The Moon Lunar phases
Lunar phases
Eight faces in one month
The angle at which the Moon’s sunlit half faces Earth changes, and a month of phases follows.
- Synodic month
- 29.53 days
- Sidereal month
- 27.32 days
- The eight phases
- New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent
01 / Overview
Meet Lunar phases
The Moon shines by reflection, and the Sun lights exactly half of it at any moment. As the Moon travels around Earth we see a changing fraction of that lit half; going from new Moon to full and back again takes 29.53 days on average — one synodic month.
Why the month of phases is longer than one orbit
Measured against the stars, the Moon circles Earth in 27.32 days. But Earth has moved about 27° along its own orbit in that time, so the Moon needs a further two days and a bit to restore the same Sun–Earth–Moon geometry. The first figure is the sidereal month; the second is the cycle of phases we know.
Telling first quarter from last
First quarter comes about seven days after new Moon, stands in the southern evening sky and has its lit side facing west. Last quarter comes about seven days after full, rises in the small hours and has its lit side facing east. Remember that the first quarter belongs to the first half of the night, and the two are hard to confuse.
02 / Key data
Core parameters
- Synodic month
- 29.53 days
- Sidereal month
- 27.32 days
- The eight phases
- New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent
- Relation to Earth’s shadow
- None at all
- Recurrence
- A cycle of about 29.5 days
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- Near the terminator around the quarters, crater shadows are longest and most three-dimensional.
- A full Moon is lit head-on, giving the lowest contrast and the poorest view of detail.
- The crescent two or three days after new Moon often carries earthshine, showing the dark limb.
04 / Sources