MoonWatch
November 1998
There's a near-perigee full Moon this month
The appearance of the near-perigee full Moon on 3 November (Peter Grego)
Every month the Moon experiences a complete cycle of phases, from one new Moon to the next, in its so-called synodic cycle lasting 29 days 12 hours 44 minutes. As the Moon orbits the Earth in an ellipse it approaches and recedes from the Earth once every revolution. When the Moon is closest it is said to be at perigee, and the interval from one perigee to the next is called the anomalistic month, and this occurs every 27 days 13 hours 19 minutes. The point of perigee actually makes one complete prograde orbit every 8.85 years, so given these two lunar periods it follows that sometimes, though not very often, the date and time of the full Moon will very nearly coincide with the perigee Moon.
On the evening of 3 November (and early hours of 4 November) the full Moon will be very near to lunar perigee - a spectacular combination that will give the viewer a sight of one of the largest Moons possible. Located in the constellation of Cetus the Whale, well south of the ecliptic, the Moon will have risen in the east by about 17:00 UT. The Moon will be nearly 34 arcminutes in apparent angular diameter, at a distance of around 352,500 km - amounting to just under 28 Earth diameters away. Saturn will be located just under 10 degrees to the west of the Moon, and the Moon culminates due south before midnight. This will be the best opportunity to observe the full Moon-perigee combination until 10 August 2014, so do make the most of it!
There will be few occasions better than this to be struck by the Moon illusion. Try to observe the Moon with the naked eye just as it rises above your local horizon and it will present a disk of apparently awesome proportions. The illusion comes about because we perceive the shape of the celestial vault as a flattened dome rather than a simple hemisphere. A Moon near the horizon has the same apparent diameter as a Moon higher up in the sky, but the low Moon is imagined to be further away - and therefore bigger - to assume such proportions.
Use this opportunity to discern some of the Moon's features without optical aid. Tonight the Moon's northern hemisphere will be very well presented, and with the naked eye you should be able to make out the dusky line of Mare Frigoris forming the brow of the Man-in-the-Moon simulacrum. Telescopically there will be a little of the terminator running around the northern lunar limb, so it would be worthwhile attempting to observe some of the craters near the north pole that are now known to harbour vast quantities of lunar ice.
