parallaxis

Parallaxis is a term used in astronomy to describe the apparent shift in the position of a star or other celestial object due to the Earth's orbit around the Sun. This phenomenon is a result of the change in the observer's perspective as the Earth moves from one point in its orbit to the opposite point six months later. The amount of shift, or parallax, can be used to calculate the distance to the star.

The concept of parallaxis is based on the principle of trigonometric parallax, which involves measuring the angle between the apparent position of the star from two different points in the Earth's orbit. The larger the parallax, the closer the star is to the Earth. This method is particularly effective for measuring distances to relatively nearby stars.

The first successful measurement of stellar parallax was made by Friedrich Bessel in 1838 for the star 61 Cygni. This achievement marked a significant milestone in astronomy, as it provided a direct method for determining the distances to stars. Today, the European Space Agency's Gaia mission is using parallaxis to create the most accurate three-dimensional map of the Milky Way galaxy.

Parallaxis is also used in other fields, such as geodesy and surveying, to measure distances and positions on the Earth's surface. In these applications, the principle is similar, but the distances involved are much smaller. The term "parallaxis" itself comes from the Greek word "parallaxis," meaning "alteration." This reflects the change in the apparent position of an object due to a change in the observer's perspective.