The last Full Moon of northern winter happens this week on March 5, 2015. This is the “Worm Moon” or the “Sap Moon”, so called because earthworms start to churn up the slowly thawing soil as winter comes to an end, and sap starts to run in the maple trees. This month’s Full Moon is also a “mini moon”, the opposite of a so-called ‘supermoon’. It will appear about 14% smaller than the largest full Moon of the year.
What is a supermoon and a minimoon?
The term “supermoon” was coined in 1979 not by an astronomer, but by an astrologer, Richard Nolles. He defined it as a New or Full Moon that happens when the Moon within 90% of its closest approach to Earth in its orbit each month. The opposite, which some call a minimoon or a micromoon, happens when the Moon is within 90% of its furthest distance from Earth in its orbit. It’s a totally arbitrary definition, but the size difference is noticeable in images and to keen-eyed stargazers.
This month’s minimoon happens at 18:07 Universal Time on March 5, just 10 hours after the Moon reaches apogee, its farthest point from Earth this month, at a distance of 406,385 km.
Two weeks later, at New Moon, the Moon will reach perigee, its closest point to Earth, at a distance of 357,583 km. So this month’s New Moon will be a supermoon, though you won’t be able to see it because the Earth-facing side of the Moon appears dark.
As for observing this month’s minimoon for yourself, it doesn’t get much easier. The full Moon rises in the east as the Sun sets in the west, then moves well above the horizon in the mid evening hours east of the bright star Regulus in the constellation Leo. If the sky is clear, you can’t miss it.
As an interesting project, you can snap an image of the minimoon on March 5, then another (using the same camera settings and focal length) during the “supermoon” of September 28, 2015, which will be the largest Full Moon of the year. For stargazers in most of the Americas and western Europe, that supermoon will occur during a total lunar eclipse.
There will be a Clubhouse Clip of this minimoon for Slooh members on March 5 when Will Gater and Eric Edelman will talk about the Full Moon and revel in live images from the Slooh Canary Islands Observatory.