmosphere. The ocean of air under which we live never holds still, and at times is downright jittery. You can see this without a telescope. Look up at night and watch the stars twinkle. That’s called scintillation, and as pretty as it may seem to the casual sky-glancer, it isn’t a well-loved phenomenon among astronomers. Telescopes magnify everything, including that jittery glitter you sometimes see at night, which goes from a pretty sparkle on high to a glaring blob of bright mush in the eyepiece of a telescope. When the seeing is bad the sky can be absolutely cloud free and the amateur astronomer will still have limited options.

Wind can be a hardship as well, complicating everything from getting a good view to using a star chart and taking notes. Breezes are fine, especially in mosquito season, but a good stiff wind battering the tube of a 203mm Newtonian reflector does not make for a fine night out, if your goal was stargazing. I can always do without wind, when I have a telescope set up.

And of course, there are the closely related matters of having the time and energy to take advantage of a clear night sky, when all other things are equal. Handling expensive eyepieces while in a hurry, or yawning, is not recommended.

Like so many matters of “real life,” then, it’s best not to take the wish for “clear skies” too literally. There’s more to it than a lack of cloud cover. Think of it as the amateur astronomer’s way of wishing you good luck. Something like saying “break a leg” to a performer, only a little bit more subtle.