creeps towards the apparent meridian. Shadows now begin to grow eastward as the incoming sunlight favours the western hemisphere.
Last Quarter: 22.2 days. Only the western hemisphere of the Moon appears to be illuminated. As the Moon is now leading the Sun, it rises in the early morning. The brilliant crater Aristarchus is unmistakable in its dark surroundings near the western limb.
Waning Crescent: 22.2–29.53 days. The terminator moves westward towards the western limb (east in the sky) as the apparent angle between the Moon and Sun decreases. Eventually the Moon appears to become a very thin crescent impossible to see in the Sun’s glare. As it rises shortly before sunrise, the waning crescent is the least observed lunar phase.
New Moon: 29.52/0 days. At the moment of New Moon, a new lunar synodic month begins. The New Moon is the night side of the Moon, and too dark to see through the glare of the Sun. New Moon is only directly visible during a solar eclipse.
The location and time of moonrise and moonset depends on your latitude. For the British Isles, detailed information about the lunar calendar is available from the UK Hydrographical Office, a department of Her Majesty’s Nautical Almanac Office: http://astro.ukho.gov.uk.
Moon phases as seen from the Earth and space.
More general information can be retrieved from the US Naval Observatory’s Astronomical Applications Department: http://aa.usno.navy.mil.
Almost every feature on the surface of the Moon visible from the Earth has been given a name. These names were originally assigned on a somewhat informal basis by early telescopic observers to honour great philosophers and artists of antiquity. As larger telescopes have revealed smaller features, names have been assigned more formally, and the process of cataloguing features is now governed by the International Astronomical Union (IAU.) Craters are named for notable deceased scientists, mathematicians, artists, scholars and explorers. The lunar maria (seas) have older Latin names, which reflect abstract states of mind and weather phenomena considered important to mariners. Lunar montes (mountain ranges) are typically named after terrestrial mountain ranges or nearby craters. Other features, with few exceptions, are named after nearby craters, maria or montes.
Features on the Moon fall into one of several categories. Here each type of feature is described with a typical example shown as viewed through a large telescope.
Craters – Fairly circular depressions usually formed from impacts. Occasionally, chains of craters are grouped together and collectively termed catena. Craters feature sloped walls and, on many occasions, central peaks, left over from the crater formation, at which point the lunar surface was locally molten by the energy of the impactor.
Mare (plural: Maria) – Latin for ‘sea’. Large basins of solidified, ancient lava. The maria appear dark relative to the other terrain features. There is one large lunar ‘ocean’ in the Moon’s western hemisphere known as Oceanus Procellarum (Ocean of Storms).
Mons – An individual mountain on the Moon. Lunar mountains were formed by a variety of processes, and vary greatly in size and height. The tallest are approaching 5 km in height, comparable to Vinson Massif, the highest peak in Antarctica.
Montes – Large chains of mountain ranges formed by gigantic asteroid impacts billions of years ago. As with terrestrial ranges, prominent individual peaks often have their own names.
Vallis (Plural: Valles) – A valley or system of valleys formed by lava flows and collapsed lava tubes. These features snake across the surface, often near craters connected to volcanism.
Dorsum (Plural: Dorsa) – Derived from the Latin for the ‘back’, and connected to ridges or fins on the back of an animal, dorsa are subtle features resembling wrinkles in lunar maria. They are hard to see unless illuminated at a low angle, causing them to cast shadows.
Rima (Plural: Rimae) – Latin for ‘fissure’. Rimae, sometimes called Rilles (German for ‘grooves’) are fissures or cracks in the lunar surface. Not to be confused with valleys, they are often jagged in appearance, with straight sections permeated by kinks. They are seismological features in the Moon’s crust, and are sometimes found in crater floors.
Rupis (Plural: Rupes) – Latin for ‘rock’, rupes are escarpments in the lunar surface. They appear as large rifts, where a pronounced change in elevation can be seen. In fact, most rupes are very gentle slopes that are many kilometres wide.
Lacus – Derived from the Latin for an opening, a lacus is a lake, which as its name suggests is a very small lunar mare. These features appear as dark, often patchy regions of dark, smooth plains.
Sinus – Derived from the Latin for a gulf, a sinus on the Moon is a bay formed by a rugged ‘coastline’ of lunar highlands meeting a low elevation, smooth plain such as mare.
Palus (Plural: Paludes) – Though its original Latin name is closer in meaning to a pool, paludes are generally translated as marches. They are low lying, but relatively rugged regions. Whilst they are not as dark as mare, they do have a relatively low albedo when compared with other types of rugged terrain.
Images of the Apollo landing sites captured by NASA’s Lunar Reconnaissance Orbiter from low altitude. The remains of the Lunar Modules and various scientific instrument packages can be seen, as well as tracks left by the astronauts and rovers.
North-East
Due to atmospheric seeing, we are typically limited to observing features no smaller than one arcsecond in apparent size. On the Moon, this corresponds to
