The sky — astronomy

Moon Phases: Why the Moon Changes Shape

The Moon never really changes shape. The Sun always lights half of it, but as the Moon circles Earth we see that lit half from changing angles, so more or less of it faces us. One full cycle, from new moon to new moon, takes about 29.5 days.

Look up on a few consecutive evenings and the Moon seems to transform: a thin sliver, a half circle, a glowing disc. Yet the Moon itself stays the same. What changes is our point of view. This article explains why, names the phases and shows how to recognise them with the naked eye.

The lit half and the dark half

The Moon makes no light of its own. We see it because it reflects sunlight. At any moment the Sun illuminates the half of the Moon that faces it, while the other half is in night, just as on Earth. As the Moon travels around us, the angle between the Sun, Earth and Moon keeps changing, so we see a different share of the lit half.

A very common misconception is that phases are Earth's shadow falling on the Moon. They are not. Earth's shadow touches the Moon only during a lunar eclipse, which happens only a few times a year and only at full moon. Phases are visible every month and are purely a matter of viewing geometry. For more about our satellite, see The Moon.

The eight main phases

The cycle is usually described in eight steps:

1. New moon. The Moon lies roughly between Earth and the Sun. Its unlit side faces us, so it is practically invisible. It rises and sets about the same time as the Sun.
2. Waxing crescent. After a few days a thin crescent appears low in the western sky after sunset.
3. First quarter. Exactly half of the visible disc is lit. The name refers to the Moon having completed one quarter of its cycle, not to how much we see.
4. Waxing gibbous. More than half, but not yet all, of the disc is lit and growing each night.
5. Full moon. Earth lies roughly between the Sun and the Moon, and the whole lit half faces us. The full Moon rises around sunset and sets around sunrise.
6. Waning gibbous. The lit part begins to shrink.
7. Last (third) quarter. Again a half disc, but the opposite side is lit. The Moon rises around midnight and is easy to see in the morning.
8. Waning crescent. A thin sliver in the eastern sky before dawn, after which the cycle starts over.

A full Moon near perigee (the closest point of its orbit) is popularly called a supermoon, and the term blue moon refers to a calendar curiosity. Both are ordinary parts of the same cycle.

The synodic month

The time from one new moon to the next is the synodic month, averaging about 29.53 days. It is longer than the time the Moon needs to complete one orbit relative to the stars, the sidereal month, which is about 27.32 days. The gap exists because Earth also moves along its orbit around the Sun while the Moon circles us, so the Moon must travel a little further to return to the same alignment with the Sun.

The synodic month is an average. Individual cycles vary by several hours because the Moon's orbit is not a perfect circle. That is why exact times of full and new moon come from an astronomical almanac or ephemeris (a table of computed positions) rather than from simple counting. They are listed in universal time, so the calendar date can differ by a day depending on your time zone.

The Moon's orbit is tilted by about 5 degrees relative to Earth's path around the Sun. As a result, most new and full moons pass above or below the shadow line and no eclipse occurs. See also solar eclipse.

Waxing or waning: a quick guide

From the Northern Hemisphere the rule is simple:

• If the lit side is on the right, the Moon is waxing (growing). You see it in the evening sky.
• If the lit side is on the left, the Moon is waning (shrinking). It is best seen late at night and in the morning.

In the Southern Hemisphere the picture is mirrored. Near the equator a crescent often appears lying on its back, like a bowl or a boat, rather than standing upright.

Time of day is another clue: a Moon in the evening sky is usually waxing, one in the morning sky is waning. On average the Moon rises roughly 50 minutes later each day because it moves along its orbit in the same direction that Earth rotates.

During a thin crescent you can sometimes see the rest of the disc faintly glowing. This is earthshine: sunlight reflected from Earth onto the Moon's night side. For tips on watching, read how to observe the night sky.

Because the Moon is tidally locked, we always see the same side of it. Phases change only the lighting, not which hemisphere faces us.

Moon phases across cultures

The phases are the easiest sky clock to read, so they underpin many calendars. The Islamic calendar is purely lunar, and each month begins with the sighting of the first thin crescent. The Hebrew and traditional Chinese calendars are lunisolar: months follow the phases, while added leap months keep the year in step with the seasons. In many Christian traditions the date of Easter is tied to the full moon after the March equinox.

Many cultures also link phases to festivals, customs and gardening lore. These are cultural traditions. Scientific studies have not found a reliable link between phases and human behaviour; see does the Moon affect people. How phases are read in the astrological tradition is described in the Moon in astrology.

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