Why does the Moon have phases?
The Moon does not emit its own light — it reflects sunlight. As it orbits the Earth (29.5 days per orbit), we see different illuminated portions from our point of view.
The 4 main phases
- New Moon: the Moon is between the Sun and the Earth. Its lit side faces the Sun — we see the dark side.
- First Quarter: the Moon, Sun and Earth form a 90° angle. We see half of the lit face (the right side as seen from the Northern Hemisphere; mirrored in the Southern Hemisphere).
- Full Moon: the Earth is between the Sun and the Moon. We see the whole lit face.
- Last Quarter: 90° again, but on the other side. Half lit (the left side as seen from the Northern Hemisphere; mirrored in the Southern Hemisphere).
Synodic cycle
The time from one New Moon to the next is 29.53 days on average. It is called the synodic month or lunation.
Synodic cycle = 29.530588853 days
Note that this is different from the sidereal month (27.32 days) — the time the Moon takes to complete one orbit in space. The difference exists because the Earth also moves around the Sun while the Moon orbits.
How this calculator works
We use as a reference a New Moon confirmed by astronomical data on January 6, 2000 at 18:14 UTC. From it, we calculate how many synodic cycles have passed until the chosen date — the remainder of the division by 29.53 tells which phase of the cycle the Moon is in.