Exploring Galileo’s Moon

Use your telescope to follow in the great astronomer’s footsteps.

Galileo Moon artGalileo first trained his telescope on the Moon in the autumn of 1609, but no one knows the exact date. His first lunar sketches appeared in the pamphlet, Sidereus Nuncius, published the following year. However, dating these drawings is a tricky exercise fraught with difficulty since Galileo failed to note when he made his observations. However, a little detective work and time on the computer makes it seem likely that the rendering shown below depicts the first-quarter Moon as Galileo saw it on the evening of December 3, 1609.

Galileo’s drawings shows few features — certainly much less than what you can see even in 10×50 binoculars. Although his telescope was crude by today’s standards, its greatest limitations were its low power (typically 20×) and narrow field of view (about 14 arc minutes, or half a Moon diameter).

Galileo Moon 1609

This photo of the Moon matches the phase and libration angle on the night in 1609 when Galileo made this telescopic sketch of the Moon’s “rough and unequal” surface. Photo courtesy Gary Seronik

The prominent feature along the terminator in Galileo’s sketch is most likely Albategnius. This 114-km-diameter crater is a striking sight especially when the terminator crosses it. That was the case when Galileo observed the crater and on the night the photograph above was taken. But why is Albategnius rendered so large in his drawing? It’s difficult to say with certainty what Galileo’s intentions were, but possibly he was less concerned with an accurate representation of the lunar disc than he was with conveying a general impression of the Moon’s “rough and unequal” surface. It’s also possible that his sketch doesn’t show the entire lunar disc. And given that his telescope only allowed him to observe a portion of the Moon at any one time, accurately placing features was no doubt something of a challenge.

When you’re out with your scope, why not attempt your own full-disc sketch? And as you put pencil to paper, consider that you’re viewing exactly the same lunar landscape Galileo saw more than 400 years ago — no craters big enough to show up in telescopes have appeared since then. Such is the static nature of our nearest neighbour. But given the hectic pace of modern life, perhaps a modest amount of comfort can be extracted from viewing Galileo’s Moon.

Categories: Moon and Planets
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