# Hipparchus, The Ancient Astronomer

## Hipparchus was one of the most renowned astronomers and mathematicians living in Bithynia from 190 BC to 120 BC

**Hipparchus, The Ancient Astronomer**

Hipparchus is often known as the ‘Lover of truth’ for his unquenchable thirst for knowing the unknown. He was one of the most renowned astronomers and mathematicians living in Bithynia from 190 BC to 120 BC. Most of his important research works have vanished with the time flow. His “Commentary on the Phenomena of Aratus and Eudoxus” is the only surviving book of this great scholar.

As he belonged to the time before Christ, we do not much about his personal life. Many scholars took the responsibility to assess the works of Hipparchus in their writings like Ptolemy. His “Almagest’’ is a great astronomical compendium and is genuinely a second-hand report of Hipparchus’ findings.

In the initial stage of seeking the truth, Hipparchus emphasized on the local weather patterns. He categorized a weather calendar on the basis of wind, rain, and the storm of the astronomical seasons. He also considered the risings and settings of the constellations.

Hipparchus continued his astronomical observation program in the Rhodes island. A list of 20 observations is cited by Ptolemy that is obviously a short part of the truth seeker’s research. We can assume how numerous his research was when we see the published annotated list of them.

He even made critical commentaries on the works of his predecessors and contemporaries. One of them is “Phaenomena”, a poem by Aratus. “Treatise of Eudoxus of Cnidus” that is now considered to be lost is another one that he criticized. It described the constellations. He communicated with the astronomers of Alexandria and Babylon. If he found new evidence, he was ready to revise his own beliefs.

The orbits of the sun and the moon was the most important astronomical work by Hipparchus. He determined the size and distance of them from the Earth. He also studied eclipses. He opined that the Earth was spherical and stationary. It was at the center of the universe and the sun, moon, planets, and the stars revolved around it every day.

According to him, the sun moves in a circular path from west to the east direction relative to the stars. Today, we all know that the sun’s path is ecliptic and it was found him many centuries ago. The plane of the ecliptic way passes through the earth’s center. The vernal and autumnal equinoxes are the two points when the ecliptic and the equatorial plane intersect. The summer and winter solstices are the two points when the ecliptic farthest north and south from the equatorial plane intersect. All these divide the ecliptical path into four parts. He had the explanation that how the sun moves in a regular circular way creating different seasons with different lengths.

When the sun completes a passage through the ecliptic, we know it as a tropical year. He wanted to measure the length of the period precisely. He had some inconclusive results while he was observing consecutive equinoxes and solstices. The main reason was he was unable to distinguish observational errors and variations in the tropical year. But he never lost his heart. He took his own observations from the 5th and 3rd centuries BC, compared them and estimated the length. The duration was 6 minutes longer than the modern estimation. No one got that accuracy till now!

Then he concentrated on the equinox and solstice dates for any year. For doing this, he took the information recorded 150 years before this genius. When researching on the observations, he unexpectedly discovered some of the stars’ movement relative to the equinoxes. They moved two degrees eastward. His discovery lost accuracy after Nicolaus Copernicus established the heliocentric model of the universe where the stars have a fixed frame of reference.

Another subject of his research was to find out the moon’s pathway. He worked on finding the motion of the moon in a more complicated way. It diverges in the ecliptic at varying speeds. The periodicities are also not the same. His contemporary Babylonian astronomers assessed the Moon’s periodicities. He confirmed that they were pretty accurate. All these helped Ptolemy to finally finish the lunar model.

There is a relation between the moon’s orbit and to the size of the earth. He mentioned it in his lost book “On Sizes and Distances”. He used two methods to do so. In the first one, he focused on an observation of a solar eclipse. That particular eclipse was total near the Dardanelles and it was partial at Alexandria. This research allowed him to make an assumption about the difference between the moon’s observable parallax (that is the apparent displacement of an object when viewed from different vantage points) against the stars. It may be the best possible way to assume that the sun is also indefinitely far away like all the stars. This method results in finding the average distance between the earth and the moon is 77 times the Earth’s radius.

The second method detects the distance between the sun and the center of the earth. It is 490 times the earth’s radius. He also used visually identical lunar and solar disks to observe the earth’s shadow during lunar eclipses. Therefore, he got the idea about the relationship between lunar and solar distances. Eventually, he concluded that the average distance between the moon and the earth is nearly 63 times the radius of the earth. It was also the nearest to the modern measurement which is about 60 times.

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