Posts Tagged ‘astrometry’


The branch of astronomy dealing with the movements and positions of celestial bodies.

Astrometry dates back to the earliest days of astronomy when the first star catalogues were being produced, e.g. that of Hipparchus in 190BC. Early measurements were probably made using cross-staffs to measure the relative positions of stars from one-another and from features on the horizon. As time progressed, more sophisticated instruments were used, such as the astrolabe.

The same principles are still used in modern astronomy but have become very precise and can measure the wobbles in the movements of stars that could indicate the presence of extrasolar planets or to find astrometric binary star systems.

Astrometry also includes the measurement of parallax. If you observe an object from two widely spaced locations, you will measure a slight difference in position. From the annular differences in position and knowing the distance between the two locations of observation, the distance of stars can be determined. That was taken to a new level with the Hipparcos satellite launched by the ESA. Early parallax measurements were limited to relatively close stars, however, the distances of several cepheid variable stars was measured. Thes can then be used as ‘standard candles’ when seen in distant galaxies to give a reasonable estimate of their distances. Further refinement to astrometric determination of distant objects came with the advent of interferometry.

As with all measurements of the extremely large and extremely small, astrometric measurements require careful error correction. As knowledge and instrumentation improves, distances and speeds of star movements are constantly being refined and it is believed that it is now possible to see the peturbations in stellar motion caused by planets not much more massive than the Earth.

Astrometric Binary

A binary star system in which the fainter component can not be seen

In an astrometric binary, the secondary component is invisible to the eye but is observable from the gravitational effects on the proper motion of the brighter companion. The components of a binary system revolve around a common centre of mass. As the pair move through space, in an astrometric binary, the visible star will be seen to move with a wobble. The path through space will be a ‘wave’ shape instead of a straight path.

There can be various reasons why the secondary component might not be seen. It may be too far away for the light to be detected, it could be a very cold star; the two stars could be too close and too far away to be resolved by telescopes or the primary may be much brighter and simply drown out the light from the secondary.

This method of looking at the movements of stars has been applied to the search for extrasolar planets. As instruments become ever more sensitive, smaller and smaller perturbations can be detected meaning smaller and smaller orbiting masses can be ‘seen’.

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