PacSci Perspectives


Super-Blue-Blood Moon!

by | Jan 12, 2018

Lunar Eclipse Diagram

January is going out with a phenomenal lunar event. There is a lunar eclipse early in the morning on January 31. Lunar eclipses happen during a full moon and this will be the second full moon of the month, so that makes it a “blue” moon. Also this full moon is happening when the moon is at its closest point toward the Earth. That means it’s what is called a “Super” Moon. Let’s take a look at all three.

The term Supermoon has become popular over the last few years. It describes a phenomenon that while not particularly rare, is interesting. The Moon does not orbit the Earth in a complete circle, it’s an ellipse. Sometimes the moon is closer, other times further. We call the closest point in its orbit the perigee and its furthest point the apogee. On average the Moon is 238,000 miles away, but at perigee it is almost 30,000 miles closer than it is at apogee. While a Full Moon can occur at any point in the Moon’s orbit, a Full Moon at perigee appears 12% larger than a Full Moon at apogee. Supermoons are not particularly rare; they happen about once a year and sometimes more. The last Full Moon of 2017 and the first two Full Moons of 2018 are all Supermoons.

Full Moon Blue Moon
A Full Moon happens once every lunar orbit. The Moon takes about 28 days to orbit the Earth. When the Moon is directly opposite the Sun from the Earth, the entire near side of the Moon is in daylight and we see a Full Moon. Since the Moon’s orbit is only 28 days, it is possible for a month with 30 or 31 days to experience two Full Moons, commonly called a Blue Moon. Since there was a Full Moon on January 1, the Full Moon of January 31 is considered a Blue Moon.

Eclipse and Blood Moon
On the morning of Wednesday, January 31, 2018 Seattle residents will be able to witness the Earth’s shadow pass over the Moon causing a total eclipse. The eclipse begins at 2:51 a.m. and will reach its maximum at 5:29 a.m. During the eclipse, Seattleites will be able to see the curvature of the Earth’s shadow across the face of the Moon. The Ancient Greeks witnessed this and, using simple geography, along with their knowledge of the circumference of the Earth, were able to estimate the distance between the Earth and the Moon at 240,000 miles, remarkably close to our current calculation of the average distance at 238,000 miles. The total eclipse will end at 6:07 a.m. and the partial will end just before Moonset at 7:11 a.m. Given a clear sky Seattleites could see the entire eclipse against the silhouetted Olympics.

During the eclipse the Moon may appear blood red. While in the past this was seen as ominous, we now know it is because of the Earth’s atmosphere. The light from the Sun passes through the edges of the atmosphere. By passing through the atmosphere the blue light is scattered and the red light makes it through reflecting off of the Moon back to us. This is the same effect that we witness at sunset and sunrise.

Super-Blue-Blood Moon!
To best view the Moon, find an area with an unobstructed view to the West. If it is clear and you find a place with a view across Elliott Bay or Puget Sound there could be enough light for the silhouetted Olympic Range to be in the background.