Popular Astronomy January 1997
"Enjoy those good librations"
Today's Moon appears virtually indistinguishable from that strange bright disc once scrutinised by the keen eyes of our distant hominid ancestors. Since long before humans evolved, the Moon has kept the very same sphinx-like face turned in the Earth's direction as it has orbited its sister planet.
In one orbit around the Earth, every 29 days, 12 hours and 44 minutes, the Moon turns once on its own axis. This is known as synchronous rotation, a phenomenon displayed by various other of the solar system's planetary satellites.
But the terrestrial observer is able to chart a little more than half of the Moon's surface. An apparent rocking motion of the Moon's globe called libration (Latin librare, to balance), means that from the Earth a total of 59 percent of the Moon can theoretically be observed over a period of time. The remaining 41 percent constitutes the permanent lunar far-side and can never be seen from the Earth. The process of libration is too slow to be noticeable in real time, but is evident when the Moon is inspected over days.
Libration has two causes - optical and physical. Optical libration results from the Moon's ever-changing presentation to the terrestrial observer, combined with the position from which the Moon is viewed. Physical libration is an actual wobble of the Moon around its own centre of gravity - a real, though exceedingly small oscillation rather than an apparent one.
In fact, the Moon's physical libration is so tiny that it need not bother us in this discussion, for it never amounts to an angular displacement of under a second of arc at the centre of the Moon's disc. Because their magnitude is so tiny, physical librations do not have any appreciable impact upon the observability of lunar features. Though Newton predicted this phenomenon in 1686, it had to wait over 150 years to be first detected by Friedrich Bessel who secured accurate astrometric measurements with a special telescope (which he himself designed) called a helimeter.
On the other hand, optical libration - a term first applied by Isaac Newton - brings about obvious effects on the apparent position of features in the libration zones around the edge of the lunar disc, and there are three kinds - libration in longitude, libration in latitude and diurnal libration.
Libration in longitude (affecting the "sides" of the Moon) was described by Galileo in 1637. It is produced because the Moon's angular velocity varies as it orbits the Earth in an ellipse, accelerating on its way towards perigee (closest point in its orbit to the Earth) and slowing down on its outward leg to apogee (furthest point from Earth), whilst at all times maintaining a constant rate of rotation upon its own axis.
Take the Moon as it completes a "fast" quarter section of its orbit after perigee; it will have covered this distance in less time than it took to make a quarter of a turn on its own axis. The Moon's axial rotation is at this point lagging behind, so to speak, and the mean centre of the Moon's disc appears displaced towards the west limb; far-side features on the east limb will have been brought onto the Earth-facing hemisphere, and if the illumination is right their favourable aspect will be viewed by telescopic observers eager to take advantage of this presentation.
Half an orbit later, when the Moon is moving away from apogee in a slower section of its orbit, the opposite situation occurs. The Moon has made a quarter of a turn on its axis before it has made a quarter of an orbit around the Earth; the apparent displacement of the Moon's surface features will be towards the east, and features which are normally on the far-side make their appearance on the Moon's western limb. Twice every anomalistic month libration in longitude creates an east-west displacement of + 7°54'.
The Moon's "top and bottom" areas are brought into view by libration in latitude because the Moon's axis of rotation is not perpendicular to its orbital plane around the Earth. The lunar poles are tilted slightly and remain fixed in one direction in space, a fact which enables the observer each month to peer a little over the mean northern limb, and a fortnight later to view under the mean southern limb of the Moon. Libration in latitude was described by Hevelius in his "Selenographia" (1647). The phenomenon displaces the mean centre of the Moon north-south by + 6°51'.
On top of these two kinds of optical libration, the observer's view of the Moon changes because the Earth is revolving, carrying the observer around with it. The extent of this so-called diurnal libration depends on the observer's terrestrial location and is restricted by the Earth's apparent diameter of some 2° as seen from the Moon, but it can appear something like the parallax effect produced by an alternate left and right eye view of a tennis ball placed ten metres away - just under a degree in lunar longitude.
The Moon's libration zones lie around the edge of the lunar disc on either side of longitude 90°E and 90°W; over time all the features within these zones (amounting to 18 percent of the surface of the near-side) eventually make an appearance on the lunar limb. Whenever the Moon is observed, a narrow crescent of the Moon's far-side (ie., that which is further than the 90°E or 90°W line) is at all times being presented to the observer to some extent, though this region is not always visible because it might be on the limb of the unilluminated hemisphere. Strange to think that the "mean" Moon - one which has the 90° longitude line positioned exactly on the visible limb, as depicted on most lunar charts and atlases - has probably never been observed!
