Total Lunar Eclipse: What to Expect

January 19, 2019 clifftop CliffNotes

Written for Clifftop by Mike Krawczynski, assistant professor of planetary science at Washington University in St. Louis.

2019 marks the 50th anniversary of humans walking on the Moon for the first time.  And as of January 3rd 2019, humans have now landed on the far side of the Moon.  Our celestial companion, the Moon, still holds our attention as we casually gaze at the night sky, and as we scientifically explore the wonders of our solar system.  In the first month of the 50th year of lunar exploration, we have a spectacular total lunar eclipse on the evening of January 20th.  Total lunar eclipses happen relatively infrequently, the last one visible from greater St. Louis was in September 2015, and eclipses require the alignment of 3 things, the Sun, Earth, and Moon.

The phases of the Moon are very familiar to us, as we watch on a monthly basis the transition from New Moon, to crescent to gibbous to full and back again.  Full moons occur every 29.53 days, and always occur when the Moon is on the opposite side of Earth from the sun.  Because the orbit of the Moon is slightly tilted with respect to the plane of the Earth’s orbit around the Sun, only occasionally does the full moon occur when these orbits align.  These are the conditions of the lunar eclipse.

During a total lunar eclipse, the Moon moves into the shadow the Earth casts from the Sun.  Unlike a solar eclipse, like the one that occurred in the greater St. Louis area in August of 2017, lunar eclipses do not require any special equipment to view.  There is no need for eye protection, a telescope, or a camera (although binoculars, telescopes, and cameras can be fun!).  Because the Earth and Moon are about the same distance from the Sun, and the Earth is much bigger than the Moon, this means that lunar eclipses last much longer than solar eclipses.  The totality portion of a lunar eclipse may last up to almost 2 hours, compared to less than 10 minutes for the longest solar eclipses.    For the eclipse of January 20th, totality will last 62 minutes.

What to expect to see:

During a total lunar eclipse, the Moon will gradually shift into the shadow of the Earth.  At approximately 9:33 PM on January 20th, the partial phases of the lunar eclipse will begin.  As the Moon enters the shadow it will appear to go through ‘phases’ of a sort, with a progressively larger amount of the Moon going dark, looking like some huge invisible space creature is taking progressively larger bites out of the full Moon.  However, the ‘dark’ part of the Moon, which is in the Earth’s shadow, is not totally dark.  The Moon is lit by two main light sources during a full Moon, one source is direct sunlight, and the other is indirect sunlight filtered through the atmosphere of Earth.  The Atmosphere of the Earth refracts sunlight so that the reddish hues of all the Earth’s sunrises and sunsets illuminate the Moon during a full Moon phase, but only during totality in a lunar eclipse is this dim reddish light visible.  You can think of this as someone holding a match right next to a bright flashlight that is shining directly in your eyes.  Both sources of light reach your eyes, but even though the match is lit, the light from the flashlight is so much brighter, that your eyes can’t perceive it.  When the flashlight is turned off, you can easily see the light from the match.  So it is with lunar eclipses.  The reflected direct sunlight of the full moon is so bright, that you cannot see the reddish glow of the light coming through Earth’s atmosphere and reflecting off the Moon until the Moon has totally entered Earth’s shadow.

Total lunar eclipse courtesy

Totality will begin at approximately 10:41 PM local time.  The Moon will transition from a sliver of silver similar to a crescent moon to a full Moon that has a deep reddish color.  This is why total lunar eclipses are often called Blood Moons.  The Moon will stay full and red until 11:43 PM when the Moon begins to leave the Earth’s shadow and the partial phases will repeat.  At 12:50AM on January 21, all the phases of the eclipse will be over.

The total lunar eclipse is a very nice event to watch, but remember since this will be occurring on a January night, it might be even nicer to watch with a warm jacket and a cup of hot cocoa!

The Dark Side of the Moon:

Allow me to leave you with one last little fact about the Moon and eclipses that you can use to impress your friends at dinner parties.  There is no such thing as a permanent dark side of the Moon.  The Moon, in fact, rotates just like the Earth, but at a slower rate.  The rate the Moon rotates in fact matches it’s orbital period exactly, meaning that the same side of the Moon always faces the Earth.  This rotation means that the far side of the Moon receives the same amount of light as the nearside except that the near side of the Moon occasionally gets eclipsed.  Which means that over the course of many years, the nearside actually receives LESS light than the far side.  In addition, the far side of the Moon is far from dark colored and looks different than the side that faces Earth.  The familiar dark marks on the Moon that often are imaginatively thought of as the face of the man on the moon, are actually darkly colored basalt lava flows.  The far side of the Moon almost completely lacks these dark lava deposits. So if there were such a thing as ‘the dark side’ of the Moon, it would, in fact, be accurate to say that the side facing the earth is the dark(er) side!

CLIFFTOP, a local nonprofit organization, is focused on preserving and protecting area blufflands.

A version of this article appeared in the January 18, 2019 edition of the Monroe County Independent.

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