Moongazing: Beginner’s guide to exploring the Moon

Waxing Crescent: 0–7.4 days. The Moon’s eastern limb emerges (east on the Moon is west in the sky). The crescent can first be sighted after sunset, once the Moon and Sun are separated by approximately seven degrees, centre to centre. Only one or two per cent of the Moon as seen from Earth appears to be illuminated at this time, as we are mostly viewing the night side of the Moon. The Waxing Crescent follows the Sun to the western horizon.

An illustration of the Moon’s inclined orbit. Eclipses will occur when the Full or New Moon crosses the ecliptic at the lunar nodes.

First Quarter: 7.4 days. Exactly half of the Moon’s near side appears to be illuminated. The terminator, where day meets night, runs vertically along its apparent meridian. Shadows are cast from the east to the west on the Moon at this time.

Waxing Gibbous: 7.4–14.8 days. When illumination is greater than 50 per cent, but less than 100 per cent, the Moon’s phase is said to be gibbous. Its western hemisphere is gradually revealed as the Sun begins to rise there.

Full Moon: 14.8 days. At 100 per cent phase illumination, the day side of the Moon is directly pointed at the night side of the Earth and the Moon is full. As the terminator is not visible, there is very little sense of relief on the lunar surface. Full Moon is the only time at which lunar eclipses can occur, when the Moon passes through the shadow of the Earth. The Moon must be close to its ascending or descending node for this to happen.

Waning Gibbous: 14.8–22.2 days. The Moon’s terminator emerges on the eastern limb and 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 (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 (http://aa.usno.navy.mil/).

Surface Features (#ulink_fd79fc1c-3749-53ed-a133-92fed46768b3)

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 just over a mile at best. This means that very small objects, such as those left on the Moon by the Apollo astronauts, cannot be resolved through the Earth’s atmosphere, but they have been imaged using the Lunar Reconnaissance Orbiter, which was only a few kilometres above the lunar surface.

It’s useful to memorise prominent features on the Moon and consider its near side being divided into four quadrants as shown here. A good knowledge of the locations of the largest lunar maria will help you when choosing areas to explore during your moongazing sessions.

Readers in the southern hemisphere should note that due to the convention of the coordinates on the Moon, it appears upside down in the sky compared with the charts in this guide. From Sydney, the Moon’s North Pole appears to be at the bottom. East and west are also reversed. From the northern hemisphere, the Moon’s eastern limb is on the right; from the southern hemisphere, it is on the left.

South-East

North-East

The north-eastern quadrant of the Moon is dominated by the dark Mare Serenitatis (Sea of Serenity), landing site of Apollo 17, and Mare Tranquillitatis (Sea of Tranquility) where Neil Armstrong and Buzz Aldrin became the first humans to step foot on the lunar surface during the Apollo 11 mission. Mare Crisium (Sea of Crises) lies near the eastern limb, and south of it Mare Fecunditatis (Sea of Fertility).

South-East

The south-eastern quadrant of the Moon is remarkable for having no maria, save for the southern region of Mare Nectaris (Sea of Nectar). It’s mostly composed of rugged highlands, pockmarked with high energy impacts ranging right across the history of the Moon. The prominent Tycho Crater is found in this region, with its bright rays of ejecta visible over 1,600 km (1,000 miles) from the impact site. Astronauts on the Apollo 16 mission collected material from these rays to be analysed on Earth.

South-West

South-West

The south-western quadrant of the Moon features several small maria and a portion of the giant Oceanus Procellarum (Ocean of Storms) as well as the landing sites of Apollo 12 and 14. Near the western limb, the dark crater Grimaldi can be seen, having the appearance of a small lunar sea in its own right. Byrgius crater, a popular sight, is to the south of Grimaldi.

North-West

North-West

The north-western quadrant of the Moon is home to prominent craters such as Kepler, Copernicus and Plato, as well as Aristarchus, the brightest crater on the entire surface. The bright craters and dark plains of Oceanus Procellarum and Mare Imbrium (Sea of Showers) make this one of the most striking, high-contrast regions of the Moon. Nestled in the western edge of Mare Imbrium is Sinus Iridum (Bay of Rainbows.)

History of Lunar Observation (#ulink_8cb2b1c4-3b6b-5130-a0e3-13da960e952a)

The Moon has lingered in our skies as long as life has existed on Earth. It predates the earliest murmurings of consciousness by billions of years. When our ancestors began to take notice of the objects in the sky, the Moon was already a familiar sight, whose regular visits were probably a source of comfort, providing a source of natural light at night. But as the earliest humans attempted to explain the world around them, events in the sky became steeped in superstition. Then unpredictable, aberrant changes in the appearance of the Moon, such as eclipses, terrified ancient people as harbingers of doom. Today we flock to see them and enjoy the spectacle.

Eventually this perception of the Moon as a god of many moods began to fade, when as early as the fifth century BCE, astronomers in ancient Babylon learned to predict its eclipse cycle through careful record-keeping across generations. Meanwhile, in Asia, astronomers in what is now modern-day India worked out how to describe the apparent elongation between the Moon and Sun throughout the full synodic month.

The Mesopotamian god of the Moon, Sin, depicted on a tablet at the British Museum.

A great deal of progress was made when mathematical cosmologies – descriptions of what we now call the Solar System, but what was once considered to be the centre of the Universe – were developed in ancient Greece. Notably, Aristarchus in the second century BCE, considering the Moon as a spherical ball of earth or rock, computed its size and distance. His results weren’t precise, but they were certainly better than anything that had been achieved before. In the same century, Seleucus had correctly linked the Moon to the tides, long before Newton would describe the force responsible.

A few hundred years later, Ptolemy significantly improved on previous calculations about the size and distance of the Moon, but with the transition to the Middle Ages, this progress of understanding slowed down.

The next leap in lunar observation would occur with the invention of the telescope in the early seventeenth century. Skilled observers of the pre-telescopic era, such as Tycho Brahe, had attempted to discern the Moon’s surface, but even the best of their visual acuity was no match for the telescope. Many people incorrectly believe that Galileo Galilei pioneered the use of the telescope in astronomy, but he was actually beaten to it by Thomas Harriot, an English scientist then living in London. Harriot was credited with making the first sketch of the Moon with the aid of a telescope in July 1609, approximately four months before Galileo’s first use of such an instrument. Nevertheless, Galileo was the first to make a systematic study of the sky using the telescope, and in the following year published his ground-breaking Sidereus Nuncius (Starry Messenger), including sketches of the Moon, showing unprecedented detail.

Over the next 400 years, rapid improvements in telescope technology would greatly improve our perception of the lunar surface. Generations of skilled observers discovered and catalogued features large and small, attempting to explain their origin. The Moon became the subject of the very first successful astronomical photograph, which was a daguerreotype captured by John Draper in 1840.