Why do observers in New York State looking due south at the night sky see a different group of constellations from what they had seen six months earlier?
1 Answer
The observers are on the opposite side of the sun, so they are seeing stars that were previously obscured by the sun's light.
Explanation:
Objects in space are constantly moving. The moon is orbiting the Earth, which in turn is orbiting the sun, which is orbiting the center of the Milky Way galaxy. As such, our perspective of the universe is always changing.
At any given time we can see about half of the sky. The Earth blocks the other half. As the Earth rotates, however, that view of the sky changes. For someone standing on the equator, one full rotation of the Earth will show the entire celestial sphere. For someone standing on the north pole, however, the visible portion of the sky will only appear to rotate. New York is somewhere in between, so the south part of the sky will change and the north part will always be visible.
As the Earth rotates around its axis, however, the sun becomes visible, and its light outshines all of the other stars in the sky. The other constellations are still up, they are just up during the day when they cannot be seen. But the Earth is also orbiting the sun, so from night to night the part of the sky that is up during the day will slowly move around to the night side.
After about six months, the constellations that were previously obscured by the sun will be visible, and the ones that were visible will be obscured by the sun.
