Lunar Phases and Motions
The Moon's phases result from the changing geometry between the Sun, Earth, and Moon as the Moon orbits our planet. Understanding this system explains not just why the Moon looks different each night, but also when and where in the sky you can expect to see it.
Cycle of Lunar Phases
As the Moon orbits Earth, the Sun always illuminates exactly half of it. But from our vantage point on Earth, we see different amounts of that lit-up half depending on where the Moon is in its orbit. That's what creates the phases.
Here are the eight phases in order:
- New Moon: The Moon sits between Earth and the Sun (a position called conjunction). The sunlit side faces away from us, so the Moon is essentially invisible.
- Waxing Crescent: A thin sliver of light appears on the right side of the Moon. "Waxing" means the illuminated portion is growing.
- First Quarter: Exactly half of the Moon's Earth-facing side is lit. The Moon is at a 90° angle from the Sun as seen from Earth. Despite the name, you're one-quarter of the way through the full cycle, not seeing a quarter of the Moon.
- Waxing Gibbous: More than half is illuminated, still growing toward full. "Gibbous" means the lit area is larger than a semicircle.
- Full Moon: The Moon is on the opposite side of Earth from the Sun (opposition). The entire Earth-facing side is illuminated.
- Waning Gibbous: The illuminated area starts shrinking. "Waning" means the lit portion is decreasing.
- Last Quarter: The opposite half from First Quarter is lit. The Moon is again at 90° from the Sun, but on the other side.
- Waning Crescent: A thin sliver remains on the left side, shrinking until the next New Moon.
The time it takes the Moon to orbit Earth relative to the stars is about 27.3 days (the sidereal period). But because Earth is also moving around the Sun, the Moon needs a little extra time to get back to the same phase. The time between two identical phases is the synodic period, which is about 29.5 days. That's the length of one complete cycle of phases.
One more detail: the Moon's orbital plane is tilted about 5° relative to Earth's orbit around the Sun (the ecliptic). This tilt is why we don't get eclipses every month. Most of the time, the Moon passes slightly above or below the Sun's position at New Moon, and slightly above or below Earth's shadow at Full Moon.
Synchronous Rotation (Tidal Locking)
The Moon rotates on its axis in exactly the same amount of time it takes to orbit Earth. This means the same side of the Moon always faces us. This isn't a coincidence.
Over billions of years, Earth's gravity created tidal bulges on the Moon. These bulges generated a torque that gradually slowed the Moon's rotation until it matched its orbital period. Once those two periods synced up, the system reached a stable state called tidal locking (or synchronous rotation).
The result:
- The near side always faces Earth and is the only side we ever see from the ground.
- The far side permanently faces away from us. (It's sometimes incorrectly called the "dark side," but it gets just as much sunlight as the near side.)
Moon's Position and Appearance in the Sky
Each phase corresponds to a predictable position in the sky relative to the Sun. This means you can figure out roughly when a given phase rises, sets, and is highest overhead:
- New Moon: Rises and sets with the Sun. Not visible because the unlit side faces Earth.
- Waxing Crescent: Visible in the western sky after sunset. Sets a few hours after the Sun.
- First Quarter: Rises around noon, is highest at sunset, and sets around midnight. Visible during the evening.
- Waxing Gibbous: Rises in the afternoon and is up for most of the night.
- Full Moon: Rises at sunset, is highest around midnight, and sets at sunrise. Visible all night.
- Waning Gibbous: Rises during the late evening and stays visible into the morning.
- Last Quarter: Rises around midnight, is highest at sunrise, and sets around noon. Visible during the morning.
- Waning Crescent: Visible in the eastern sky just before sunrise. Rises shortly before the Sun.
The pattern to remember: as the Moon progresses through its phases from New to Full, its rise time shifts later and later, from sunrise all the way to sunset. Then from Full back to New, rise times continue shifting later, from sunset back to sunrise.
The Moon's altitude in the sky also varies with the seasons. A Full Moon in winter appears high in the sky (following the same path the Sun takes in summer), while a Full Moon in summer hangs lower. Higher altitude means the Moon is visible above the horizon for longer.
Lunar Orbit and Surface Features
The Moon's orbit around Earth is not a perfect circle; it's an ellipse. This means the Earth-Moon distance changes throughout each orbit:
- Perigee: The closest point in the orbit (about 363,000 km). The Moon appears slightly larger.
- Apogee: The farthest point (about 405,000 km). The Moon appears slightly smaller.
When a Full Moon coincides with perigee, it looks noticeably bigger and brighter than average. This is often called a "supermoon" in popular media.
The lunar surface has two main types of terrain, easily visible even with the naked eye:
- Maria (singular: mare): The dark, smooth plains. These formed when ancient volcanic eruptions flooded large impact basins with lava that cooled into basalt. They're mostly found on the near side.
- Highlands: The lighter, heavily cratered regions. These are older than the maria and represent the Moon's original crust, battered by billions of years of impacts.