If you've studied math at a high school level, you probably have experience with trigonometry. It's a fascinating branch of mathematics, and it all came about through the genius of Hipparchus of Rhodes. Hipparchus was a Greek scholar considered the greatest astronomical observer in early human history. He made many advances in geography and mathematics, specifically in trigonometry, which he used to construct models to predict solar eclipses. Because math *is* the language of science, his contributions are particularly important.

### Early Life

Hipparchus was born around 190 BCE in Nicaea, Bithynia (now known as now Iznik,Turkey). His early life is mostly a mystery, but what we do know about him comes from Ptolemy's *Almagest. *He is mentioned in other writings as well. Strabo, a Greek geographer and historian who lived around 64 BCE to 24 AD called Hipparchus one of the famous men of Bithynia. His image, usually depicted sitting and looking at a globe, has been found on many coins minted between 138 AD and 253 AD. In ancient terms, that's a pretty important acknowledgement of importance.

Hipparchus apparently traveled and wrote extensively. There are records of observations he made in his native Bithynia as well as from the island of Rhodes and the Egyptian city of Alexandria. The only example of his writing that still exists is his *Commentary on Aratus and Eudoxus. *It's* *not one of his major writings, but it's still important because it gives us an insight into his work.

### Life Achievements

Hipparchus's major love was mathematics and he pioneered a number of ideas we take for granted today: the division of a circle into 360 degrees and the creation of one of the first trigonometric tables for solving triangles. In fact, he very likely invented the precepts of trigonometry.

As an astronomer, Hipparchus was curious about using his knowledge of the Sun and stars to calculate important values. For example, he derived the length of the year to within 6.5 minutes. He also discovered the precession of the equinoxes, with a value of 46 degrees, which is fairly close to our modern number of 50.26 degrees. Three hundred years later, Ptolemy only came up with a figure of 36".

The precession of the equinoxes refers to the gradual shift in Earth's rotation axis. Our planet wobbles like a top as it spins, and over time, this means that the poles of our planet slowly shift the direction in which they point in space. It's why our north star changes throughout a 26,000-year cycle. Right now the north pole of our planet points to Polaris, but in the past it has pointed to Thuban and Beta Ursae Majoris. Gamma Cepheii will become our pole star in a few thousand years. In 10,000 years, it will be Deneb, in Cygnus, all due to the precession of the equinoxes. Hipparchus's calculations were the first scientific effort to explain the phenomenon.

Hipparchus also charted the stars in the sky seen with the naked eye. While his star catalog does not survive today, it is believed that his charts included around 850 stars. He also made a careful study of the motions of the Moon.

It's unfortunate that more of his writings do not survive. It seems clear that the work of many who followed was developed using the groundwork laid by Hipparchus.

Although little else is known about him, it is probable that he died around 120 BC most likely in Rhodes, Greece.

### Recognition

In honor of Hipparchus's efforts to measure the sky, and his work in mathematics and geography, the European Space Agency named their HIPPARCOS satellite in reference to his accomplishments. It was the first mission to focus exclusively on *astrometry*, which is the accurate measurement of stars and other celestial objects in the sky. It was launched in 1989 and spent four years on orbit. Data from the mission have been used in many areas of astronomy and cosmology (the study of the origin and evolution of the universe).

*Edited and updated by Carolyn Collins Petersen.*