Lunar eclipse observations throughout the ages

The total lunar eclipse on June 15th provides us with an unprecedented opportunity to advance our knowledge of the Moon's surface. This is not the first time that science has relied upon observations of a lunar eclipse; eclipse observations have been used throughout the ages as a way to formulate, test and prove theories about our solar system. Here are just a few examples:

Ancient Greek Astronomy

Aristotle

Aristotle (384-322 BC)

In the 4th century BC, Aristotle used his observations of lunar eclipses as proof that the Earth was spherical. He had noticed that during lunar eclipses, the Earth always casts a circular shadow on the Moon, and argued that the Earth must therefore be spherical because a sphere is the only shape to cast a circular shadow from every angle (even a circular disc casts an elongated shadow from certain angles).


Aristarchus

Aristarchus of Samos (circa 310-230 BC)

Barely a century later, Aristarchus correctly calculated the Moon's distance from the duration of a lunar eclipse. Observing that Earth's shadow was around twice the size of the Moon, he also deduced that the diameter of the Moon was half that of the Earth. Although his assumption wasn't quite correct - the Moon's diameter is actually one third of Earth's (see this explanation), his conclusions are astounding given the fact that he didn't have a telescope, and trigonometry had not yet been discovered. It's even more impressive when you consider that Artistarchus' observations led him to be the first to proclaim that the Earth orbited the Sun and not vice versa.


Hipparchos

Hipparchos (circa 190-120 BC)

During a lunar eclipse, the center of the Earth's shadow is at the point on the celestial sphere which is exactly opposite that of the Sun. In the 2nd century BC, Hipparchos used this knowledge to determine the position of the Sun relative to the star Spica. When he compared his eclipse observations with observations of lunar eclipses from more than 150 years before, he noted that the position had changed by 2 degrees. He had discovered the precession of the equinoxes - a 26,000 year cycle caused by the wobble of the Earth on its axis.


Modern Planetary Science

The first infrared observations of the Moon during a lunar eclipse were carried out in 1965. During this time, scientists used recently developed infrared technology to scan the entire nearside of the Moon from Earth. Upon doing so they observed that there were hundreds of 'hot-spots' - regions that stayed warmer as the surrounding landscape cooled off. They concluded that this was due to enhanced surface roughness in these regions, and the presence of large numbers of rocks - findings which were later confirmed by the Lunar Surveyor and Lunar Orbiter missions.

To find out how Diviner is helping to advance our knowledge of the Moon during the eclipse on June 15 2011, read this article.



The Diviner Lunar Radiometer Experiment is funded by NASA through the Lunar Reconnaissance Orbiter Project at NASA Goddard.
The instrument is built and operated by JPL.

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