Super Blue Blood Moon

(Photo: NASA)

By Chuck McPartlin

Eek! It’s a Super Blue Blood Moon!

This Wednesday, January 31, we will experience a Super Blue Blood Moon. OK, that sounds pretty dramatic, but it just means that the Full Moon will be near the closest point to Earth of its elliptical orbit (Super), it’ll be the second Full Moon this month (Blue), and that it will be totally eclipsed (Blood). The eclipsed Moon will be dimly illuminated by the refracted light of the sunrises and sunsets occurring around the rim of the Earth, giving it a reddish hue, and thus the reference to blood.

The phases of the Moon are caused by the changing angle of illumination of the Sun on the surface of the Moon, and have nothing to do with the shadow of the Earth. The dark area of the Moon is, in effect, the shadow of the Moon itself. A Full Moon rises at sunset, opposite the Sun in our sky, and sets at dawn.

And, as you can see, there is no permanently Dark Side of the Moon. A typical spot on the Moon’s surface sees about 14 days of sunlight, followed by 14 days of night. Because the Moon’s rotation is tidally locked to the Earth, it rotates once around its axis in the same amount of time it takes to revolve around the Earth, and thus we always see essentially the same face of its surface. The other side is the Far Side, a much better term for the side we don’t see, except from spacecraft.

A lunar eclipse, however, does involve the shadow of the Earth. Watching the orbital animation might make you wonder why we don’t see a total lunar eclipse with every Full Moon, when the shadow of the Earth would extend in that direction. At New Moon, you might expect to see a solar eclipse, with the Moon blotting out the Sun. However, there is a third dimension, or depth into the image, that isn’t apparent.

The orbital plane of the Earth around the Sun, corresponding to the circle around our sky that the Sun appears to follow, is called the Ecliptic, because that’s where eclipses happen. The orbital plane of the Moon is tilted relative to the ecliptic by just over 5 degrees. The apparent angular size of the Moon is about half a degree, or 30 arcminutes. Therefore, the Moon can easily pass above or below the Earth’s shadow or the visual disk of the Sun, which is also about 30 arcminutes across.

The points where the lunar orbital plane intersects with the ecliptic are called its nodes, and it is only when the Moon is near the nodes of its orbit at Full and New phase, so the line of the nodes points toward the Sun, that we have eclipses. For this diagram, at the time of this lunar eclipse, that means that the Sun is where you are, and the Moon is farther “inside” your display, behind the Earth.

Weather permitting, go out on the morning of January 31, and you can watch orbital motion in action as the shadow of the Earth sweeps across the face of the Moon. The shadow has a lighter penumbra and a darker central umbra, since the Sun is an extended light source, rather than a point, so the Sun’s rays are not parallel as they reach our neighborhood. The penumbral phases start at 2:51 AM PST, as the lighter part of the shadow starts to touch the Moon. The darker umbra reaches the Moon at 3:48 AM, and the Moon will be totally in the umbra from 4:51 AM until 6:08 AM. The receding umbral phase will be well along when the Sun rises at 7:01 AM, and the Moon sets at 7:03 AM (if you have a flat western horizon). See whether you can detect the curved edge of the umbra, from which even the ancient Greeks realized that the Earth is spheroidal.

The Santa Barbara Astronomical Unit is not planning any public observations of the lunar eclipse, since it is occurring in the wee hours when it is difficult to find an appropriate venue with parking and restroom facilities that is open. Given that this event is happening early in the morning, and is a great precursor to a nice breakfast, I’ve even heard it referred to as a Super Blue Blood Bacon Moon.