You may have heard of the Hipparcos satelite. Launched in 1989 by the European Space Agency, Hipparcos was the first space instrument dedicated to measuring the accurate and precise positions of celestial objects, in particular stars. Knowing the positions of stars allows for the determination of their motions and ultimately their distances from Earth. “Hipparcos” is an acronym for HIgh Precision PARallax Collecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea.
Hipparchus who probably invented trigonometry and discovered the precession of the Earth lived from 190 B.C.-120 B.C. His exploits in mathematics and astronomy are vast, but what concerns us today is his quest to measure the distances and sizes of the Moon and the Sun. These are values that we take for granted today, but in Hippachus’ time there were big question marks about them. Hipparchus was able to use parallax (the apparent movement of an object caused by the movement of the observer–put your pointer finger out in front of your face at arm’s length and blink back and forth; the apparent motion of your finger against whatever is behind it is an example of parallax), careful geometry, and observations of a total solar eclipses that probably took place in 129 B.C. to measure the distance to the Moon from the Earth. Although his measurements were only in terms of the size of the Earth he was able to get a measurement of the Moon’s distance to the Earth to within 10% of the actual figure we know today! To find out more about how he did those measurements check out the following NASA webpage: https://www-istp.gsfc.nasa.gov/stargaze/Shipparc.htm