Moon phase calendar 2020

2020 is a thirteen-full-moon year. Thirteen full moons and 12 new moons fit into the twelve months, which means one month gets two of them and the year carries a Blue Moon.

The moon phase calendar 2020 below lists all of them, in your own timezone, with the exact instant rather than the date alone. Every time on this page is searched for individually in the ephemeris rather than stepped forward from the one before, because the lunation length varies enough that adding 29.53 days repeatedly drifts off within months.

One thing the table will not do is round to the day. Instants matter here more than they look like they should: the difference between a full moon at 23:40 and one at 00:20 is a whole calendar date, and which side of midnight it falls on depends on where the reader is standing rather than on anything about the Moon.

The shortest lunar cycle of 2020 runs 29.35 days and the longest 29.75 — a spread of about 10 hours between them. That variation comes from the Moon's elliptical orbit: it moves fastest at perigee, so the cycles that contain a perigee near new moon finish sooner.

October is the month with two full moons in 2020, which makes the second one a Blue Moon under the modern definition. It happens because the lunar cycle is 29.53 days and most months are longer: a full moon in the first day or two of a long month leaves just enough room for another before it ends.

This is a calendrical event rather than an astronomical one. Nothing about the second full moon is different — it is not bigger, brighter or bluer, and the Moon does not know which month it is. What is genuinely unusual is the arithmetic that produced it, and that only happens about every two and a half years.

An older seasonal definition — the third full moon in a season containing four — gives different dates entirely. This calendar uses the month-based one, because that is what almost every calendar a reader might compare against uses, and because the seasonal rule was already being misreported by the 1940s.

A lunation is not a fixed length. In 2020 the shortest runs 29.35 days, from the new moon on August 19, and the longest runs 29.75 days, from the one on February 23 — a difference of about 10 hours between two consecutive months of the same cycle.

The cause is the shape of the orbit. The Moon travels along an ellipse, moving fastest near perigee and slowest near apogee, so a cycle that happens to contain its perigee close to the new moon gets through the 360° of elongation sooner. Over a year the effect is large enough to matter: adding 29.53 days repeatedly to a known new moon drifts off the real dates within a few months, which is why every instant on this page is searched for individually rather than stepped forward from the one before.

This is also the answer to a question the table above raises for anyone who studies it. The gaps between consecutive full moons are not equal, the gaps between a full moon and the next first quarter are not equal, and neither pattern repeats from year to year. The moon phase calendar 2020 above spaces them where they actually fall; one that spaces its events evenly is drawing the idea of a lunation rather than the real one.