Parallax is the word used to describe the apparent shift in the position of a nearby object relative to the distant background when observing the object from two offset positions.
If you close one eye and hold up your finger so that its tip appears to be directly over the top of some distant object on the horizon, then quickly close that eye and open the other one, your finger’s position relative to the distant object will seem to have jumped. This is a demonstration of parallax.
Parallax is also exhibited by stars that are closest to the Earth, which seem to shift their positions slightly, relative to the background distant stars, when observed from different points on the Earth’s orbit.
Since even the closest stars are a long way away, this apparent shift in position is only small, but it can be used to directly measure how far a star is from the Earth. As the size and shape of the Earth’s orbit around the Sun is known, measurements of the star’s observed position in the sky at different points in our orbit can be used to determine the distance to the star using trigonometry.
Annual parallax measurements of stars are given as half the angular distance across the sky by which the star appears to shift in position throughout the year.
The nearest star to the Sun, Proxima Centauri, has a parallax of around 0.7687 arcsec.
A parsec is a unit of distance used by astronomers, which is equivalent to around 3.26 light-years. The word parsec is a contraction of the words “parallax of one arcsecond”, since a parsec is defined as the distance from Earth that a star would need to be in order to exhibit an annual parallax of 1 arcsecond.
Since the Moon is much closer to the Earth than the stars, it exhibits a parallax even with respect to different positions on the Earth’s surface. This “diurnal” parallax, is usually quoted as the angular shift in the Moon’s apparent position relative to the background stars from two different points on the Earth’s surface, where the Moon appears to be directly overhead and where the Moon appears to be on the horizon.
This parallax can be as large as one second of arc, roughly twice the apparent angular size of the Moon’s disc. This lunar parallax is the reason that a total solar eclipse is only visible at certain locations on the Earth’s surface.