Perigee Apogee and the Far Side

The Moon's monthly orbit around the Earth is not circular, it is elliptical. At its closest (perigee), the Moon's center is only 356,410 kilometers from the Earth's center. At its furthest (apogee), the centers are 406,697 kilometers away. That is a considerable variation of ±7%. On average, a tiny crater on the Moon, say one kilometer across, will only span an angle of around 0.55 arc-seconds and will be close to the limit of detection in average seeing conditions, with an amateur astronomical telescope. Having said this, tiny high contrast rilles, seen at low sunset and sunrise illumination angles, will still be detectable even though they may be well below a kilometer in width.

Libration is a phenomenon by which we are able to peer around the lunar limb and see, at a painfully shallow angle, the edges of the Moon's far side. In fact, we can theoretically see 59% of the lunar surface, not just 50%, by using libration tilts. Let us dispel one myth here: contrary to popular terminology there is not a lighting orientation that results in a permanent "dark side of the Moon," so viewing features on the far side is not complicated by this misconception. When the Moon appears full to us, the opposite side is dark; but when the Moon appears new (i.e., a hair-thin crescent) the opposite side is almost fully illuminated. However, the Moon does have a near side and a far side because the Moon's rotation on its axis has become "locked," due to tidal forces; thus, the lunar side that we can see stares permanently down at the Earth (if we ignore the libration effects that is). The "man in the Moon" never rotates out of view.

The Moon rotates around the Earth every 29.5 days (from new Moon to new Moon) and, with respect to the stars, every 27.3 days (remember the whole Earth-Moon system orbits the Sun, too). However, it is constantly rotating on its axis such that the same face always points towards the Earth . . . well, almost. Imagine you are talking face to face with someone, but occasionally they lower their head, so you see more of their badly fitting wig, or raise their head so you see more of their chin. Now and again they shake their head too, so you see a bit more of one ear and then the other. This is analogous to what librations do to our view of the Moon. Because the Moon's orbit is elliptical, its angular position with respect to the Earth does not vary constantly, even though its axial rotation is always the same. The velocity of the Moon around the Earth is faster at perigee than apogee. Because of this, we can sometimes peer around either the eastern or western limbs and see almost 8 degrees more Moon (7° 54' to be precise). This is called a libra-tion in longitude.

There is an additional effect called diurnal libration, caused by the fact that the Earth has a radius of over 6,000 kilometers and so, depending on whether the Moon is rising or setting (or if you are at the north or south poles for that matter) you are standing on a platform that gives you an extra ability to peer round the limb. However, most observers will view the Moon when it is near their meridian and will not travel to the Arctic or Antarctic to get a tiny bit more north-south advantage!

The main librations in latitude (excluding traveling to the poles of the Earth) are caused by the fact that the lunar equator is tilted with respect to the lunar orbital plane, much as the Earth's equator/axis is tilted with respect to the ecliptic/ecliptic pole. Thus, as the Moon orbits the Earth, first one pole and then the other tilts by 6° 41' toward the Earth (the absolute extreme librations are actually 6° 50').

In practice these monthly librations in latitude and longitude form a vector sum, peaking in a maximum libration effect of 10 degrees when latitude and longitude librations peak together and swing a feature on the NE, SE, SW, or NW limb toward Earth. Of course, that is not much use if the feature is in darkness, but it is highly exciting when a favorable Sun angle picks out the feature well and the sky is clear.

Under extreme libration conditions the lunar webcam user can secure some rare shots of regions usually invisible from the Earth. Of course, the Moon has been fully mapped by various spacecraft, most recently by Clementine and Smart-1. Even the southern polar regions have at last now been fully imaged. However, this does not take away the enjoyment of observing and imaging the Moon. Despite the Moon being fully mapped, it is true to say that it has certainly not been mapped at high resolution at every illumination angle. Following the progress of mountain peak shadows as they grow and shrink under sunset and sunrise conditions can be a fascinating pastime and can be carried out at unprecedented resolution in the webcam era. Lunar craters can easily become like "old friends" to the dedicated lunar observer and, with webcams, it is now possible to capture the appearance of a crater without any hint of observer bias.

Unlike any other planetary body, the Moon is an extremely photogenic subject, even under poor seeing conditions, simply because it is so large. When seeing is poor, just reduce the f-ratio (remove the Barlow lens or Powermate) and shoot some images at a scale of, say, 0.5 or 1.0 arc-seconds per pixel. The rugged southern highlands and the jagged southern limb regions are especially good wide-field targets in this respect. Using a webcam on a small-aperture Newtonian, a mosaic of half a dozen frames can produce a spectacular picture of an entire lunar crescent. Alternatively, a digital camera or digital SLR can be used to good effect too.

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