Once in a Blue Moon

By Dr Russell Cockman

Most of us have heard of the expression once in a blue moon, which is an expression used to signify a rarely occurring event. In astronomical terms a blue moon is used to describe a calendar month featuring two full moons. The moon's synodic period of approximately 29.5 days means that if a full moon occurs early on the first of a month (containing 30 or 31 days) then the next full moon will occur within the same calendar month and so will be a blue moon.

The expression is actually true in reality because blue moons are reasonably rare-- there are no blue moons in 2000. Last year the UK had two blue moons, in January and March, when full moons occurred on the 2nd and 31st of each month. Strangely, Australia had only one blue moon in 1999 (in May!), a situation arising because of the difference in local time between the two locations.

So, no blue moon in 2000. However, we do have a so-called black moon in July, since there will be two new moons in the month. It will be a doubly special black moon, too, because each new moon will be seen passing across the face of the sun, although the eclipses will only be partial and visibility limited to the polar regions. The eclipse on the 1st, which was visible from the southern Pacific Ocean, had a magnitude of 0.48, while the one on the 31st has a magnitude of 0.60 and will be visible from the Arctic. It is unlikely that many hardy souls will make the effort to travel to view these events!

A more accessible event took place in the middle of the month when the full moon entered the Earth's shadow on the 16th. Sadly, the eclipse happened during daylight hours in the UK and therefore was not visible, but viewers in the Pacific Rim saw the moon turn a deep copper red colour during the period of totality of just over 100 minutes. We will have to wait until 9th January 2001 for our next opportunity to see a lunar eclipse.

In summary, July 2000 was a rather unusual month for Earth-Moon-Sun alignments and their appearance when viewed from Earth; a black moon eclipsing the sun twice at each end of the month and a moon turned deep copper-red in between.