Adler Planetarium

Eclipses this April and throughout History

Eclipses are among the most spectacular and easily viewed astronomical events. Nothing is required but a clear sky, and even the most unobservant person who happens to be out-of-doors when one takes place is likely to notice these dramatic encounters between the two brightest objects in our sky: the Sun and the Moon. 

  The Geometrical Construction of Solar and Lunar Eclipses from James Ferguson’s Astronomy (Adler Planetarium Collection, P-65k).


The Geometrical Construction of Solar and Lunar Eclipses from James Ferguson’s Astronomy (Adler Planetarium Collection, P-65k).

An eclipse of the Sun is the more rare of the two types of eclipses. (The next total solar eclipse will not be visible in the United States until August 2017.) Because the Sun is so much larger than the Moon, the Moon’s shadow during a solar eclipse travels along a fairly small strip of the Earth’s surface. Any observers who happen to be within the path of totality are treated to the awe-inspiring sight of a few minutes of total darkness in the middle of the day.

Eclipses of the Moon are more common and visible from wider swathes of the globe. They also last longer. In a lunar eclipse, the full Moon swings slowly into the Earth’s shadow. This year observers in the Midwest are treated to two of these events. The first takes place in the early morning hours of April 15th. 

  Page from Peter Apian’s Cosmographicus Liber (Adler Planetarium Collection, P-279b).


Page from Peter Apian’s Cosmographicus Liber (Adler Planetarium Collection, P-279b).

A lunar eclipse officially begins when the Moon makes contact with the Earth’s partial shadow, or penumbra, but this slight darkening of the Moon’s glow is often barely visible. The real action starts around 1:00 AM as the Moon begins to slide into the Earth’s full shadow, or umbra. As the Moon enters the umbra, it does not become totally dark but instead takes on a dull reddish color as the Earth’s atmosphere scatters sunlight instead of blocking it completely. The Moon during a lunar eclipse is red for the same reason the sky at sunset is red. An observer on the Moon looking back at the Earth during the eclipse would see a sunset glow completely circling the Earth’s silhouette.

About an hour later, at around 2:00 am, the Moon is completely within the Earth’s umbra. It reaches its point of greatest darkness, at the heart of Earth’s shadow, by 2:45 am. The Moon will still be fairly high in the western sky at this time. By 3:30 am the Moon has dropped lower and begins sliding out from the Earth’s shadow. Finally, at around 4:30 am, the Moon is completely free of the umbra and returned to its normal full-moon splendor.

Both solar and lunar eclipses captured the imagination of observers and spurred early astronomical inquiry. The curve of the edge of the Earth’s shadow during lunar eclipses, for example, was used as an argument for the spherical shape of the Earth. Accurate prediction of eclipses was a test for astronomical models and, when eclipses were predicted correctly, offered strong evidence for the power of such models to explain the complex interplay of Sun, Moon, and Earth. 

This month, the Adler features a display of historic representations of eclipses from throughout history. While the awe and wonder of an eclipse can be observed first hand this April, these depictions in art and manuscripts show how that wonder has translated into inquiry and understanding over time. Events that once inspired fear were reduced to geometrical arrangements of light and shadow. And yet, as these beautiful images show, even as eclipses were explained and their cause communicated to the public or to readers of astronomical texts through pictorial representation, they lost nothing of their sense of majesty and wonder.

Don't miss our free Lunar Eclipse Viewing Party on Tuesday, April 15 from 12:01 to 4am. 

Written by Stephen Case, curatorial and research intern at the Adler Planetarium.

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