Hipparchus was the most important Greek astronomers of his time. He very accurately cataloged over 1,000 stars and invented the mathematical science of trigonometry. Ptolemy was a great admirer of Hipparchus's research and recorded some of it in his book the "Almagest". Hipparchus studied the finding of earlier astronomers and compared them with his own. He discovered the procession of the equinoxes and was able to calculate the tropical year within 6.5 minutes of modern measurements. The tropical year is the length of the year measured by the Sun. Hipparchus devised a method of locating geographic positions by means of latitudes and longitudes which is still in use today.
Categorizing the Stars
Telescopes would not be invented for almost 1,500 years after his death, so in order to focus his attention on a certain area in the sky, Hipparchus used a thin tube. While he observed the stars he divided them into six different categories according to their brightness. He called the brightest stars "first magnitude" and the faintest "sixth magnitude". In the 1850's the magnitude system was changed in order to make it more precise. Scientists found that some of the brightest objects in the sky exceeded the first magnitude, so the system began to include a rating of zero and recently even negative magnitudes were added. Sirius, the brightest of all stars has a magnitude of -1.46 as seen from Earth. A magnitude number that is greater than 6 means that an object is only visible through a telescope.