The Moon's Phases, Finally Explained Properly
There is one common wrong explanation for lunar phases (Earth's shadow) and one common right one that is often stated too vaguely to actually help. Here is the geometry, with real numbers.
Not Earth's shadow
Earth's shadow does cross the Moon — during a lunar eclipse, which happens two or three times a year and lasts a few hours. The phases are something different and continuous.
The Sun always illuminates exactly half the Moon (the same as it always illuminates half of Earth). Phases are simply the changing angle from which we see that lit hemisphere as the Moon orbits us.
The geometry, step by step
Put the Sun far to the right of the page. Earth sits at center. The Moon travels in a nearly circular orbit around Earth, taking 27.3 days relative to the stars and 29.5 days from one full moon to the next (the difference is Earth also moving around the Sun).
When the Moon is between Earth and the Sun, we see its dark far side: new moon. When the Moon is on the far side of Earth from the Sun, we see its fully lit near side: full moon. In between, we see partial phases.
| Phase | Sun–Moon angle | Best time up |
|---|---|---|
| New | 0° | Not visible |
| First quarter | 90° east | Afternoon, sets midnight |
| Full | 180° | All night — rises at sunset |
| Last quarter | 90° west | Rises midnight, up until dawn |
Waxing versus waning
Between new and full the illuminated portion grows — the Moon is waxing. Between full and new it shrinks — waning. In the northern hemisphere the lit side is on the right when waxing and on the left when waning. The reverse is true in the southern hemisphere, which is why phase diagrams from a Miami sky look mirrored to a Melbourne reader.
Why 'quarter' when it looks half?
'First quarter' does not describe how the Moon looks; it describes where it is in its cycle. At first quarter the Moon has traveled one-quarter of the way around Earth from new. It happens to look 50% lit because we are viewing it from 90° off the Sun–Moon line.
Why we can't see one whole side
The Moon rotates on its axis once per orbit — a phenomenon called tidal locking. That means one hemisphere always faces us. There is no 'dark side' of the Moon in a permanent sense; the far side gets exactly as much sunlight as the near side, on the same 29.5-day cycle. It just also happens to be pointing away.
Frequently asked
- Why do we sometimes see the Moon during the day?
- For the same reason we see it at night — it's up. Around first quarter, the Moon rises around noon and is highest in the sky in the late afternoon. Anytime it is above the horizon and not too close to the Sun, it is visible.
- How long is a lunar month?
- Two different lengths depending on definition: sidereal (relative to stars) is 27.32 days; synodic (new moon to new moon) is 29.53 days. Calendars use the synodic period.
- Why is the Moon sometimes orange?
- Same reason sunsets are — light traveling through more atmosphere near the horizon has more of its blue scattered away. A full moon rising through humid Miami air looks distinctly warm-toned.