Oneday Moon

The one-day moon, if seen at all, is a very slender crescent indeed. Moreover, its altitude is low, i's light is feeble, and the sky background is bright-All these factors conspire to thwart the observer. The most favorable time for viewing it in thi I

two-day moon 81

latitudes of the United States is in March or April, for with the sun near the vernal equinox the young moon stands higher in the sky at sunset than at any other time of year. Unless we had a total eclipse of jie sun to inaugurate the present lunation, the moon recently has passed either north or south of llie sun in the sky. Consequently, the line joining the horns or cusps of the crescent still may be inclined at a considerable angle to the north-south line on the moon's disk. If the lunation began near noon yesterday the angle should not exceed about 15 degrees, but if the moon is much younger than 30 hours the angle may be considerably greater. Since that angle is equal to the displacement of the midpoint of the crescent from the east point on the moon's disk, there is some uncertainty as to what portion of the limb the illuminated crescent will cover.

My advice to you is to wait until tomorrow evening before trying to identify any lunar surface features even if you have succeeded in locating the one-day moon, a major triumph for anyone on his first observation evening. After you have observed through a lunation and have become experienced in locating surface objects, you may wish to try your skill on the one-day moon.

Get ready to observe before sunset, and be sure you get out in time to note precisely the point on the horizon where the sun sets. Don't point your binoculars toward the sun before atmospheric scattering and absorption have reduced it to a dull red disk at the horizon. Your binoculars probably have a field diameter of seven or eight degrees. You can check their area coverage almost any night by observing the Pointers of the Big Dipper which arc a little more than five degrees apart. If the field diameter is about seven degrees, the average one-day moon should be found two field diameters east of the sun. From the sunset point on the horizon lay off two field diameters up and •0 the left. Then through that point sweep your binoculars in a circular arc centered on the sunset point from a low to a high altitude. In a few minutes you should pick up the faint, thin sliver of a crescent. Your first impression will be that it is completely blank. However, the skv still is bright and the moon isn't. The relative brightness will change rapidly in the next few minutes, and you will begin to notice surface detail.

"The largest and best-defined crater visible this evening is Humboldt, 125 miles in diameter and '5.400 feet deep. Its dark inner east wall shows as * snort black streak parallel to the terminator and not far from it. Its apparent location depends upon the cusp line inclination, and so it may be found anywhere from the midpoint of the crescent to % the way from the midpoint to the south cusp. As more detail appears we can check the identification of Humboldt by looking for another black streak, thicker but only % as long and located between it and the terminator. That is phillips, 80 miles in diameter and 10,500 feet deep. Just north of Humboldt and appearing almost as a continuation of it is a stronger black streak marking the east wall of hecataeus, 87 miles in diameter, 15,700 feet deep, and joining another crater on the north for a total length of 115 miles. Just north of Hecataeus a shorter, stronger black dash marks beiiaim, 33 miles in diameter and 11,000 feet deep. A little farther north may be seen a pair of black specks. The farther southern one is anscarius, 58 miles in diameter and 10,000 feet deep. The nearer, weaker, northern one is lapeyrouse, 46 miles in diameter and 10,000 feet deep.

In a telescope the craters we have been looking at can be seen as very narrow, cigar-shaped ellipses with some irregularities here and there. The reason that we see only black specks and streaks through binoculars is that we are picking up only their strongest parts—black-shadowed inner east walls. Their bright inner west walls are tipped away from us, and their illuminated floors blend with the light surroundings.

If now we jump north from Lapeyrouse about twice the IIumboldt-Lapeyrouse distance we should find another strong black dash which is Plutarch, 41 miles in diameter and 9200 feet deep. North of Plutarch, a distance equal to that between Humboldt and Behaim, is the mountain-walled plain causs, 112 miles in diameter and 13,000 feet deep. Its dark streak is not so strong as the others, and you will have to look hard to see it. Just before the moon disappears behind the trees or into the thickening haze we get the impression that the whole crescent is pocked with craters and by no means blank.

0 0

Post a comment