Encyclopedia of Astronomy And Astrophysics

the literaturn of astrometric surveys may

even be carried out over the web (se. Although the term survey in

astronomy is quite often applied to observational programs,

some recent astrometric surveys han of astrometric surveys may

even be carried out over the web (sve made use

of the material published in the avain of astrometric surveys may

even be carried out over the web (slable literature. What

is more, the new generation of astron of astrometric surveys may

even be carried out over the web (sn of astrometric surveys may

even be carried out over the web (smetric surveys may

even be carried out over the web (sn of astrometric surveys may

even be carried out over the web (see ‘The new millennium’,

below). Therefore, when speaking about surveys,

we may refer to observational astrometric surveys, bibliographic

astrometric surveys and the coming computer

astrometric surveys.

When can a survey be called astron of astrometric surveys may

even be carried out over the web (smetric? In other

words, what are the main characteristics of an astrometric

survey? To answer these questionsn of astrometric surveys may

even be carried out over the web (s, the first point to

consider is what is astrometry? Astrn of astrometric surveys may

even be carried out over the web (sometry is that part of

astronomy dealing with the positions (absolute or relative),

motions and trigonometric distances of celestial

objects (see also ASTRONOMY: TELESCOPES AND TECHNIQUES).

Each of these parameters has its on of astrometric surveys may

even be carried out over the web (swn importance in the

study of our stellar environment.

What should be the accuracy of a given astrometric

parameter? It is important to define n of astrometric surveys may

even be carried out over the web (sthe level of accuracy

because that value will, to some extent, determine the

quality of the survey. For example, no one would attempt

to carry out an astrometric survey tn of astrometric surveys may

even be carried out over the web (soday with the accuracy

level of the Durchmusterungs (Bonner, Córdoba or

Cape) since such a level would be extremely low for

today’s standards; however, the importance of these surveys

at the time they were published is un of astrometric surveys may

even be carried out over the web (sndeniable.

What is an astrometric survey? Putting all the above

considerations together, an astrometric survey can be

defined as a search oriented to findn of astrometric surveys may

even be carried out over the web (sthose celestial objects

meeting a given accuracy in their absolute or relative

positions, motions, or trigonometric distances. Some of

these surveys have been or are bein of astrometric surveys may

even be carried out over the web (sng conducted with

ground-based meridian circles, ASTROLABES or astrographic

telescopes, while others—already completed or

planned for the future—have used n of astrometric surveys may

even be carried out over the web (sor will be using

space-based instruments.

Brief historical account of astrometric surveys

The compilation of catalogs includinn of astrometric surveys may

even be carried out over the web (sg positions and

PROPER MOTIONS is among the best-known activities in

astrometry. In the early beginnings of astronomy, providing

positions of celestial bodies was a rn of astrometric surveys may

even be carried out over the web (seal necessity for

many of the ancient cultures. With roots dating back to

the pre-Greek civilization, astrometry provided the information

for computing solar and lunar eclipsn of astrometric surveys may

even be carried out over the web (ses and, more

importantly, for the determination ofn of astrometric surveys may

even be carried out over the web (stime.

Thanks to the positions obtained by his predecessors

and his own observations, HIPPARCn of astrometric surveys may

even be carried out over the web (sHUS was able to identify

the precession of the Earth’s axis of rotation. It was

also thanks to old and new astrometric catalogs that

Edmund HALLEY introduced—in 1718—the proper

motion of the stars. Some years laten of astrometric surveys may

even be carried out over the web (sr, in 1802, HERSCHEL

noted the relative orbital motion for the two components

of Castor, marking the beginning of double-star astrometry.

The next landmark appeared in 1838, when Friedrich

Wilhelm BESSEL was able to determine the PARALLAX of 61

Cygni. The history of astrometric catalogs—taking the

detector used in the observational process as a parameter—

could be divided in three different pn of astrometric surveys may

even be carried out over the web (seriods: visual

(with or without an instrument), photographic and CCD

based (Earth- or space-based instruments). With regard

to the epochs in which the different detectors were

employed, it could be said that, in general, the unaided

human eye was used as a detector n of astrometric surveys may

even be carried out over the web (sfrom the very beginning

of astronomy until the year 1610, when GALILEO

GALILEI started to use the telescope during his observations.

After 1610, meridian circles or some other types of

optical instruments were used to aidn of astrometric surveys may

even be carried out over the web (sthe human eye in

most of the astrometric surveys. This approach lasted for

over 300 years.

One of the first catalogs ever compiled was prepared

by Hipparchus. It was completed aron of astrometric surveys may

even be carried out over the web (sund 129 BC (there

has been a debate concerning the number of objects listed

by Hipparchus: some historians have agreed that

there were 1080 stars while others consider that there

were only 850 stars). Hipparchus’ cn of astrometric surveys may

even be carried out over the web (satalog was included

in Ptolemy’s Almagest, the biggest compendium of

astronomy of that time, which was in use for about 14

centuries. Previous compilations by Greek and Chinese

astronomers date back to roughly 3n of astrometric surveys may

even be carried out over the web (s00 BC and even to 360

BC (Eichhorn 1992). During the Middle Ages the most

important compilation worth mentioning—although it

may not be considered a catalog in the current sense—is

called the Alfonsine Tables, compilen of astrometric surveys may

even be carried out over the web (sd under the reign of

Alfonso the Wise and first published—probably—after

1277. Actually, the real purpose of the Tables was to offer

a means to calculate the positions on of astrometric surveys may

even be carried out over the web (sf the planets; one of

the main users of the Alfonsine Tables was Copernicus

(Gingerich 1992). Another catalog compiled in the first

half of the 15th century was that of n of astrometric surveys may

even be carried out over the web (sULEGH BEG.

TYCHO BRAHE is among the first astronomers who—

using his mural quadrant—started to make accurate systematic

observations during the pre-telescon of astrometric surveys may

even be carried out over the web (spe years. His

observations include the supernova of 1572, the comet of

1577 and a large collection of positin of astrometric surveys may

even be carried out over the web (sons for Mars. After

Brahe’s death in 1601, these positions were used by hisn of astrometric surveys may

even be carried out over the web (s

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