# Hipparchus, The Greek Astronomer and Mathematician

Updated on January 23, 2019

## Background

Hipparchus was a Greek astronomer and mathematician who made fundamental advancements as a mathematical science and to the foundations of trigonometry during the 5th and 3rd centuries BC. He was considered one of the greatest astronomical observers in early human history. Hipparchus used his advances in geography and trigonometry to construct models to predict solar system mysteries.

## Hipparchus’ Astronomical Work

Hipparchus’ most important astronomical work done concerned using parallax to determine the distance from the Earth to the Moon in 150 BC. Parallax is the difference in direction of a celestial object as seen by an observer from two wildly different points, when a person looks at something from a different location, it appears at a different place against the background. For Hipparchus, the locations in which he viewed the moon were Greece and Egypt and the background was the stars. When a body shows a significant amount of parallax, astronomers can use geometry to calculate its distance from earth. Hipparchus thus calculated that the mean distance of the Moon from the earth is 77 times the Earth’s radius. Then Hipparchus hypothesised that the distance from the center of the Earth to the Sun is 490 times the Earth’s radius because it is the shortest distance consistent with a parallax that is too small for the naked eye. He found a relationship between these two findings enabling him to calculate the Moon’s mean distance from Earth being approximately 63 times Earth’s radius, the actual value is about 60 times or roughly 380,000 km.

## His Impact on Science and our Lives

Hipparchus’ discoveries during this event have impacted the development for science since then greatly. His foundations in mathematics helped future scientists use algebra and trigonometry as a way to observe and analyze the solar system, extending the development of mathematical science. Because he estimated that the Moon is 380,000 km away from Earth, this information helped Aristarchus later calculate the Earth-Sun distance. Which in turn most likely laid the foundation for modern scientist to create a spacecraft that would successfully cover the distance to the Moon and land on it, such as Luna 2, launched by the Soviet Union on September 12th, 1959 being the first spacecraft to successfully reach the Moon.

Also in 134 BC, Hipparchus calculated the length of the year to within 6.5 minutes, which to this day is applicable to our daily lives, and with this information he discovered the precession of the equinoxes with a value of 46 degrees, very close to our modern number of 50.26 degrees. The precession of the equinoxes refers to the gradual shift in Earth’s rotation axis, over a period of time our North Pole points to a different place in space. Hipparchus was the first person to notice the Earth’s precession by noting the locations stars rose and set during equinoxes, the twice yearly dates when the night and day length are exactly 12 hours each. He noticed that every few years the stars were rising and setting in slightly different location; at least one degree per century, while also influenced Hipparchus’ star map also in 134 BC.

Hipparchus’ calculations lead to a better understanding of the phenomenon of why our North Star changes throughout a 26,000-year cycle; right now the North Pole points to Polaris, but in the past it has pointed to Thuban and Beta Ursae Majoris. The calculations and star map have decided that Gamma Cephei will become our pole star in a few years.

Overall Hipparchus' discoveries during his time have impacted how scientists study astronomy and his theories further help scientists.