The challenges of measuring everything from a fast-moving, wobbly platform through a haze.

Nowadays besides the obvious need for astronomers to know where in the sky to point their telescopes, accurate star positions are needed to keep artificial satellites pointing the right way and to help in the navigation of spacecraft.

The Earth is a very wobbly platform

The positions of stars are usually measured from the Earth's surface, and occasionally from satellites orbiting the Earth. The Earth is not a very stable platform from which to make measurements of directions in space because it wobbles on its axis, revolves around the Sun, and follows the Sun, as a member of the Solar System, on its journey around the Galaxy.

Besides these wobbles, there are more complicated gravitational effects due to the fact that the Earth is not rigid and deforms under the attraction of the Sun and Moon (tidal deformation). Added to this, the Earth's atmosphere bends the rays of light from stars, such that the positions of stars as seen from the Earth's surface are distorted in a systematic way.

Astronomers measuring the positions of stars, planets and other objects have used considerable ingenuity in removing from the observations the effects of the motion of the Earth and the distortion caused by its atmosphere.

Set the controls from the heart of the Sun

When all the effects above have been removed from observations astronomers find the true directions of the stars and planets as they would be if viewed from the centre of the Sun.

Motions of stars

The small motions of individual stars on the sky (proper motions) are determined from accurate mapping of their positions at intervals of time stretching from a few years to decades. With a knowledge of their distances, astronomers can build up a 3-dimensional picture of the distribution and velocities of stars in the Galaxy.

Solar System exploration

Repeated measurement of the positions of objects in the Solar System lead to the accurate prediction of their orbits, which is essential in planning space missions such as the famous Voyager I and II trips to the outer parts of the Solar System. A notable success closer to the Earth was the encounter of the European Space Agency's spacecraft Giotto with Halley's comet when it returned to the inner Solar System in 1986/87 after a period of 76 years. Giotto passed very close to Comet Halley and provided a wealth of information on the structure and composition of the comet.

Global doom alert system

Similar measurements are needed to search for and track small bodies in the Solar System whose paths make it possible for them to come close to the Earth. The collision of a body even 100 metres across could have a devastating effect around its collision site. Bodies bigger than that would represent disasters on a global scale.

Measurement techniques

Astronomers use many different techniques to map the heavens. At optical wavelengths they use specially designed telescopes to ensure stability of pointing direction, such as the Carlsberg Astrometric Telescope in the Canaries. Special telescopes for photographing the entire sky are operated by Britain such as the UK Schmidt Telescope in Australia.

Britain has participated in the European Space Agency's satellite Hipparcos, which has mapped the sky with unprecedented accuracy at optical wavelengths. Satellites are also used to map the stars through capturing infrared and X-rays.

Great advances have been made in the accuracy of mapping at radio wavelengths using the technique of Very-Long-Baseline-Interferometry (VLBI). This technique has become so highly developed that the primary reference points for mapping the sky are no longer the bright stars, but the faint, remote, starlike galaxies (quasars). Apart from the great accuracy of their radio positions, they have the advantage over stars of not having any angular motion because they are so remote.

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