SPA Lunar Section

Some Notes About Intensity Estimations

Lunar craters change markedly in appearance during the course of the lunar cycle (New Moon to Full Moon and back to New Moon). The greatest detail can be seen just after a crater emerges from the terminator (or again two weeks later just before the terminator crosses back over that crater) because of the long shadows cast by all relief features. It is around this time that a drawing of a particular crater is usually made.

On the other hand, the "look" of a crater around a week after its first appearance is utterly different. Now there are no shadows to be seen at all, and no relief detail is directly visible. Instead, what you see are genuine brightness variations of the Moon's surface, indicating how good various parts of the lunar rock and soil are at reflecting the sun's light. A drawing of a crater made under these conditions is a useful complement to one made when the crater is full of shadow near the terminator.

The intensity estimations technique is simply to sketch the boundaries of areas of different brightness, and in addition to estimate those brightnesses on a scale of 0 to 10. In Elger's book The Moon, the author quotes a scale of intensities adopted by the lunar observer Schröter. Although it is a very old system, it cannot be much improved upon, and this is the scale which is used by SPA Lunar Section members. The examples of tones described below are for a general low-power telescopic view (x40). Each area will, of course, break down into further gradations of tone under higher powered scrutiny.

0 Black -
for the darkest of lunar shadows

1 Very dark greyish black -
dark features under extremely shallow illumination

2 Dark grey -
the southern half of the floor of Grimaldi

3 Medium grey -
the northern half of the floor of Grimaldi

4 Yellow grey (subtle) -
general tone of area west of Proclus

5 Pure light grey -
general tone of floor of Archimedes

6 Light whitish grey -
the ray system of Copernicus

7 Greyish white -
the ray system of Kepler

8 Pure white -
the southern floor of Copernicus

9 Glittering white -
the rim of Tycho

10 Brilliant white -
the bright central peak of Aristarchus

These tints are obviously a little hard to picture, but you'll get used to them with practice. The method is a little like that used by planetary observers, and although intensity estimates of this type depend heavily on the individual observer, excellent results can be obtained if the observations of a large number of people are combined.

The following craters should be included in the observing programme of anyone hunting for TLP (transient lunar phenomena) which often cause colour and intensity variations.

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 Maintained by Alan Mayer Email: A.D.Mayer@Swansea.ac.uk
Last modified 27 Nov 1996