Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to foreshortening, nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances.
In this photograph, the Sun is visible above the top of the streetlight. In the reflection on the water, the Sun appears in line with the streetlight because the virtual image is formed from a different viewing position.
Parallax measurements may be an important clue to understanding three of the universe's most elusive components: dark matter, dark energy and neutrinos.
Hubble Space Telescope precision stellar distance measurement has been extended 10 times further into the Milky Way.
Contax III rangefinder camera with macro photography setting. Because the viewfinder is on top of the lens and near the subject, goggles are fitted in front of the rangefinder and a dedicated viewfinder is installed to compensate for parallax.
Stellar parallax is the apparent shift of position (parallax) of any nearby star against the background of distant stars. By extension, it is a method for determining the distance to the star through trigonometry, the stellar parallax method. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).
The Dollond heliometer of the late 1700s
Bessel's heliometer
The split lens of the Bamberg Heliometer (late 19th century)
Diagram of a heliometer from the 1911 Encyclopædia Britannica, which would be a view looking towards the split lens of a heliometer
