Full Moons and other matters

Fig. 1: Near full Moon. Imaged with 100mm refractor May 12 2006 at 22h 30m UT.
Fig. 2    The Moon imaged with 175mm Maksutov February 2 2006 at 17h 27m UT.
Fig. 3    The crater Aristarchus imaged with 175mm Maksutov May 9  2006 at 20h 24m UT.
Fig. 4    The crater Proclus imaged with 175mm Maksutov February 2 2006 at 17h 29m UT
Fig.5: The crater Proclus imaged with 175mm Maksutov May 9  2006 at 21h 39m UT.

To the casual observer one full Moon looks much the same as another, apart from its size and position in the sky of course.

Essentially the Moon circles the Earth in what is called synchronous motion. That is, the Moon's period of revolution about the Earth is almost exactly the same as the time it takes to rotate upon its axis. .

Indeed but for other factors affecting the movement of our natural satellite we would see from Earth only half the Moon's surface.

Owing to what is called the Moon's librations, around 18% of its surface is rendered alternately visible and out of sight. The three principle librations are termed the librations in latitude and longitude, and the diurnal libration. It is a salutary fact that before the advent of space probes we knew nothing of the features on the far side of the Moon.

Of course at the time of full Moon we see only half the Moon's surface, but careful examination in the telescope will show that slightly different features are visible close to the limb at different full Moons and at other times. Libration is clearly demonstrated by comparing Figs. 1 and 2 and the position of Mare Crisium in relation to the Moon's western limb.

A full Moon is a not the best time to appreciate the craters since from Earth we are observing the Moon's surface illuminated from the Sun at our back. In other words, there are no shadows to be seen and therefore no relief.

At full Moon even a low telescopic power will show that in addition to the extensive dark areas (mare) the surface is peppered with numerous craters many of which now appear as bright spots.

It soon becomes apparent that not all craters reflect sunlight to the same extent. Naturally contrast with the surrounding area has to be considered.

The many bright crater in the region of Delambre (Fig. 1) would appear even brighter against the background of the darker mare.

The craters also stand out from their surroundings to a varying degree depending upon the Moon's phase. It is instructive to note the appearance of craters throughout the Lunar month.

Proclus in the west, close to Mare Crisium, and Aristarchus in the east on the edge of Oceanus, both exhibit bright features including rays. These craters appear bright at times other than full Moon (Figs 2 & 3). The case of Proclus is particularly interesting (see Fig. 4).

Other bright craters include Goldschmidt in the north and the impressive Tycho in the south. The large crater Copernicus appears exceptionally bright when the Moon is close to full (Fig.4).

I am frequently asked for ideas on how best to remember the names of astronomical objects. There is no easy answer.

I guess we all tend to recall the names of things (and people!) which most impress us. In my own case I came to identify several hundred stars by name and catalogue number simply through repeated observation when working on the meridian circle.

The technique of association is also useful. Thus, the 110 Messier objects might be compared to the fully authenticated 108 symphonies of Franz Joseph Haydn.

The case of M102 is intriguing in this regard. M102 is in fact identical to M101. In Haydn's case the Symphony No. 96 is sometimes referred to as The Miracle.

However the event at the Hanover Square Rooms concert, in which a chandelier fell into the audience without harming anybody, in fact took place at a performance of Symphony No. 102 in 1795, according to the newspapers of the day. (Symphony No. 101, incidentally, is known as The Clock - an important piece of astronomical equipment!) I have no real favourites in Haydn, a composer I have studied with increasing amazement and delight all my life. But Symphony No. 42 (without a name) is a miracle in its own right and not at all diffuse like Messier 42.

JV 15/05/06