Last quarter

How the phases work

The Moon does not produce light; it reflects the Sun's. What changes through the month is not the Moon but our angle on it — how much of its lit half happens to face the Earth. The full cycle takes 29.53 days, which is longer than the 27.32 days the Moon takes to orbit, because the Earth has meanwhile moved along its own orbit and the Moon must catch up to the same alignment.

The four phases named above are instants, not periods. Full moon is the exact moment the Moon reaches opposition to the Sun — the times on this page are given to the minute for that reason. What people call "a full moon" lasts a night or two either side, but the moment itself passes in a second.

Blue moon

A blue moon is not a colour and not an astronomical event. It is a calendar accident: because 29.53 days is slightly shorter than most months, a full moon occasionally falls on the 1st or 2nd and a second one lands at the end of the same month. It happens roughly every two and a half years. Nothing about the Moon looks different.

Blood moon

This one is real and visible. During a total lunar eclipse, the Earth passes between the Sun and the Moon and its shadow covers the disc entirely. The Moon does not go dark, because the Earth's atmosphere bends some sunlight around the planet and into the shadow — and that light has had the blue scattered out of it, the same reason sunsets are red. What reaches the Moon is the combined light of every sunrise and sunset on Earth at once.

Partial and penumbral eclipses do not produce this. Only a total one does, which is why this page marks a blood moon only when the eclipse is genuinely total.

Where these times come from

Every time on this page is computed with the Swiss Ephemeris, the same engine professional astronomical software uses, rather than copied from a table. The moment of each phase is found by solving for when the Moon–Sun angle reaches exactly 0°, 90°, 180° or 270°. Checked against independent sources, the times agree to within two minutes.