Jan 30

What’s Up in the Sky

What’s Up in the Sky – February, 2018

A Super Blue Moon Eclipse

I’m going to cheat a little this month. Although the focus of this column is usually on astronomical events occurring in February, I will be describing an event that happens tomorrow. It’s always cloudy in February anyway.

I am talking about a total eclipse of the Moon. Lunar eclipses occur when the Moon passes through the Earth’s shadow. Unlike a solar eclipse where the Moon casts its shadow on the surface of the Earth, blocking the Sun from the view of only those in the shadow’s narrow path, a lunar eclipse is visible to anyone on the side of the Earth facing the Moon. Unfortunately, that won’t be us, at least not for long because the Moon will be setting over Lake Michigan just as totally begins at 8:00 a.m. And the chances for a clear horizon in late January around here are slim. But if it is clear and you have a view of the horizon, go for it.

But there’s more. I have written before how this column is sometimes inspired by locker room conversations after noon ball and this is no exception. The question was in regards to a “Super, Blue Moon”, a relatively rare event. The discussion morphed into questions about frequency and observability.

First some background. A “Super Moon” occurs when the Moon is at perigee (the point in its orbit closest to Earth) and is also opposite the Sun (a full Moon). The closeness will make it appear slightly larger although the effect is not noticeable to the naked eye. That is why there were no “Super Moons” twenty years ago – the term was invented by the Weather Channel to increase viewer interest. It has no astronomical significance.

The term “Blue Moon” originally referred to the third Full Moon in a season with four Full Moons (rather than the usual three). This is still known as a “seasonal” Blue Moon. But in a 1946 article in Sky & Telescope magazine a misunderstanding of this definition caused the author to erroneously describe the Blue Moon as the second Full Moon in a month. This definition was repeated on a program called “StarDate” in 1980 and even made it into Trivial Pursuit as an answer in 1986. Today, rather than a mistake, it is considered to be a second definition.

Both the seasonal and modern Blue Moons occur once every two or three years on average. That may seem puzzling considering that this year there is a Blue Moon tomorrow and then another one March 31. But that’s because February has too few days to ever have a second full Moon so a Blue Moon in January is always followed by another in March. The next one will be in October 2020.

Now add in the fact that tomorrow the Super Blue Moon will pass through the Earth’s shadow and we have the “Super Blue Moon Lunar Eclipse”. I don’t know how rare that is, but it is definitely very, very cool, just like most things up in the sky.

My thanks to Dr. David Myers, whose questions inspired this article.

This month in history:

Feb 6: Alan Shepard hits first golf balls on the Moon – 1971
Feb 15: Galileo Galilei born – 1564
Feb 18: Clyde Tombaugh discovers Pluto – 1930
Feb 19: Nicholas Copernicus is born – 1473
Feb 20: John Glenn is first American to orbit Earth – 1962
Feb 24: Detection of first pulsar (by Jocelyn Bell in 1967) is announced – 1968