C. 170–125 BC

HIPPARCHUS spent long periods taking measurements of the earth’s position in relation to the stars. The results enabled him to make several important findings and calculations.

. The Precession of The Equinoxes

He discovered what is now known as the “precession of the equinoxes” by comparing his own observations with those noted by Timocharis of Alexandria a century and a half previously together with earlier recordings from Babylonia. What Hipparchus soon realised was that by taking into account any observational errors made by his predecessors, the points at which the equinox (the two occasions during the year when day and night are of equal length) occurred seemed to move slowly but consistently from east to west against the backdrop of the fixed stars. He gave a value for the annual precession of around 46 seconds of the arc, which is exceptionally close to the modern figure of 50.26 seconds, given the tools and data then available to him.

. The Distance of The Moon

From these observations, Hipparchus was able to make much more accurate calculations on the length of the year, producing a figure that was accurate to within six and a half minutes.

He was also able to correctly determine the lengths of the seasons and offer more exact predictions of when eclipses would take place.

He made observations of the sun’s supposed orbit and attempted to do likewise with the more irregular orbit of the moon. Although partially successful, he could not make entirely accurate calculations.

Using measurements and timings related to the earth’s shadow during eclipses, other attempts were made to determine the size of the sun and moon and their distances from the earth. Again, while not entirely accurate, Hipparchus proposed that the distance of the moon from the earth was 240,000 miles. This is remarkably close to the modern figure.

. A Catalogue of Stars

Perhaps Hipparchus’ most important astronomical achievement was his plotting of the first known catalogue of the stars, despite warnings from some of his contemporaries that he was thus guilty of impiety. He was inspired to begin this work in 134 BC after allegedly seeing a “new star” which prompted his speculation that the stars were not fixed as had previously been thought.

He went on to record the position of 850 stars in the remaining years of his life, a significant achievement given the resources available to him. What is more, he devised a scale for recording a star’s magnitude or brightness: from the most visible (the first magnitude) to the faintest (the sixth). Though amended considerably, it is a scale still used today.

. Developing Trigonometry

Because of the accelerated developments Hipparchus was making in astronomy, he was required to break new ground in other disciplines, particularly mathematics, to facilitate his celestial observations and calculations. Most notably of all, he developed an early version of trigonometry. With no notion of sine available to him, he constructed a table of chords which calculated the relationship between the length of a line joining two points on a circle and the corresponding angle at the centre.

. Further Influence of Hipparchus

Although Hipparchus is considered to be one of the most influential astronomers of the ancient world, it is arguable that his most impacting achievements lay in the areas of mathematics and geography.

The geographer and astronomer Ptolemy cited Hipparchus as his most important predecessor, and he is most often revered for his astronomical measurements and cataloguing. Yet, as the attributed inventor of trigonometry, as well as being the first person to plot places on the earth’s surface using longitude and latitude, his influence has been long lasting and widespread.

He was able to apply his work on the trigonometry of spheres to the planet from which he made his observations. Significantly, he was the first person to use longitude and latitude in his mathematical calculations to position where places were on the earth’s surface. Like so many of Hipparchus’s achievements, it is his further pioneering work that still resonates today.

Hipparchus was born in Nicaea, Bithynia, now in modern Turkey, where he undertook some of his astronomical observations, along with sustained periods in Rhodes and to a lesser extent in Alexandria.

Most of the detail of Hipparchus’s life that has come down to us is taken from Ptolemy’s record of his achievements (because the vast majority of Hipparchus’s original work has been lost).