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.
I went out this morning just before sunrise, telescope ready, expecting this to be the last morning of observing the waning Moon. Turns out yesterday morning was the last day, after all. The Moon was a delicate curl of bright light all of 20° above the horizon, set in a pale blue not quite sunrise sky. Sliding along the ecliptic, it had moved more to the north than down toward the horizon, but the… Sun is rising earlier and the sky was quite bright. Unfortunately that bright sky was infested with thin grey clouds that first robbed the lunar crescent of detail in the eyepiece, then stole the show as the rising sun lit them with sweeping strokes of apricot and gold. What can you do? After thinking I could make out Bailly and the western shore of the Ocean of Storms, I stepped back to take in the beauty of the sunrise and watched the silvery crescent fade into the blue, as the apricot clouds slowly turned white.
The Dawn Patrol is proceeding as planned. So far there’s been one observing session that started between 2am and 3am (MST), and the rest have begun closer to 4am. I’m finding it much easier to rise an hour and a half earlier than normal, rather than to stay up until midnight or so, as I did a few days back. Except for a day that involved high winds in the wee hours of the morning, these Dawn Patrol sessions have involved pleasant and beautiful conditions. Little or no wind, and then just light breezes, combined with mild temperatures between 58°F and 65°F. (14°C and 18°C). The skies have been clear and the transparency good, and as the Moon has waned more and more stars are out and easily seen. And it is quiet, very quiet, in the cool pre-dawn hours. The sounds of the city around me are muted, and on this Sunday morning almost inaudible. No voices, no cars on the street, with only a mockingbird singing loudly in the light of the Moon. He is singing less, now, and starting later, as the light of the Moon fades. Now and then a White-winged Dove awakens early. These calls and songs do not banish the morning quiet so much as define it. The awareness of how quiet the world is at such an hour is accented when the Mockingbird’s last note fades.
This morning the lunar terminator was a complicated thing, in mountainous places rough and ragged, with bright arcs of brightly lit stone and black bays of inky shadow where the divide between day and night passes through cratered terrain. It’s easy to think of the lunar terminator as a simple dividing line between lunar night and day, and where it crosses the surface of maria this may come close to the truth. But more often than not the terminator is anything but simple. This morning served as a fine example to prove the point. Half lit craters with shining west facing walls to the north of Mare Frigoris, with the shadows within clinging as much to the north as to the west. Plato broad and dark and smooth, with no sign of craterlets using such modest aperture (102mm) under mediocre seeing conditions. The lunar Alps presented a crazy jumble of gleaming bright peaks in a black matrix made of the mingled shadows at their feet. Farther south the proximity of craters such as Ptolemaeus, Alphonsus, and Arzachel created bulges of shadow reaching to the light of day, and bright broken curves of light where high crater rims and the tips of central peaks were bathed in the light of the setting sun. South to more heavily cratered terrain, where outlines grew more confusing, and the black bays of intruding shadow were smaller and rimmed with silver light.
Observing the Moon is always about tricks of the light. For this morning’s Dawn Patrol, it added up to quite a show.
It’s a thing I’ve always wanted to do, and life has handed me the opportunity to get it started and then see it through. Starting a day past New Moon I’ve been out each night to spend an hours or so observing the Moon with either a 60mm or a 102mm refractor – whichever seemed most expedient on a given night. (The larger telescope has seen the most use so far.) Each night, when the Moon has risen high enough for convenient observation, I put whichever telescope I’m using on an Orion AstroView EQ mount and go out. With the Moon in the eyepiece, I work my way down the lunar terminator from north to south, identifying craters, mountains, rilles, and other features as I go. I call this process a terminator slide. I’ve repeated the process every night, now, starting on April 22nd. (I missed on the 26th due to clouds.) I have revisited many familiar sights during this set of observations, and seen things on the Moon for the first time. One session each night of this lunation, weather permitting. It’s been something of an adventure.
And now the easy part is behind me. Each night the Moon rises above the horizon later, which means I go out ever later to make the next slide. Now that full Moon has passed (the much hyped Super Moon of 2012), I’m pushing midnight before I can get a good look at the Moon. Tonight, as I type these words, I’m waiting for midnight, a lunar witching hour. Past this point I will be on “dawn patrol,” for it will make more sense to get up early than to stay out late.
Amateur astronomers do the strangest things!