Lunar observers take optical librations into account because they determine the practical visibility of the Moon's limb features to a great extent; foreshortening effects can favour or disfavour useful observation. For example, it is no good planning in advance to observe Mare Orientale, which lies largely past the 90°W line near the Moon's southwestern limb, without knowing whether libration will have brought it onto the Moon's near-side.
Similarly, it is best to plan to observe near-side limb features when they are presented favourably. There isn't much point, for example, in attempting to observe the floor of the walled plain Gauss, near the northeastern near-side limb, when there is a strong libration favouring the southwest.
Features closer to the centre of the lunar disc are affected not so much by alteration in apparent shape, but by their proximity to the Moon's terminator, hence their individual times of emergence from and immersion into the lunar night. Numerous astronomical almanacs (including the BAA Handbook) contain tables for the calculation of libration, and some astronomical programmes available for the PC are capable of displaying it graphically. Popular Astronomy's Sky Diary features a list of near-limb objects favoured by libration.
Mare Orientale
A splendid lunar feature named Mare Orientale (the Eastern Sea) is perhaps the most famous of the Moon's libration features. At a favourable libration the formation may be discerned in binoculars, making a recognisable "dent" on the edge of the Moon.
Orientale lies on the "near-far-side", as it is situated beyond the mean western lunar limb (the whole of the 300 km diameter sea being past 90° west) and therefore a good libration is needed to bring it sensibly onto the visible hemisphere.
Orientale's discovery is generally credited to Percy Wilkins and his friend Patrick Moore nearly half a century ago. This comparatively recent find illustrates how infrequent the conditions are for the sea to be discerned well, for it had gone unrecognised (though not unrecorded) as a major lunar feature for nearly 3½ centuries of intensive telescopic lunar observation.
The Orientale basin consists of several fractured mountain rings, and it was formed 3.8 thousand million years ago by a huge asteroidal impact. It is the youngest of the Moon's maria. Unlike the near-side basins, which have all been filled with lava, the Orientale basin is only partly flooded. Mare Orientale itself is only 300 km in diameter. The basin's outer regions are stained by lava flows, and of these Lacus Veris and Lacus Autumni (Lakes of Spring and Autumn) are situated just on the near-side and may be seen whilst Mare Orientale is not on the disc. The true majesty of the whole formation only came to light when spaceprobes photographed the area.
Orientale had undoubtedly been seen many times before, and is even captured on a plate in Pickering's 1904 lunar photographic atlas. Wilkins and Moore were first to realise that the feature was a very substantial one, and they named the feature Mare Orientale (Latin for Eastern Sea), nomenclature which was correct at the time since the system of lunar co-ordinates then in use meant that all features on the left hand side of the Moon were deemed to be in the east. The old classical system had lunar east and west pointing towards east and west on the celestial sphere, thus favouring telescopic observers. In 1961 the International Astronomical Union (IAU) chose to reverse east and west on the Moon to favour "astronautical" convention where an observer on the Moon would see the sun rise in the east and set in the west, as it does on Earth.
Seas on the Moon's Eastern Limb
Mare Humboldtianum, on the northeastern limb, is a small crescent-shaped sea some 300 km across, situated wholly on the near-side from 90°E to 70°E longitude. Humboldtianum occupies the central part of a larger impact basin 650 km in diameter which extends onto the far-side. Like all near-limb features its visibility is affected by libration. At a good libration the whole of the mare and the mountains beyond can be discerned.
East of Mare Crisium lie two small irregular-shaped seas, Mare Marginis and Mare Smythii, both of which extend onto the far-side. Mare Smythii is one of only six lunar maria known to possess a central mascon - a concentration of mass beneath the surface whose gravitational pull is enough to make Moon-orbiting spacecraft deviate slightly from their orbit.
Mare Australe at the southeast limb is a circular sea around 1,000 km in diameter which is composed of a network of flooded dark-floored craters and plains. It is probably a very ancient impact basin which experienced vulcanism after subsequent meteoritic bombardment breached the pre-fractured crust in the basin's interior. Australe sits more-or-less squarely on the 90°E line, half on the near-side and half on the far-side.
Observing near-limb and true far-side lunar features is a challenge which appeals to those interested in taking their astronomical studies into places rarely viewed by human eyes. Even if the chances of new discoveries are very unlikely (now that intensive Moon-mapping has been completed by spaceprobes) those who make this area of research their speciality will gain a great insight into the near-limb topgraphy and librational behaviour of our satellite.
