So, the Full Moon of January 31 was red, white, and blue? A Blood Moon because it went into eclipse in the Earth’s reddened shadow. White because, as a Full Moon, it glared white on either side of the eclipse. And blue because it was the second Full Moon in the month. And it loomed large toward us, a Supermoon!
How much of that is true?
If you were where the total eclipse could be seen, let us know how reddened the shadow on the Moon became on this occasion. Reports so far suggest it was only slightly tinged.
As for being a Blue Moon, that has two or more meanings. I’ve put my full explanation into a new “Moon” section of “Astronomical Calendar 2018,” to which you can get by clicking that tab at the top. Briefly, it was not a Blue Moon in the older sense of being a fourth Full Moon in a season; but it was, in the later and now more widely-known sense of being a second Full Moon in a calendar month. March, too, will have two Full Moons, the second being “blue” in this sense, because the month in between, February, has no Full Moon at all.
And was it a Supermoon, looming outstandingly large because of being exceptionally near? No. It was the fourth nearest Moon of 2018. The nearest was its immediate predecessor, the Full Moon of January 2. That occurred only about four and a half hours after a perigee, or nearest moment in the Moon’s orbit; so the Earth-Moon distance (between their centers) became as small as 55.91 Earth-radii. The Jan. 31 Full Moon occurred nearly 28 hours after a perigee, so the distance reached 56.28. Small, but by no means the smallest.
This graph, which has also been put into “Astronomical Calendar 2018,” can make things clear.
The curve is the Moon’s distance. The circles are the Moon – to scale – at its Full, Last Quarter, New, and First Quarter moments. The distance varies over each lunar month, between perigee (minimum) and apogee (maximum). But these extremes vary in a larger wave taking somewhat less than a year. Especially the perigees, which this year are nearest-in around January (near Full Moon moments) and July-August (near New Moons). So the Moon is near around the Jan. 2 and Jan. 31 Full and July 13 and Aug. 11 New times, but the nearest is the Jan. 2 one. It happens to be the only one of the four at which an eclipse does not occur.
Newspapers showed gorgeous “Supermoon” photos from around the world, giving the impression that the Moon got stuck between skyscrapers or that cyclists on a hilltop could reach out and touch it. Such photos may bebetter suited for illustrating the “Moon Illusion,” whereby the Moon when near the horizon appears larger to the mind than it geometrically does to the eye. But their trick is narrow angle of view, so that something distant and small seems huge when a small piece of foreground is superimposed on it.
I’ve also put into “Astronomical Calendar 2018” the beginning of a section about eclipses.