Relive the Discoveries of Galileo September 2014 | Page 6

Relive the Discoveries of Galileo

Discovery No. 1
The Mountains, Valleys, and Craters of the Moon
When Galileo aimed his telescope at the
Moon, he expected to see a smooth surface. Since
Aristotle’s time in ancient Greece, it was widely
believed that all the heavens, that is, everything from
the Moon and beyond, were perfect and never
changing spheres. Only on Earth would perfection be
ruined with mountains and valleys. Only Earth,
where mere mortals lived, was imperfect, changing,
and doomed, while the Heavenly bodies were perfect,
never changing, and immortal.
Instead, Galileo saw what looked like mountain
ranges, mares (Latin for “seas”), valleys, and the
scars of many craters. On consecutive weeks in
November and December 1609, as the Moon passed
through its phases and different features were
highlighted by long shadows at the day-night
terminator, Galileo sketched some of the mountains
and valleys he was able to discern through his small
telescope.
That the moon was not a perfect sphere, but instead
Earth-like, profoundly challenged conventional
wisdom that the heavens were perfect and separate
from Earth. Galileo’s discovery, by showing that the
Moon has features like those found on Earth, served
to unite Earth with the Moon, and by extension, with
the rest of the universe. It opened the door to the
possibility that one should not divide the universe

Galileo’s sketches of the Moon in November 1609.

between Earth and everything else, but instead
consider our planet as part of the universe, subject to
the same laws of nature that apply everywhere.
Galileo’s discovery connected our Earth to the rest of
the universe.

How to relive Galileo’s discovery
Try your hand at sketching the moon like Galileo did,
while observing it through a telescope. Choose a
low-power eyepiece (try a 26 mm or higher eyepiece
- the higher the “mm”, the less it magnifies) so you
can see the whole moon. Try sketching the moon at
three phases:
crescent
firstwaxing

quarter

gibbous

Tips:
1 Slip cardboard or stiff paperboard (like a collapsed
cereal box) behind page 7 so you can sketch.
2 Use a pencil with an eraser, not a pen.
3 Start by slowly drawing the terminator: the line
separating the sunlit side from the dark. The terminator is not smooth. Where it comes to a crater,
the crater’s floor remains in darkness, appearing
-6-

4

5

6

7

like a dark bay, taking a bite out of the sunlit side.
On the sunlit side close to the terminator, look for
craters still in partial shadow and add them to your
drawing as dark spots or curves.
On the dark side near the terminator, look for
mountain peaks reaching up into sunlight. Circle
these before filling in the black.
Next, outline the smooth dark “seas” appearing on
the lighted side. Rub the pencil tip on its side to fill
the sea and smear with a fingertip.
Don’t worry about your finished drawing. Erase
and re-draw features as necessary. The purpose
of sketching is to make you carefully examine what
you see in the telescope and improve your observing skill.

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