The Venus Clue

If you happen to be awake very early, Jupiter and Venus are close together in the eastern sky before sunrise. These two planets are brighter than any star, brighter than any objects in the sky except the sun and moon. Venus is the brighter of the two.

Until several centuries ago the night sky was very confusing. Everything seemed to move around the earth but there were several stars that appeared to "wander" among the other stars. These  wanderers became known as "planets", which means "wanderer". The planets all followed the same path, known as the ecliptic. The twelve constellations through which the planets pass are known as the Zodiac.

To make it more confusing one of these "wanderers" moved very differently than the others. The other planets displayed retrograde motion. They were moving among the other stars when they suddenly reversed direction and moved backwards. The one that moved differently, the brightest of them all, would appear in the evening sky after sunset, and then switch to appearing in the morning sky before sunrise. This "wanderer " was called "Venus ".

With some insight, some "outside the box" thinking, and a little bit of geometry, there was a simple solution. These "wanderers" were planets like the earth. Everything did not revolve around the earth, it revolved around the sun. The planets were at different distances from the sun, and the closer they were the more quickly they moved around it.

The planets that underwent retrograde motion were further from the sun than the earth and were thus moving around the sun more slowly. The apparent retrograde motion is as the earth passes them in it's orbit, like passing a car on the highway so that it appears to be moving backward.

Venus is closer to the sun than the earth, and thus revolves around it faster. We can tell that Venus is closer to the sun than earth because we can never see it in the middle of the night, only after sunset or before sunrise. The earth rotates eastward and so would logically revolve around the sun in the same direction. When Venus is in the western sky, after sunset, it is "catching up" to the earth in it's orbit around the sun. When Venus has overtaken the earth it appears in the eastern sky, just before sunrise.

In the following diagram, the green circle represents the earth and the blue circles represent Venus. The earth is seen as if looking down from above the north pole, which is represented by the black dot. The day side of the earth is indicated by the D, and the night side by the N.

The earth rotates eastward, as shown by the inner arrows. Point 1 is the point of sunrise and Point 2 of sunset. The earth revolves around the sun in the same direction as it rotates, as indicated by the outer arrows. If you were watching the sunrise, the direction overhead is the direction that the earth is moving in it's orbit through space. If you were watching the sunset, the direction overhead is the direction that the earth is coming from in it's orbit.

Venus, represented by the blue circles, is closer to the sun than the earth so that it moves faster in it's orbit. Since it is closer to the sun it appears only after sunset or before sunrise, it is never seen in the middle of the night. When at Position A, Venus is seen after sunset. Remember that Point 2 represents the sunset position. When Venus overtakes the earth, at Position B, it appears before sunrise.

I am certain that Venus provided the most important clue that the earth revolves, along with the other planets, around the sun.