Astronomy Now July 1997
The Moon is an object of beauty, and free to observe!
A darkening twilight sky adorned with an earthshine-tinted crescent Moon is one of the most beautiful sights in the heavens. Add a sprinkling of bright stars and planets, and the inspiration behind Beethoven’s "Moonlight Sonata" becomes palpable. Such celestial inspiration is absolutely free of charge; moreover, the careful naked eye observer can learn a great deal about our natural satellite into the bargain, providing one’s vision isn’t seriously impaired.
Earthshine
Earthshine is the faint illumination of the Moon’s dark side, caused by sunlight being reflected onto the Moon by the Earth. The phenomenon is most obvious when the Moon is a thin waxing crescent in a reasonably dark sky - a sight sometimes referred to as "the old Moon cradled in the young Moon’s arms". Viewed from the Moon’s surface at this time the Earth would appear as a bright waning gibbous sphere lighting the lunar landscape with as much as sixty times the brilliance of full Moon.
With the naked eye the Earthshine may be followed for several days until around first quarter phase and picked-up again in the last quarter of the lunar month. The brightness of the Earthshine actually varies according to Earth’s phase, geography and global weather conditions - continents are more reflective than oceans, and a cloudy globe is highly reflective. Anyone who regularly observes the Earthshine will soon come to notice that there are times when the phenomenon seems especially prominent.
Moonlight
When our eyes are dazzled by the full Moon riding high in the sky, it is difficult to acknowledge the fact that the Moon is really a very dark object. In fact, it is one of the least reflective worlds in the entire solar system, with an average albedo (reflectivity) of just 0.07 - in other words, only 7 out of every 100 photons which hit the lunar surface manage to bounce back into space. The actual intensity of moonlight is only a quarter that of a burning candle placed a metre away (0.25 lux). If the Moon were a smooth sphere then its average albedo would be raised a little. But its surface is rough and irregular, and largely because of the shadows thrown up by these features towards the terminator, the brightness of the half-Moon of first or last quarter phase is not one half of the full Moon’s value as suggested by its area, but just one ninth.
Colour perception in moonlight
As bright as the full Moon appears, sunlight is half a million times brighter. A terrestrial landscape bathed in moonlight has a ghostly monochrome appearance because the scene is not bright enough to trigger all of the colour receptors in the human eye. To test this, prepare ten pieces of different coloured paper - white, grey, pink, light brown, pale green, lilac, sky blue, bright red, bright green, and yellow - and label them on the back. On a clear night, select a dark spot outside which is illuminated only by the light of the Moon and (after shuffling the paper and without looking at their labels!) attempt to identify each colour. The red one may turn out to be the only one which can be identified with certainty, and only then if the moonlight is bright enough.Those living near sodium street lighting will be able to perform another experiment, this time illustrating a psychological aspect of human colour perception. When the Moon is high and near full, stand close to an orange street-lamp and face the Moon. Observe the colour of your shadow being cast on the ground by the lamp. It will seem indisputably blue, though in reality your shadow is being faintly illuminated with moonlight which is white, not blue.
This odd effect comes about because our human brains are "wired-up" to acknowledge that the brightest source of illumination is white light - in nature, the Sun and the Moon. We therefore perceive our sodium orange surroundings as being illuminated with white light, and the blueness of the shadow is simply caused by colour contrast perception - an effect obvious in double stars like Albireo (Beta Cygni) whose brighter yellow component much enhances the blueness of its companion.
Atmospheric effects
Lunar rainbows form under the same circumstances as those produced by the Sun - a coloured arc of 42° radius situated diametrically opposite the light source. Lunar rainbows, however, have a maximum brilliance of a mere 1/500,000 of their day-time counterparts and as such they are rare, dim and somewhat hypochromic.The white corona often seen in the Moon’s immediate vicinity is caused by moonlight’s reflection and diffraction amid water droplets in the lower clouds. The corona is sometimes encircled by one or two (on rare occasions even three) rainbow-coloured lunar haloes caused when moonlight is diffracted by countless water droplets or ice crystals in the upper atmosphere.
