# What is parallax good for?

## parallax

parallax, general apparent lateral displacement Δl of objects of different visual distances when changing the viewing direction by the angle

, in which a is the distance between the object plane and the projection plane (see Fig. 1). The parallax of the retinal images is the cause of that stereoscopesSee. It is also understood to mean those occurring within a stereoscopic image pair Coordinate differences between homologous, i.e. related points. A distinction is made between the Horizontal- and the Vertical parallax.

In the astronomy Parallax generally describes a method for determining the distance of celestial bodies. The absolute or primary Methods are based on geometric principles and do not require any knowledge of the physical properties of the stars or the distances to other reference bodies. This includes in particular the trigonometric parallax. Both relative or secondary Methods, the distance of at least one other star must be known. Here are the photometric, spectroscopic and Variable parallax really important.

Basis of primary Methods is the fact that all celestial bodies appear projected onto the celestial sphere. If the observation location changes its position, the projected location shifts in the sky. This parallactic shift is greater, the greater the change in position of the observer and the closer the star is (see Fig. 2). A change of location of an observer on earth results from their rotation (daily parallax), the movement of the earth around the sun (annual parallax) and the movement of the sun together with the planets in the Milky Way system (secular parallax). The daily parallax can only be measured on the sun and the moon. They are 8.8 '' or 57 'at the Earth's equator. The annual parallax, also trigonometric parallax called, is the decisive method to determine the absolute distances of stars up to a distance of about 1,000 pc. This was first reliably achieved in 1838 by the German astronomer Friedrich Wilhelm Bessel at Stern 61 Cygni. It has a parallax of 0.125 ", corresponding to a distance of 8 pc.

In the secular parallax one takes advantage of the fact that the sun covers a distance of 6 · 10 in a year8 km, corresponding to twice the diameter of the earth orbit. This shifts the projected locations of the stars. But since the stars themselves have their own motion, the motions of the sun and the star cannot be distinguished from the outset in a single star. This method is therefore applied to groups of stars whose members are roughly equidistant, but whose movements are random and therefore averaged out statistically.

The parallax can also be reversed. The parallactic displacement of a star is the same as the angle at which the connecting line of the two places on earth appear on the base line when viewed from the star. This is used with the Stern current parallax out. Here one observes a star cluster at time intervals, the members of which move through space with roughly the same direction and speed (Moving star clusters). To determine the distance, the angle between the line connecting the earth-star cluster and the direction of movement of the cluster (Vertex) as well as the proper motion and radial velocity of at least one representative cluster member. Secular parallax and stellar flow parallax reach up to about 5 kpc distance.

Both relative or secondary Methods is the photometric parallax of great importance. The absolute brightness must be used hereM. of a star, for example, be known by its color. Then results from the measured apparent brightnessm the distance r from the distance module (without absorbance): mM. = -5 + 5 lg r.