52 Chapter 1 Is Earth the Center ofthe Universe?
That’s a clever
idea, isn’t it?
Why are Mercury, Venus, Earth, and
the Sun all lined up in a straight line
in Ptolemy’s model??
This is how Ptolemy explained why
Mercury and Venus always appear
closer to the Sun.
What’s the matter?
The retrograde motion of Mars in the night sky, as observed from Earth
Of course. Since they revolve around
Earth together with the Sun, they
will always appear closer to the Sun.
The ancient Greeks believed that the Moon, the Sun, and the other planets
revolved around Earth, and Ptolemy’s geocentric theory didn’t challenge this.
However, the earlier geocentric theories from ancient Greece were unable to
explain the retrograde motion of the planets.
What do we mean by retrograde? Planets appear to move in an eastward
direction most of the time. However, planets sometimes appear to reverse directions
in the night sky, which we call retrograde motion. All planets periodically exhibit
this behavior. Ptolomy's geocentric model explained this motion as a revolution
around a point in a planet's orbit. (In reality, this apparent backward motion is
caused by Earth "lapping" the planet in a revolution around the sun.)