Blue Moon
An extra Full Moon: the second in a calendar month or the third of four in a season. Its counterpart at New Moon is called a Black Moon. In 2027 the Blue Moon falls on 20 May.
Next in the sky · May 20, 2027 at 11:59 AMBlue Moon: the third of four full Moons in spring →What is a Blue Moon?
From Full Moon to Full Moon an average of 29.5 days pass, one lunar month. Twelve lunar months are just under eleven days shorter than a year. That is why every two to three years a thirteenth Full Moon fits into the year, and it has to go somewhere. This extra Full Moon is called a Blue Moon. There are two rules for deciding which Full Moon is the extra one.
- By calendar month: the second Full Moon in a month is the Blue Moon. This only works if the first Full Moon falls in the first days of the month.
- By season: an astronomical season, such as spring from the equinox to the solstice, usually has three Full Moons. If it has four, the third is the Blue Moon.
On average both rules give seven Blue Moons in 19 years, but on different dates.
Where do the two rules come from?
The seasonal rule appeared in the Maine Farmers’ Almanac in 1937. In 1946 the amateur astronomer James Hugh Pruett read it differently in the magazine Sky & Telescope and wrote that the second Full Moon of a month was called a Blue Moon. This version spread by radio from 1980 onwards and is the better known one today. Both rules are in use.
Does the Moon look blue then?
No. A Blue Moon looks like any other Full Moon. The Moon can appear blue when fine particles in the air scatter the red light, for example after large volcanic eruptions or forest fires. This happened after the eruption of Krakatoa in 1883 and after the forest fires in Canada in 1950. It has nothing to do with the calendar.
Black Moon
The counterpart at New Moon is called the Black Moon: the second New Moon in a calendar month or the third of four New Moons in a season. A New Moon cannot be seen. The night stays without moonlight, good for shooting stars and the Milky Way.
Blue Moon and Black Moon in the planting calendar
In the Dyyna calendar Blue Moon and Black Moon are recorded as events in the sky. For sowing and planting they count like any other Full Moon and New Moon. How the Moon’s phase is used in the garden is explained under Moon phases in the garden.
Whether a Full Moon counts as the second in the month depends on the time zone. The Black Moon on 30 June 2030 falls at 23:34 in Germany. One time zone further east it is already 1 July, and the Black Moon does not happen there.
Dates up to 2030
| Date | Time | Event | Rule |
|---|---|---|---|
| 20 May 2027 | 12:59 CEST | Blue Moon | third of four Full Moons in spring |
| 31 Aug 2027 | 19:41 CEST | Black Moon | second New Moon in August |
| 20 Aug 2028 | 12:44 CEST | Black Moon | third of four New Moons in summer |
| 31 Dec 2028 | 17:48 CET | Blue Moon | second Full Moon in December |
| 24 Aug 2029 | 3:51 CEST | Blue Moon | third of four Full Moons in summer |
| 30 Jun 2030 | 23:34 CEST | Black Moon | second New Moon in June |
The Full Moons in spring 2027 fall on 22 March, 21 April, 20 May and 19 June. Under an older almanac calculation with the mean instead of the true Sun, the Blue Moon of 2027 falls on 20 February.
Sources
- Blue moon, Wikipedia
- Six Millennium Catalog of Phases of the Moon, Fred Espenak, AstroPixels
Frequently asked questions about the Blue Moon
When is the next Blue Moon?
On 20 May 2027 at 12:59 CEST, under the seasonal rule. Under the calendar-month rule it is 31 December 2028.
How often is there a Blue Moon?
On average seven times in 19 years, so roughly every two and a half to three years.
What is the difference between a Blue Moon and a supermoon?
The Blue Moon is a matter of the calendar, the supermoon a matter of distance. A Blue Moon can be a supermoon at the same time.
What is a Black Moon?
The second New Moon in a calendar month or the third of four New Moons in a season.