What is known a parallax ?
Both answers are correct, but there’s one more important concept: Parallax refers to the fact that an object’s position in the sky appears to shift when you change your location, but how do you change your location in the universe? When astronomers talk about parallax, they usually mean viewing a star from opposite sides of the earth’s orbit (separated by 186 million miles). This is the oldest method of measuring positions to stars, but the stars are so incredibly far away that it works only for the closest stars. (Space-based measurements of parallax in recent years have improved measurements of stellar distances.) For objects in the solar system, astronomers talk about the parallax from observers in different locations on the earth. If two observers in different locations — say, New York and California — look at the moon at the same time, they’ll see it in a slightly different position (relative to the background stars). They are 2500 miles apart, and the moon is about 238,000 mile