Lunar haloes can be particularly vivid when moonlight passes through a thin homogeneous cloud layer composed of very minute water droplets. Sometimes the lunar halo may be host to "mock Moons" or "Moon dogs" (more properly, parselene) which are diffuse images of the Moon located 22° either side of it, caused by the light’s refraction by ice crystals in the upper atmosphere.
The Moon illusion
When the full or near-full Moon is seen close to the horizon it often appears to assume unexpectedly large proportions; "as big as a plate" is a simile commonly used to describe this stunning effect. In reality the Moon’s apparent diameter becomes increasingly smaller as it approaches the horizon because the observer, situated on a revolving planet, is being transported away from the Moon. Whilst the phenomenon of apparent shrinkage is far too subtle to be detected with the naked eye, observation with a simple naked eye measuring device called a cross-staff will prove to anyone’s satisfaction that the Moon remains more-or-less the same size whether it is riding high in the sky or hovering over the distant horizon
Lunar cross-staff
Cross-staffs and assorted naked eye devices were used extensively in the pre-telescopic era. Their last greatest exponent was the famous 16th century Danish astronomer Tycho Brahe who made many accurate measurements of the positions of stars and planets. His observations of the movements of Mars in particular helped Johann Kepler to formulate his important laws of planetary motion.
Constructing a lunar cross-staff is simplicity itself. To a thin straight staff of wood about a metre in length, attach a small slider which can be moved up and down the main staff. The slider should be easy to move, though not so loose as to slide down the staff of its own accord when pointed upwards. At the leading edge of the slider fasten two straight pins or small nails side by side and 7 mm apart. Along the upper half of the staff draw a line 50 cm long marked at 1cm intervals. At the other end of the staff fasten a small squint hole - a used cotton reel or piece of card with a small hole punched out will do. The instrument is now ready to be turned to the Moon.
A lunar cross-staff. AN graphic by Mike Inglis.
Once sighted through the squint hole the Moon is aligned between the pins on the slider. If the Moon is not quite full, turn the staff slightly so that the widest illuminated breadth of the Moon is lined-up between the pins. The slider’s position on the 50 cm scale you have drawn is then noted.
It will be found that the Moon’s apparent diameter always remains around half a degree, regardless of its location in the heavens. Nevertheless, the Moon illusion is powerful. It occurs mainly because we perceive the sky as being shaped like the interior of a flattened dome - an illusion reinforced by the perspective effect of clouds. Celestial objects appear attached to the dome’s interior. When the near-horizon Moon is viewed we imagine that it is far away and must therefore be a large object to subtend that angle. When the Moon is above us we subconsciously imagine it to be closer to us and smaller in size. Exactly the same illusion occurs in our perception of the size of constellations. Castor and Pollux, for example, give the impression of being more widely separated when they are near the horizon.
Distance and diameter
Throughout the month the Moon’s distance ranges from 356,400 km when nearest the Earth (perigee) to 406,700 km when furthest away (apogee). This difference of 50,300 km means that the Moon’s apparent diameter at perigee is 33’ 29", compared with 29’ 23" at apogee. The perigee Moon presents an area larger by nearly 30 percent than at apogee.
Such a substantial size variation, even if the object is only around half a degree wide, can be detected with the lunar cross-staff - remembering always to line the edges of the pins with the widest breadth of the Moon’s disc when gibbous, or with the tips of the crescent if only narrowly illuminated. If the Moon’s diameter is measured in this manner for several months then it becomes clear that the period from one perigee to the next - called the anomalistic month - is shorter than the synodic month (new Moon to new Moon) by a couple of days.
The Moon's features
To the unaided eye the illuminated face of the Moon appears as a patchwork of dark and light areas. If the Moon is observed near full when it is close to the horizon, veiled by thin cloud or even wearing sun-glasses, a surprising amount of detail may be discerned. The Moon’s seas cover a third of the near-side and appear as dark patches with the naked eye. The brighter areas which surround them are mountainous cratered regions and the light coloured ray systems of a few impact craters.
Libration
With the naked eye it is possible to monitor some effects of the Moon’s libration, which is an apparent slow rocking motion of the lunar globe. This phenomenon becomes apparent when monitoring the position of Mare Frigoris in the north and the presentation of Mare Crisium near the Moon’s east limb over a lunation. Those with average eyesight will find that both features, when closest to the lunar limb, are something of a challenge.
