Introduction to Parallax

Difference in the apparent position of an object viewed along two different lines of sight.

Parallax is a fundamental concept in astronomy that has been used for centuries to measure distances to celestial objects. It is based on the apparent shift in the position of an object when viewed from two different points. This article will provide an overview of the concept of parallax, its historical background, and its basic principles.

Definition of Parallax

Parallax is the apparent shift in the position of an object when viewed from two different points. This shift is due to the change in the observer's line of sight and is most noticeable when the observer and the observed object are at significantly different distances from the background against which the object is viewed.

In astronomy, parallax is used to measure the distances to nearby stars. The apparent shift in a star's position is observed by looking at the star from two different points in Earth's orbit around the Sun.

Historical Background of Parallax

The concept of parallax has been known since ancient times. The Greek astronomer Hipparchus is credited with being the first to use parallax to estimate the distance to the Moon in the 2nd century BC. However, the small size of stellar parallax made it impossible to detect until the 19th century.

The first successful measurement of stellar parallax was made by Friedrich Bessel in 1838. He measured the parallax of the star 61 Cygni and calculated its distance to be about 10.4 light-years, remarkably close to the current estimate of 11.4 light-years.

Basic Principles of Parallax

The basic principle of parallax is triangulation. By observing an object from two different points, a right triangle can be formed with the object at the apex and the two observation points forming the base. The parallax angle is then the angle at the object formed by lines drawn to the two observation points.

In the case of stellar parallax, the two observation points are locations in Earth's orbit around the Sun six months apart. The distance from the Earth to the Sun (1 Astronomical Unit) forms the base of the triangle, and the star forms the apex. The parallax angle is then half the total shift in the star's apparent position over the six months.

In conclusion, parallax is a fundamental concept in astronomy that allows us to measure distances to nearby stars. Its discovery marked a significant milestone in our understanding of the universe and our place within it.