Blue Moons, Blood Moons, and Super Moons: Separating Fact from Fiction


[Total lunar eclipse. Courtesy NASA]

Prepare to be inundated with lunacy this week, thanks to three interesting configurations of our nearest celestial neighbor. Wednesday the 31st is the second full moon of January, making it the so-called “blue moon” (as in “once in  a blue moon”). But just how rare is a blue moon? Not very, if you consider that the time between two consecutive full moons is about 29.5 days, and all months except February have more than 29.5 days in them. For example, March 2018 also has two full moons.

The “blood moon” descriptor refers to a total lunar eclipse. When the sun, earth, and moon line up in that order in a straight line, the earth casts a shadow on the otherwise full moon. Because the red portion of sunlight refracts or bends through our atmosphere, the full moon doesn’t turn black but a coppery or bloody red. While it can be a startling sight to those who aren’t expecting it (I remember being scared to death as a child when I saw a lunar eclipse during a late night potty break), there is nothing bloody or gruesome about the event. On average one or two total lunar eclipses occur each calendar year. The January 31 total lunar eclipse will not be visible from here in Connecticut, as the moon will be setting as the eclipse begins. Our next chance to see a “blood moon” will be January 21, 2019.

Well, what about the so-called “super moon?” This refers to the full moon looking larger than average. Because the moon’s orbit around the earth is not a perfect circle, when the moon is at its closest point to earth (called the perigee) it looks 14% larger than when it is at its farthest point (called apogee). When this happens at the time of full moon, you might notice that the moon looks a little bit brighter than average. There is no scientific definition of a super moon (because it’s not really something that scientists care about, to be honest), but one running definition on the internet is that it is when the full moon occurs within a few days of perigee. So how rare is this? Considering that the first full moon of January 2018 (which occurred on New Year’s Day) was also a “super moon,” you have your answer.

Putting all three of predictable, cyclical configurations together into one event, however, is just a tad more rare. The last “blue-blood-super moon” was probably in 1866 (again, depending on your definition of “super moon”). So if you want to take this opportunity to go outside and gawk at dear old Luna, please be my guest. Just don’t howl, because you might be mistaken for a super blue werewolf!


Perihelion and Palantiri: A Reflection on January 3, 2018

[Earth’s orbit (Courtesy NASA); J.R.R.Tolkien (Courtesy  The Tolkien Society]

Given the weight of the stories burying your social media on January 3 this year (including “bombogenesis”), you might have missed two less sensational annual events. At 12:35 AM EST, the Earth passed closest to the sun in its orbit, a point called perihelion (“around sun”). The actual time of perihelion varies a little bit each year, occurring some time between January 2-4. As you can see from the above diagram, we are not that much closer to the sun in January than we are at the farthest point (around July 4-5), called aphelion (“from sun”). A common misconception is that it is the shape of the earth’s orbit that causes seasons – being closer causes summer and being farther away causes winter. This misconception is fed by diagrams in science textbooks that greatly exaggerate the shape of our orbit, drawing it as an elongated ellipse rather than the rather more circular shape it actually has. The seasons are instead caused by the 23.5 degree tilt of earth’s axis.

The second annual event that was widely publicized on my social media was the 126th birthday of author J.R.R. Tolkien (January 3, 1892September 2, 1973), best known for The Hobbit and The Lord of the Rings. Among Tolkien fans it is a tradition to raise a toast to Tolkien at 9 PM local time on January 3 and proclaim “The Professor!” You might be wondering why I have included Tolkien in an astronomy blog. What many readers of Tolkien’s works miss is that he has a plethora of (often fairly accurate) references to astronomy in his fantasy! From the moon runes of The Hobbit to the various references to the sun, moon, stars, eclipses and more in The Lord of the Rings and his grand mythology The Silmarillion, Tolkien’s use of astronomy in his fantasy reveals his impressive knowledge of the night sky. Tolkien certainly wasn’t the first or last fantasy writer to integrate the night sky into his fantasy world, but unlike his friend C.S. Lewis, Tolkien felt it important to get the astronomy “right.” In fact, when he considered revising his unfinished mythology in the last decades of his life, he toyed with completely revising it to avoid trying to describe a mythological creation of the sun and earth, and used the then-current hypotheses concerning the formation of the moon as his guide.

So as you can see, January 3, 2018 gave this astronomer two reasons to celebrate. January 4, on the other hand, is just giving me an astronomical amount of snow to shovel….