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Something unusual will happen on January 31. A total lunar eclipse, which will be visible from Asia, Australasia and North and Central America, will coincide with a “blue moon” and a “supermoon” in what some are calling a “super blue blood moon.” The event, which for western hemisphere observers happens for the first time in 150 years, will also be the last in a trilogy of supermoons over the past two months. It sounds cool, but how excited should you really be? And is there anything scientists can actually get out of it?

To be honest, I feel pretty conflicted about all the excitement surrounding the supermoon, a term which describes the moon when it is at a close point to the Earth. It’s great that people get excited about astronomy, but the trouble is that a supermoon in itself is not really all that special.

Since the moon follows an elliptical orbit around the Earth, the distance between the two bodies changes, ranging from about 360,000km to 406,000km (224,000 to 252,000 miles). At its closest point, perigee, the moon will obviously appear largest. Such a “maximoon” has an apparent diameter about 13 percent larger than a “minimoon” at its most distant point, apogee.

Blue vs blood moon

A blue moon does not appear blue. It’s just a term that, in the 1940s, was mistakenly used to describe a rare occasion: the second full moon in a calendar month. Full moons are separated by about 29.5 days and so typically occur in different months, with a blue moon appearing on average every 30 months or so.

A blood moon on the other hand actually appears dusky red. The name, which is not used by scientists, apparently derives from apocalyptic Biblical prophecy. As with supermoon, the term only became popular very recently, in this case in 2014 when a series of four over 18 months spawned a best-selling book.