The convention of compass directions on the Moon has changed and this causes some confusion, especially as some commentators still use the old convention. On this web site I try to use the modern convention throughout. The north pole of the Moon is the pole which is in the same general direction as the north pole on the Earth, so as seen from the northern hemisphere the north of the Moon is at the top. This has always been the convention. Until recent years, however, east on the Moon was taken as in the direction of east on the Earth Looking at the Moon from the northern hemisphere, one is looking south so this put the east on the left and the west on the right. This meant that the Moon rotated east to west even though it rotates the same way that the Earth rotates which is west to east. The convention now is that all astronomical bodies rotate west to east so that now the left side of the Moon (as seen from the northern hemisphere, of course) is west. This gives rise to the slight anomaly that the Mare Orientale (the Eastern Sea) is now in the west but it is not worth changing its name.
Another older convention was that pictures and maps of the Moon were shown with south at the top, because this is how it appears in astronomical telescopes from the northern hemisphere and this made it much easier to compare the view in the eyepiece with the map. The current convention, however, is to present pictures with north at the top in conformity with normal practice on the Earth. Many modern telescopes include diagonal mirrors which either correct the up-down inversion but leave the left-right inversion unaffected, or vice versa (or even something in between), so the situation is no longer that simple.
Unless otherwise stated, all my pictures have north at the top, or roughly so. The polar axis of the Moon is not parallel to that of the Earth, and my camera is aligned with the polar axis of the Earth, so my pictures are usually not accurately aligned north-south.
There are five basic types of feature on the lunar surface. These are Maria, Craters, Mountains, Rilles and Domes.
Anyone who has looked at the Moon with the naked eye must have noticed that it has dark areas and bright areas. In the days before telescopes it was believed that the dark areas were seas and they still retain the name "Mare" (plural "Maria") which is the Latin word for "Sea". The astronomers of those days gave them rather fanciful names, such as the "Sea of Tranquillity", and these names, in their Latin forms, are still used today even though it is now recognised that they are not, and never were, seas of liquid water. Seen through the telescope, these areas are found to be relatively flat compared with the bright areas which are rugged and mountainous. It is believed that these areas were formed by massive impacts either at times when the interior was molten or that the impact itself melted the underlying rock so that lava welled up and flooded the area. It is strange that the far side of the Moon, which has only become known since the development of space probes, is devoid of Maria. Maybe after the Moon's rotation became locked so that it always pointed the same face towards the Earth, the Earth itself shielded the Moon from objects which would otherwise have impacted the near side, so that the far side has had many more impacts which have obliterated any maria that were there originally.
Look at the Moon with any telescope, especially when it is not full, and one will immediately see that the bright areas are composed of roughly circular features which cast shadows across the landscape. These are the craters. Their origin was for many years a matter of great debate. Some authorities believed them to be volcanic craters, others believed they were the result of the impact of objects falling onto the Moon from space. Modern belief is that the majority of the craters, especially the big ones, were formed by impacts, but that there certainly are craters that were likely formed by some internal mechanism such as volcanism. Most impact craters have a central mountain, except those that appear to have been flooded with lava after their formation, which have relatively flat floors and one must assume that the central mountain was drowned in the lava. It is generally true that the central mountain is not as high as the outer ramparts of the crater. Studies of impact cratering indicate that craters less than about 10 Km in diameter would not be expected to have central mountains. The impacting missile would be too small to produce enough compression of the target rocks to produce significant rebound to form a central mountain.
Some craters, particularly Tycho in the southern highlands, are associated with bright radiating streaks, called "Rays". These rays appear to be composed of numerous small, bright craters and blocks of light-coloured rock. They only show up well around the time of Full Moon.
Mountains abound on the Moon. As on Earth they are often in chains, and on the Moon these are frequently named after mountain chains on Earth. Some isolated mountains have specific names, but mostly they are named from a nearby crater followed by a Greek letter. My main sources, Hatfield and the VMA (see below) do not list the latter names so they are also missing from my pictures.
Rilles are narrow channels that criss-cross many areas on the Moon. They may be several kilometres wide, but can be several hundred kilometres long. They are of two types: (1) Linear Rilles and (2) Sinuous Rilles. The linear rilles (not strictly straight, indeed often arc shaped) are found particularly around the edges of the maria. They are probably fault lines not unlike the tectonic faults on Earth. The sinuous rilles, on the other hand, are twisted and meander in a way that looks like a river bed. They certainly weren't formed by water and yet they look so like it. The most likely explanation is that they are lava tubes like those found on Earth. Lava runs down hill much like water; the top solidifies but the hot lava continues to flow beneath. Eventually the lava drains away and leaves a tube, the top of which collapses and forms a channel.
Domes are what the name implies, large humps in otherwise flat ground. They are difficult to observe as they only produce shadows when the Sun is very low in the lunar sky. They are probably volcanic in origin, being swellings pushed up by forces below but they have not fractured and let lava out.
Many features on the Moon have names. To be official, the names must be ratified by the International Astronomical Union (IAU). The larger craters have names that go back to the invention of the telescope; smaller craters are given letters following the name of a nearby large crater. If only the letters are
shown on a map, it can be difficult to know to which main crater they relate. Usually the nearest, but not always. Occasionally on my maps, if I think there is some doubt and if I can determine the truth, then I
have indicated the linkage with a dotted line or, if there is room, the full name of the craterlet. As source I have used Henry Hatfield's Amateur Astronomer's Photographic Lunar Atlas (Lutterworth Press) 1968 edition,
and the Virtual Moon Atlas (VMA), a superb and free
digital atlas. If there is a discrepancy between the two which I have been unable to resolve, then I have taken the Hatfield version*. Dimensions
come from the VMA, but have to be treated with some caution. I suspect that the ultimate source of the data is American and in the old Imperial units which are, I believe, only used today in the USA; the conversion to metric is not entirely accurate. For example all craters, whether circular or not, are shown as circular in metric units, as are the rilles. As long as this is borne in mind, the VMA is a fantastic source of information. The VMA also has a tool for measuring distances on the Moon which takes into account the perspective of a spherical Moon seen on a flat plane. (What we see is what geographers would call an orthographic projection.) I have used this tool to put scale markings on my more recent pictures. These must be taken as approximate only and will often be different in different directions so I have indicated a scale north-south and east-west. For mosaics and small-scale images, the scale will vary in different parts of the picture.
Other information about the Moon comes from a variety of sources but one I rely on extensively is The Modern Moon by Charles A. Wood (Sky Publishing 2003).
* In December 2005 I obtained a copy of Rükl's Atlas of the Moon so, as from 1st January 2006, I have been using this, more modern, reference in place of Hatfield's atlas, but the latter remains a useful reference because it's pictures are photographs.
Home Back to Larger-scale Images