Dawn at Nasmyth Crater

Two views of an object are rarely exactly the same.

Our astronomer has taken advantage of the good weather over the past week or two to pull out his small telescope and look at things in the sky.  Alexandria, Virginia, where he lives, is not a promising place for this activity.  The sky is very bright; even with good binoculars he can’t see stars that are obvious to the naked eye from a good site.  And the houses make the air decidedly unsteady.  Still, there is often something to be seen in bright objects, and he thought to show his neighbors Saturn and the Moon.

One evening he was admiring the craters just coming into dawn at the edge of the visible Moon, where the shadows and sunlight make for dramatic effects.  The walls of one crater shadowed parts of the otherwise brilliant far rim of another, with the crater floor in deep darkness; some peaks and ridges seemed to float in space beyond the sunlit globe.  The Moon has never been his special study, and the observations had no particular scientific value, but he enjoyed them nonetheless.  He toyed with the idea of taking the time to make a good drawing, because you simply see more that way, but put it off for now.  His efforts in the past have not been satisfactory.

Then he made some mental notes about where this particular configuration of craters was, using a map of the Moon in his Norton’s.  (Norton’s Star Atlas and Reference Handbook has been a mainstay of generations of amateur astronomers.)  Sitting down with other books and material later, he found he couldn’t identify the craters satisfactorily.  While his Norton’s and other works described and plotted many stars and nebulae and things well beyond anything he could actually see in his telescope, even from a good site, the material on the Moon was almost superficial.  He could go well beyond it without hardly trying (as this episode showed).  So he got out the telescope the following night in an effort to locate the region again.

Why should he bother?  Isn’t the Moon an unchanging globe, showing the same face to us all the time, repeating its phases each month exactly as the month before?  Well, yes and no.  Overall, yes, the Moon stays the same.  But as seen from any particular place on Earth it wobbles a bit, something called libration, due to its orbit not being exactly circular and other small effects.  This makes a big difference in the foreshortening of objects near the edge as seen from Earth (the “limb”).  Maybe more important is the variation in the angle of sunlight caused by the time of month and the tilt of its orbit.  A crater showing dramatic effects at dawn (as seen from the crater) will lose them in a couple of hours as the Sun rises higher.  Even the next dawn, a month later, will show different effects as the Sun rises at a slightly different angle.  So our astronomer could not count on seeing the same thing again.

The difference in light, something well-known to observant photographers, can change the appearance of something dramatically.  Even experienced astronomers in previous ages were often unaware of how much this happens; Sir John Herschel was willing to believe a crater had completely disappeared, when it was just a trick of the light.  (Our astronomer plans on looking for that one, Linnaeus, when conditions are right next month.)  But he’s not optimistic about seeing again exactly what he saw a couple of nights ago.  The same things rarely look exactly the same.