Examples of the effect of libration. ABOVE LEFT The full Moon showing the maria on the eastern limb. Mare Crisium is prominent, while ABOVE RIGHT, the Mare Orientale is visible on the western limb, and mare Crisium is "squashed". Both drawings by the author.
The man in the Moon
With a little imagination the naked eye observer can see various "faces" and "creatures" on the lunar disc. The most famous of these is the Man-in-the-Moon - familiar to human eyes since ancient times - whose eyes are Maria Imbrium and Serenitatis, with a gaping mouth formed by Mare Nubium. This character always appears (to me anyhow) to have a look of complete astonishment, with his eyes and mouth set wide open!
There are many more examples of this type, though most aren’t quite as apparent. An alternative face, nicknamed Cyrano for obvious reasons, is best seen at first quarter phase. He has eyes of Maria Serenitatis and Crisium, Mare Nectaris makes his small mouth and a his very prominent nose is formed out of Mare Fecunditatis.
After identifying Cyrano, the naked eye observer can then attempt to make out the leaping hare, an example of lunar simulacra as old as the Man-in-the-Moon. The creature is mentioned in the myths of numerous cultures, and one ancient Sanskrit word for the Moon can be translated as the “hare bearer”. The hare’s anatomy is configured thus:- ears - Mare Crisium; head - Mare Fecunditatis; front paws - Mare Nectaris; thorax - Mare Tranquillitatis; haunches - Mare Serenitatis; hind legs - Mare Vaporum; small fluffy tail - Lacus Somniorum. The crater Langrenus forms the hare’s gleaming right eye, though even those with superb visual acuity will find it difficult to discern with the naked eye alone.
The full Moon as seen with the naked eye, drawn by Peter Grego
From the very same maria can be constructed a French poodle sitting obediently at heel. As a guide, imagine Maria Serenitatis and Vaporum as its head and muzzle, with Mare Crisium its tail; the rest of the well-groomed dog should then fall into place.At full Moon a giant crab extends its pincers right across the Moon’s disc. These fearsome appendages are formed out of both the eastern and western maria, attached to the main shell which is Mare Imbrium.
One of the most difficult images to identify at full Moon is the "Kiss in the Moon"; the faces of this amorous couple, seen in profile, are located in the centre of the Moon’s disc. The mouth and eyes of the figure on the right are Sinus Medii and Mare Vaporum respectively, whilst the outline of the other’s hair may be traced from the top of Mare Imbrium all the way around the western edge of Oceanus Procellarum.
Pickering's list
The American astronomer W H Pickering gave a list of lunar features which may be used as a test of visual acuity. Starting with the easiest, becoming progressively more difficult, they are:- 1 Bright region around Copernicus; 2 Mare Nectaris; 3 Mare Humorum; 4 Bright region around Kepler; 5 Gassendi region; 6 Plinius region; 7 Mare Vaporum; 8 Lubiniezky region; 9 Sinus Medii; 10 Faintly shaded area near Sacrobosco; 11 Dark spot at foot of Mt Huyghens; 12 Riphean Mountains
Those with good eyesight will certainly be able to discern Mare Vaporum with little trouble. However, it is unclear what is meant by the supposedly easier Plinius region, since Plinius itself can only be seen with optical aid, there being nothing else of particular prominence here. Perhaps what is referred to is the bright gap in-between Maria Tranquillitatis and Vaporum, actually some distance southwest of Plinius. Last on the list, the Riphean Mountains represent a challenge for those with superb eyesight since the light grey cluster of peaks subtends an apparent angle of just 2 arcminutes.
Montes Jura at sunrise. The irregularity on the Moon's terminator can be detected with the naked eye. Drawing by Peter Grego.
Irregularities along the Moon’s terminator can sometimes be seen with the naked eye, giving us good reason to wonder why some scholars once considered the Moon to be a perfectly smooth sphere. The most noticeable of such irregularities becomes apparent when Sinus Iridum has just emerged from into the morning sunshine (Moon around 10 days old) and the brilliant curve of the bordering Montes Jura projects into the darkness beyond. The big craters Ptolemaeus and Clavius also make dents on the terminator each fortnight, though better-than-average eyesight is needed to see them.
