# Spherical astronomy

The **spherical astronomy** treats the two-dimensional measurement of the night sky and the associated calculations, star positions and cosmic reference systems . One ignores the fact that the various celestial bodies are very different distances away, and treats the stars like points on an imaginary unit sphere (the " celestial sphere ") that surrounds the earth. The third dimension is the radius of this unit sphere.

## Basis of astronomy

The most important results of spherical astronomical measurements - which essentially cover the geometric sub-area of astronomy - are the celestial *coordinates* of *right ascension and declination of* the stars and their changes over time. These measurements of locations and velocities are the basis of position astronomy and are closely related to the methods of astrometry and trigonometry .

This form of astronomy was the only possible well into modern times, as the determination of the distance from celestial bodies and their radiation was hardly possible before the 18th century. With the invention of the telescope , spherical astronomy took a hitherto unimaginable boom. It increased its measurement accuracy from around 0.02 ° ( open-eyed ) to *ten thousand times* (around 0.01 ") and has also been used on very faint stars and distant galaxies for around 100 years .

Spherical astronomy thus became the basis of all astronomical advances - especially in celestial mechanics - and for our current knowledge of the structure of the universe . The increased accuracy of the direction measurement enabled the astronomers to also determine the distance of distant “ fixed stars ” (for the first time in 1838 by Bessel's measurement of an annual star parallax ).

Until about 1870, when, after the invention of photography and spectral analysis the astrophysics began to usher made astrometry and spherical astronomy from the majority of scientific astronomy.

## Development since around 1900

The reorientation of astronomy from geometric to increasingly physical methods was tantamount to a revolution in celestial science as a whole, which was reflected in its popular astronomy and also in the construction of many new observatories - in Central Europe e.g. B. the University Observatory Vienna and the Astrophysical Institute Potsdam . However, between 1880 and 1920, strict attention was paid to ensuring that positional astronomy also remained possible - for example with the development of high-precision meridian circles and zenith telescopes . The *theoretical* part of the department dealt with the definition of ever more precise reference systems - which ultimately became the basis of space travel - and from 1900 with the irregularities of the earth's rotation (see also IPMS ) and the polar movement .

Nevertheless, between 1950 and 1975 only less than a fifth of astronomers were active in geometric methods, but since the development of satellite geodesy many geodesists have been working on related topics. This changed rapidly around 1990 when the production of optoelectronic sensors became cheaper and the potential of CCD became fully apparent. In the meantime, there are fully automatic meridian circles and astrometry satellites and a further increase in measurement accuracy, which since Hipparcos has been up to 0.001 ". With radio interferometry (see VLBI ), coordinates can be determined much more precisely and changes in the earth down to the millimeter range

- detailed studies in the field of geophysics as well
- the body and orbits in the solar system ,
- detailed movements in our galaxy ,
- also the discovery of hundreds of exoplanets

and further improvements at least tenfold through future satellites and space-based measurement campaigns such as GAIA , Galileo and others.

## See also

- Astronomy , Cosmic Geodesy
- Star catalogs , astrometric surveys
- Astronomical coordinate systems

## Web links

**Wikibooks: Astronomical Calculations for Amateurs / Positional Astronomy**- Learning and Teaching Materials

## literature

- RM Green:
*Spherical Astronomy*, Cambridge University Press, Cambridge 1985, ISBN 0-521-23988-5 and ISBN 0-521-31779-7 - Oliver Montenbruck:
*Fundamentals of the ephemeris calculation*, spectrum Akademischer Verlag, Heidelberg 2005, ISBN 3-8274-1602-7 -
Albert Schödlbauer :
*Geodetic Astronomy. Basics and concepts.*De Gruyter, Berlin / New York 2000, ISBN 3-11-015148-0