Early Astrometry

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In 129 B.C. and only with the help of the naked eye, the Greek astronomer Hipparchus was the first to complete a catalogue of a thousand stars, specifying their relative brightness and position with an accuracy of about one degree, i.e. the angle equivalent to the apparent height of a person at a distance of 100 metres. This is considered to represent the birth of the science of astrometry.

On the illustration below you can see that the angular position can be measured with the accuracy about one degree without any special tools:

After Hipparchus, progress in the accuracy of angular measurements was slight until the 16th century. A revolution came with Tycho Brahe (1546-1601), a Danish astronomer, who fixed star positions to about a minute of arc, i.e. one sixtieth of a degree. He designed, built and calibrated a wide variety of viewing instruments like the sextant or the mural quadrant and changed observational practice profoundly.

Click on the thumbnail below to explore an illustration of a sextant like the type designed by Tycho Brahe in the 16th century:

The Sextant


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