Once in a Blue Moon

The Blue Moon of 2004

Our Blue Moon Calculator says that in 2004, there is a Blue Moon in July. You may have heard or read reports which said that the Blue Moon really occurs in August. Who is right?

Well, you can see Blue Moons in both months, but it depends where in the world you live.

Let's start with the basic astronomical facts. The three Full Moons in July and August 2004 are as follows:

2 July at 11:09 GMT
31 July at 18:05 GMT
30 August at 02:22 GMT

These dates and times have been calculated rigorously using the same methods as those employed by the United States Naval Observatory and by Her Majesty's Nautical Almanac Office. They are given in Greenwich Mean Time which is the standard time zone used by astronomers worldwide.

Notice that the second Full Moon in the list occurs less than six hours before the end of 31 July. Changing the time zone by six hours or more to the east of Greenwich changes the month in which the Full Moon falls. In Australia, New Zealand and the Far East, the Full Moon is on 1 August, not 31 July.

Here are those three Full Moons again, but with the dates and times adjusted for several major time zones around the world. For each time zone, the two Full Moons which fall in the same calendar month are highlighted.

City Time zone Full Moon 1 Full Moon 2 Full Moon 3
Places East of Greenwich
Sydney GMT + 10 hours July 2
21:09
August 1
04:05
August 30
12:22
Tokyo GMT + 9 hours July 2
20:09
August 1
03:05
August 30
11:22
Islamabad
(Pakistan)
GMT + 5 hours July 2
16:09
July 31
23:05
August 30
07:22
Moscow¹ GMT + 4 hours July 2
15:09
July 31
22:05
August 30
06:22
The Greenwich Meridian
London¹ GMT + 1 hour July 2
12:09
July 31
19:05
August 30
03:22
Places West of Greenwich
New York¹ GMT - 4 hours July 2
07:09
July 31
14:05
August 29
22:22
Chicago¹ GMT - 5 hours July 2
06:09
July 31
13:05
August 29
21:22
Los Angeles¹ GMT - 7 hours July 2
04:09
July 31
11:05
August 29
19:22
Hawaii GMT - 10 hours July 2
01:09
July 31
08:05
August 29
16:22
¹ These places observe daylight saving time in July and August.

As you can see, Australia, New Zealand and the Far East see two Full Moons in August, whilst the rest of the world sees two Full Moons in July instead.