The Moon is officially ranked as the Earth's satellite. Relative to its primary, it is however extremely large and massive, and it might well be more appropriate to regard the Earth–Moon system as a double planet. Data are given in Table 3.1.

The synodic period (i.e. the interval between successive new moons or successive full moons) is 29d 12h 44m, so that generally there is one full moon every month. However, it sometimes happens that there are two full moons in a calendar month and one month (February) may have none. Thus in 1999 there were two full moons in January (on the 2nd and the 31st), none in February and two again in March (on the 2nd and the 31st as with January). By tradition a second full moon in a month is known as a blue moon, but this has nothing whatsoever to with a change in colour. (This is not an old tradition. It comes from the misinterpretation of comments made in an American periodical, the Maine Farmers' Almanac, in 1937.) Yet the Moon can occasionally look blue, due to conditions in the Earth's atmosphere. For example, this happened on 26 September 1950, because of dust in the upper air raised by vast forest fires in Canada. A blue moon was seen on 27 August 1883 caused by material sent up by the volcanic outburst at Krakatoa, and green moons were seen in Sweden in 1884 – at Kalmar, on 14 February, for 8 min, and at Stockholm on 12 January, also for 3 min.