Twice in a Blue Moon

The first day of 2018 was greeted with a beautiful full moon. Since the average time between full moons is about 29 days, that means there will be another one before the end of January, the 31st to be exact. The cool thing about the full moon on January 31st is that there will be a total lunar eclipse on that day. You probably know that a lunar eclipse occurs when the moon passes into the shadow of the earth (recall from last August that a solar eclipse occurs when the earth passes into the shadow of the moon). Lunar totality on the 31st will occur for us in North America as the full moon is setting. Set your alarms early that day so you can enjoy the spectacle before sunrise!

The second full moon in a calendar month is sometimes called a Blue Moon. There are various and contradictory definitions of the term “Blue Moon,” but I prefer to think of a Blue Moon as the second full moon in a month. Two full moons in one month is a relatively rare occurrence. I don’t remember how often it happens, probably every couple of years, but January is one of those months. You’ve undoubtedly heard the phrase “once in a Blue Moon” to describe something that happens rarely. Well, one explanation (there are several) for that phrase lies in the second monthly occurrence of a full moon. Please share other explanations if you are so moved.

In an interesting quirk of our Gregorian calendar, when January has two full moons, it turns out that a full moon will not occur in February. Poor February! It also turns out that March, another month with 31 days, will also have two full moons. So, two out of the first three months in 2018 will have a Blue Moon. So much for the rarity of this event! Perhaps for 2018 we should use the phrase twice in a Blue Moon! I did some research on this sequence and it turns out that 1999 had two full moons in January, none in February, and two in March. The next time will be 2037. How about that for some cool, meaningless trivia?

Finally, it also turns out that our orbital companion is at its closest point in its orbit about the earth, so both full moons in January are “super moons,” as the Weather Channel likes to call them. Personally, I’m not a big fan of the whole “super moon” phenomenon. The reason is that it is very difficult to distinguish a “normal full moon” from a “super full moon.” Here’s why: there is only about a 10 to 15% difference in the size of the moon between a “normal full moon” and a “super full moon.” Plus, our visual perception of the size of the moon is quite subjective. Think about it – the moon always “appears” to be larger near the horizon than at its apex. I’m sure you’ve noticed it. The moon always looks bigger on the horizon than when straight overhead. But it is exactly the same size. The size difference is an illusion. If you doubt me, try this experiment. The next time you see the moon rise or set, hold out your arm and use your thumb as a reference point to gauge the size of the moon. Typically, the diameter of the moon will be about one half the width of your thumb. Later, when the moon is near its apex and looking much smaller than it did when it was on the horizon, perform the same test with your thumb and you will see that the size of the moon relative to your thumb has not changed. It is exactly same size when it is overhead as when it is on the horizon! The actual size of the moon has not changed. Yet it is such a powerful illusion that I still do the thumb test occasionally to remind myself.

You might ask, why then is the “super moon” such a big deal? Well, I think there are two reasons. First, people hear about it on TV, then make a point to check out the moon at moonrise. Of course, the moon appears large at moonrise for the reasons I mentioned above and everyone says, “wow, look at that super moon!” The second reason is that photographers sometimes create composites in Photoshop with an artificially enlarged moon. Usually, these attempts are not very good, but most viewers don’t realize it and say, “wow, look at that super moon!” Please forgive my cynicism. I’m not saying that a “super moon” is a myth. It really does happen. And, if it gets people out to look at a moonrise, I’m all for it! I’m just saying that it is very difficult to know a “super moon” when you see it. Except on the Weather Channel.

Here is a shot of the moonrise on January 1st that I took from Fort Point at the base the Golden Gate Bridge. In the foreground below the moon, you can see the famous Alcatraz Island. Pretty cool juxtaposition of the moon and Alcatraz. I used a 400 mm lens for this image. Alcatraz and the moon are both far away, which explains why the moon appears so big. Enjoy!

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