Moongazing: Beginner’s guide to exploring the Moon. Royal Greenwich Observatory

Moon take their turn to appear. We see different amounts of the lit and unlit side of the Moon because the apparent angle between the Moon and Sun changes continuously.

      The Moon rotates on its own axis once every 27.3 days, in the same direction (anticlockwise as seen from above the North Pole) as its orbit around the Earth. This results in the misleading illusion that it doesn’t appear to rotate at all, as we always see one side from Earth. In fact, the Moon keeps its familiar side facing the Earth throughout its orbit, and the far side is never seen. Only the Apollo astronauts have seen the far side with their own eyes, but robotic orbiters have mapped the entire lunar surface in incredible detail. This synchronised orbit and rotation of the Moon is no accident, but speaks to its ancient relationship with the Earth. The two became tidally locked, carefully curating the Moon’s rotation period. Tidal locking occurs elsewhere in the Solar System, for example between Pluto and its large companion Charon.

      The distance between the Moon and the Earth also changes, as the Moon’s orbit is not perfectly circular, varying between 356,500 km (221,500 miles) at lunar perigee and 406,700 km (252,700 miles) at lunar apogee, if measured between the centres of both bodies. When Full Moon occurs around lunar perigee, it appears slightly larger and somewhat brighter than average – an event colloquially known as a supermoon. These events are not very rare, but perigee and Full Moon do not always align, because lunar perigees are separated by a period of approximately 27.5 days. This is known as an anomalistic month. The difference between this period and a sidereal month means the perigee point of the Moon’s orbit undergoes gradual precession, taking nearly nine years to move all the way around the Earth once. This is known as precession of the line of apsides.

      Because the Moon’s orbit is elliptical, its speed changes as it moves. However, the rotation rate of the Moon remains constant. This discrepancy produces an effect called libration – an apparent ‘wobbling’ of the Moon – which allows us to see slightly more of its eastern or western sides (known as its limbs) as it moves ahead, or falls behind in its orbit relative to its rotation. Accounting for both extremes, we can see a total of 59 per cent of the surface of the Moon, but how much we see of the east and west depends on how favourable the libration is with respect to that limb. Because this effect is small, and features on the extreme limbs are less well-known, maps of the Moon in this guide do not show any overall libration bias.

       The far side of the Moon captured by NASA’s Lunar Reconnaissance Orbiter, heavily cratered but with far fewer maria than the near side. Mare Orientale (Oriental Sea) is shown in the top left.

       The Moon’s orbit around the Earth.

      When the Moon crosses the ecliptic, it is said to be crossing a node. The ascending node is the point where the Moon moves northward from the southern celestial hemisphere into the northern celestial hemisphere. The opposite point in the Moon’s orbit is called the descending node. Eclipses are possible, and indeed inevitable, only when the Moon is full or new at one of the nodes. When the Moon is not crossing a node, it cannot coincide with the Sun in the sky (a solar eclipse) or the Earth’s shadow (a lunar eclipse.)

       A diagram of lunar libration caused by the elliptical shape of the Moon’s orbit.

Lunar Cycle and Phases

      The Moon is the second brightest object in the sky after the Sun. Unlike the Sun, it is safe to observe directly without filters. If you own a telescope, the Moon alone can provide a lifetime of mesmerising views, ever changing as the shadows play across its surface. In the observing section, you can find advice on choosing and using a telescope to explore the Moon.

      Around New Moon, there is an approximate period of 1.5 days when the Moon is an extremely thin crescent near the Sun and too faint to be seen, even around sunset or sunrise. For the remainder of the Moon’s 29.5-day synodic month, the phases are said to wax on to Full Moon and wane off to the next New Moon. The phases are illustrated on page and explained below, with the approximate number of days that separate them. Note that there is no half-moon, as this is not an astronomical term, but rather two quarter-moon phases. The number of days given are averages, and vary slightly due to the Moon’s elliptical orbit.

      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.

       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.