Moon phases happen because, as the Moon orbits Earth, we see different amounts of its sunlit half.
Cause
The Moon orbits Earth every 27.3 days. We see half of it lit by the Sun, and the part we see changes with its position. A full cycle of phases takes 29.5 days.
Phases
New moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent.
Not Earth’s shadow
Earth’s shadow causes lunar eclipses, which are rare.
Waxing and waning
Waxing means growing, waning means shrinking.
Tides
Full and new moons cause higher spring tides.
Fun fact
The Moon’s pull on tides slows Earth’s rotation.
Cause
The Moon has phases because we see different portions of its sunlit half as it orbits Earth. The Moon does not make its own light. It reflects sunlight, and half of it is always lit by the Sun. As the Moon travels around Earth over about 29.5 days (a synodic month), the angle between the Sun, Earth and Moon changes, and we see more or less of the lit half.
Phases
- New moon: the Moon is between Earth and Sun, and its lit side faces away from us. It is not visible.
- Waxing crescent: a thin sliver of light grows on the right side in the Northern Hemisphere.
- First quarter: half the disk is lit, about 7.4 days after new moon.
- Waxing gibbous: more than half is lit.
- Full moon: Earth is between the Sun and Moon, and the whole face is lit, about 14.8 days after new moon.
- Waning gibbous, last quarter and waning crescent: light shrinks.
The cycle takes about 29.5 days to repeat.
Not Earth’s shadow
A common myth is that phases are caused by Earth’s shadow on the Moon. Earth’s shadow causes lunar eclipses, which happen only when the Sun, Earth and Moon line up exactly, a few times a year at full moon. Phases happen every month.
Waxing and waning
“Waxing” means growing, “waning” means shrinking. In the Northern Hemisphere, the right side lights up first during waxing. In the Southern Hemisphere, the pattern is reversed. A crescent moon with the bulge on the right is waxing in the north.
Tides
The Moon’s gravity causes ocean tides, with two high tides and two low tides daily. At new and full moons, the Sun, Moon and Earth align and produce the highest high tides and lowest low tides, called spring tides. At the quarter moons, the Sun’s and Moon’s pulls partly cancel and give neap tides, with smaller tidal ranges.
Fun fact
The Moon is about 384,400 kilometers away, and takes 27.3 days to orbit. It is a bit longer for the phase cycle because Earth also moves around the Sun. The same side always faces Earth. The Moon is quite dark: it reflects only about 12% of the sunlight that hits it, similar to worn asphalt, and looks bright only against the night sky.
Orbital cycle and numbers
The Moon’s phases are caused by the changing angle between the Sun, Earth and Moon as it orbits Earth, so we see different portions of its sunlit half. The cycle from new moon to the next new moon, called a synodic month, takes 29.53 days, a bit longer than the 27.3 day orbit, since Earth moves around the Sun in the meantime. There are eight named phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter and waning crescent. At a full moon, the Sun, Earth and Moon are roughly aligned, and a lunar eclipse occurs only when the Moon passes into Earth’s shadow, which happens a few times a year. Lunar phases affect ocean tides, since spring tides, with the highest high tides, occur at new and full moon. Waxing means growing, and in the Northern Hemisphere, the lit part grows from the right side.
Based on data from NASA and the US Naval Observatory
