Relive the Discoveries of Galileo September 2014 | Page 8

Relive the Discoveries of Galileo

half-lit like the moon at its first- and thirdquarter phases - if only Venus were large
enough to show its phases, like the moon, to
the ancient philosophers.

Discovery No. 2
The Phases of Venus

Ptolemy’s model of the universe had Venus
and the Sun (indeed everything) orbiting Earth.
Meanwhile, it was well known that Venus never
strays far from the Sun, either setting or rising within
minutes or a few hours of each other. In other words,
one could never see Venus at midnight, or on the
opposite side of the sky from the sun. So it would
stand to reason that the two bodies would have to
orbit Earth at the same rate, together. But sometimes
Venus is to the east of the Sun, appearing in the west
as the Evening Star as the sun sets. At other times, it
lies to the west of the Sun, rising before it as the
Morning Star. To match observation, Ptolemy put
Venus on an epicycle, a circular path superimposed
on top of its “regular” orbit around Earth. As Venus
followed its epicycle path, it would alternatively
appear east or west of the sun.
Ptolemy’s solution would imply that an observer on
Earth would never see a fully-illuminated Venus
since the side of Venus directly facing the sun would
always be turned away from Earth. One would
always see Venus as a crescent, or at most, nearly

In the autumn of 1610, with Venus
appearing as an Evening Star, setting closely
after the Sun, Galileo turned his crude
telescope at Venus and was surprised to see
it full (position B in the illustration of the
Copernican model). Over the following
weeks, Galileo watched Venus move from
full to gibbous (slightly less than full) to half
-lit and on to crescent before moving too
close to the sun to be seen. As Venus
moved into the pre-dawn sky, Galileo
watched it pass through the same phases in reverse
order,
eventuall y
reaching full again
(position A).
Galileo,
already
familiar
with
Copernicus’s theory of
a sun-centred universe,
realized that to see a
fully-illuminated
Venus, it would have to lie on the opposite side of
the Sun from Earth (as in positions A and B). And to
see it later as a crescent, Venus would need to pass
between the Sun and Earth. Put it all together, and
you have Venus in orbit around the Sun!
This was a monumental discovery, putting the
Ptolemaic system in serious doubt. It presented a
much simpler model in which the planets trace
circles around the Sun instead of riding on epicycles
imposed on orbital circles, a questionable idea at
best.

How to relive Galileo’s discovery
In March 2010, Venus will appear as the “Evening Star”,
visible in the South-West, close to the horizon shortly after
sunset.
It will be in position B as depicted in the
Copernican model in the illustration above. Viewed through
a telescope (wait for the sun to set!) Venus appears “full”,
as a small round disk in early March.

The path of Venus
March-September,
2010, in evening
twilight, facing SW

From March through August 2010, you can observe Venus
slowly change phase from Full to half-lit while at the same
time grow in size. The reason Venus appears to grow
Image created in Starry Night Pro
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