UGB 240M Astronomy Assignment 1
There are two reasons supporting the astronomers’ suggestion.
First, it is because Venus can mainly be seen at dawn and dust. It cannot be seen at midnight, unlike the moon, which can be seen through the night. When at night, the Earth turns from the sun and Venus, thus we cannot see Venus at night. When at dawn and dust, the sky is not brightened up yet and the Earth is turning to the sun again, thus we can see Venus reflecting lights from the sun.
Second, Venus appears to be “changing its size” over time but the moon doesn’t when observed from the Earth. This difference can be explained by the fact that Venus orbits around the Sun and its distance from the Earth varies along its elliptical orbit while the moon orbits round the Earth and does not vary much in its distance from the Earth. Astronomy Structure question
The astronomers conclude that the Venus must orbit around the Sun but not the Earth mainly because of two reasons. Firstly, it’s observed that the Venus can only be visible at dawns and dusts because at those times, the Venus gets the reflected lights from the Sun, and therefore, we can see it. However, during the nights, the Earth doesn’t face the Sun and the Venus. So the Venus is invisible at nights. On the other hand, it’s rather different from the moon. As it’s concluded that the moon must orbit around the Earth, therefore, we can see the moon at nights.
Secondly, it’s observed that the size of Venus is changing in different time. It’s because the distance between the Venus and the Earth varies they orbit the Sun respectively. Thus, it leads to the change of Venus’s size. Meanwhile, since the moon orbits around the Earth and so its distance from the Earth remains stable. Therefore, the size of the moon is almost the same all the time.
From the above reasons, the astronomers don’t say that the Venus orbits around the Earth as well as the moon.