Why does stellar parallax exist?

1 Answer

it exists because even though the distances between stellar objects is large, so is the distance covered by Earth's orbit and the changing view of an observed object.


Parallax is the phenomenon of seeing an object apparently move against its background due only to the movement of the observer.

When someone takes a picture of the Moon from the North Pole, it shows the Moon against one set of background stars. Another picture taken at the same time from the South Pole will show the Moon against a different set of stars - comparing the 2 photos will show the Moon moving "up" and "down" against the background stars. This is parallax.

When we start talking about objects that are very far away, like stars, we assume that the distance is so great that there won't be a shifting of the image of photos taken from different points on Earth or at different times. But that's not the case - stars that are closer to Earth will appear to move (or move more) against the background of stars that are tremendously far away. Taking a photo of a field of sky in January and then again in July will show it most because the Earth will have moved from one side of its orbit around the Sun to the opposite side - a large distance in its own right.

As the distances increase between the Earth and the star being observed, the parallax will decrease until at some point it is undetectable.