Heliometers

A heliometer is an astronomical instrument used to measure the angular diameter of celestial objects, particularly the Sun and planets. It was invented by Benjamin Martin in 1752. The most distinctive feature of a heliometer is its objective lens, which is split into two halves. These halves can be moved relative to each other along a diameter. By adjusting the separation of these two halves, an astronomer could bring the images of two distinct points in the sky, or two edges of a single large object, into coincidence. The amount of separation required to achieve this coincidence directly corresponds to the angular distance between the two points or the angular diameter of the object.

Historically, heliometers were crucial for measuring the apparent size of the Sun and planets with greater accuracy than was possible with simple telescopes. They were also used to determine the distances to nearby stars by measuring their parallax, the apparent shift in position due to Earth's orbital motion. Famous astronomers like Friedrich Bessel used heliometers to make precise measurements that contributed to our understanding of stellar distances. While modern instruments like CCD cameras and digital imaging techniques have largely superseded the heliometer for many applications, its historical significance in the development of astronomical measurement is undeniable. The principle of splitting an optical element to measure displacement remains a concept explored in various optical instruments.