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Positional Astronomy

Particle Physics and Astronomy Research Council

Royal Greenwich Observatory

Information Leaflet No. 16: 'Positional Astronomy'
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Why Do We Want To Measure the Positions of Stars in the Sky?

Three hundred years ago the Royal Observatory was founded at Greenwich, not to do astrophysical research, but to solve a very practical problem that was of considerable importance to England's life as a trading nation. That problem was determining the Longitude of a ship at sea so that it could be navigated properly. This needed accurate measurements of the positions of the stars as landmarks. As these positions were not known, the Royal Observatory started making accurate measurements of the stars, the Sun, the Moon, and the planets.

A little later Sir Isaac Newton realised the scientific value of these observations, and encouraged the observatory to continue them in order to check his famous law of gravity as it applied to the Moon and planets.
After many years it became possible to combine the observations over a period of a century or so, in order to show up the small individual motions of the stars relative to one another. Nowadays it is possible to work out, from these motions, various theories of how old the stars are, and how the galaxy of stars evolved into its present state and shape.
In the modern era, besides the obvious need for astronomers to know where in the sky to point their telescopes so as to be able to observe faint galaxies and quasars, accurate star positions are needed to keep artificial satellites pointing the right way, and to help in the navigation of spacecraft such as Giotto, which successfully acquired such dramatic pictures of Halley's comet in 1986.

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.
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, etc., have used considerable ingenuity in removing from the observations the effects of the motion of the Earth and the distortion caused by its atmosphere.

The motion of the Earth is complex.
Its shape is a flattened sphere whose equator is tilted at 23 degrees to the plane of its orbit around the Sun. As a result, the gravitational attraction of the Sun causes the Earth to wobble like a spinning top, with a period of 26,000 years (precession).
The Moon also causes smaller wobbles having a whole range of periods, from days to 18.6 years (nutation).
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).
Finally, the distorting effect of the atmosphere has to be considered, although this can be obviated by making the measurements from a satellite, such as the European Space Agency's satellite Hipparcos.

As the Earth moves around the Sun, the direction to the stars changes slightly. The size of this effect (parallax) is greatest for the stars nearby and decreases with distance. The velocity of the Earth in its orbit, and to a lesser extent, its rotation on its axis, combined with the finite velocity of light, cause the apparent direction of incoming light from the stars to vary systematically in direction (aberration).

True Directions in Space:

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.
Note that here we are only considering the angular direction and motion at right angles to the line of sight. The determination of distances and motion along the line of sight is another branch of astronomy, although purely angular measurements within the Solar System, combined with a knowledge of the masses of the planets, etc., can lead to a complete description of their orbits, including distance.

Cross-Identification of Objects on Maps at Different Wavelengths:

Maps of the sky at various wavelengths, made from different parts of the Earth, and at different parts of its orbit, need to be reduced to a common point of reference from which they can be compared and scientific deductions made.
Once this has been done, cross-identification can be made of objects on maps of the sky made at X-ray, optical, infrared, and radio wavelengths. In this manner, for example, a star which radiates at radio as well as optical wavelengths can be identified as one and the same star; radio jets can be placed relative to the bright nuclei of active galaxies, and X-rays can be found coming from the centres of dense agglomerates of stars (globular clusters) where black holes may lurk.

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. These motions across the sky, combined with their motions along the line of sight (from the Doppler shift of spectral lines) give their space motions. With knowledge of their distances, astronomers can build up a 3-dimensional picture of the distribution and velocities of stars in the Galaxy. From such a picture, they have found relationships between the chemical composition of stars and their positions and velocities within the Galaxy.

Solar System Exploration:

Repeated measurement of the positions of objects in the Solar System, reduced to the centre of the Sun as described above, 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.
Another notable success in this area was the fly-by of asteroids Gaspra (in 1992) and Ida (1993) by the NASA spacecraft Galileo on its way to Jupiter. Accurate predictions of the positions of these asteroids were required so that Galileo could point its cameras in the right direction as it sped past. In the event, the most detailed close-up pictures of asteroids ever obtained were secured.

ARVAL Notes:
See Galileo To Jupiter in the Image Gallery.
See Voyager Mission (Voyager 1 & 2: 1977-2020? in NASA-JPL)

The study of the recent collisions of Comet Shoemaker-Levy 9 with Jupiter depended on accurate measurements of the positions of each of the fragments, and of Jupiter. 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.

Techniques of Measurement in Positional Astronomy:

Astronomers use many different techniques to map the heavens. The techniques are dependent on the wavelength at which the observations are made.
At optical wavelengths they use specially designed telescopes to ensure stability of pointing direction, such as the Carlsberg Astrometric Telescope at the Observatorio Roque de los Muchachos on the island of La Palma in the Canaries.
Special telescopes for photographing the entire sky are also operated by Britain, such as the UK Schmidt Telescope in Australia.
The greatest drawback to ground-based telescopes is the distorting effect of the atmosphere, and occasionally this is overcome by launching a satellite above the atmosphere. Britain has participated in the European Space Agency's satellite Hipparcos that has mapped the sky with unprecedented accuracy at optical wavelengths.

Great advances have been made in the accuracy of mapping at radio wavelengths using the technique of Very-Long-Baseline-Interferometry (VLBI), where the baselines are intercontinental. Indeed, 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, star-like galaxies called quasars. Apart from the great accuracy of their radio positions (measured in milliarcseconds), they have the advantage over stars of not having any angular motion because they are so remote.

X-rays, and to some extent infrared rays, do not penetrate the Earth's atmosphere, so mapping the sky at these wavelengths has to be undertaken from orbiting satellites. Britain has participated in building instruments for measuring positions with the infrared satellite ROSAT and the X-ray satellite IRAS.