HeadsUp for Sunday's Blue Moon. 4.45am EDT

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The Moon will reach its full phase on May 31, and because this is the second full Moon to occur within a single calendar month, it qualifies as a Blue Moon. Not blue in colour. That part is almost never true. The name comes from the relatively rare occurrence of fitting two full Moons into one calendar month, which happens only when the lunar cycle of roughly 29.5 days lines up in just the right way with our 30 or 31-day months. On average it occurs once every two to three years, which is where the phrase once in a blue moon originally came from.

The Moon takes 27.3 days to complete one orbit of Earth, but the full lunar cycle from one full Moon to the next takes 29.5 days, because Earth is also moving around the Sun during that time. When a full Moon falls in the first day or two of a month, another full Moon can just barely squeeze in before the month ends. May had its first full Moon earlier this month, and the second arrives Sunday.

This particular Blue Moon also coincides with the Moon being relatively close to Earth, making it a supermoon as well. A supermoon occurs when the Moon is near perigee, its closest orbital point, making it appear up to 14 percent larger and 30 percent brighter than when it is at its farthest point. The combination of a Blue Moon and a supermoon in the same event is genuinely uncommon.