More than 2,000 years ago, the Greek astronomer Hipparchus calculated the number of days in a year with an error margin of less than 0.005. At that time, telescopes had not even been invented.
Around 170-120 BC, Hipparchus discovered a new method to track the movements of the stars, determine the coordinates of celestial bodies, and this method is still applied to this day.
Great astronomers have studied and explored the vast universe with their own eyes and intellect. (Photo: Alamy).
He viewed the Earth as the center and used a calculation system similar to our latitude and longitude to estimate the specific position of stars. From this, he calculated the precise time it takes for the Earth to orbit the Sun.
Who is Hipparchus?
Hipparchus was born in 190 BC in Bithynia, located in the northwestern part of present-day Turkey.
Accurate information about his life is not well-documented. However, based on the legacy Hipparchus left behind, researchers believe he died in Rhodes and had visited Alexandria.
Hipparchus is considered a genius in many fields. He invented trigonometry, created comprehensive star charts, calculated the size and distance between the Earth, the Moon, and the Sun, and expanded studies in geography.
One of Hipparchus’s greatest achievements was helping to design the Antikythera mechanism, a complex astronomical device used to predict eclipses and other celestial events.
Hipparchus is also known as the “father of astronomy.” (Photo: Fermat’s Library).
“Hipparchus is the greatest astronomer of antiquity, at least the greatest among the ancient scientists whose names we know,” stated historian Victor Gysembergh from the French National Center for Scientific Research (CNRS).
Before him, many great Greek astronomers had emerged, such as Eratosthenes, who accurately calculated the circumference of the Earth, and Aristarchus, who hypothesized that the Earth revolves around the Sun. However, Hipparchus is regarded as the greatest among them. There are many reasons for this, including his calculation of the number of days in a year with a very small error margin.
Error Margin of Only 0.5%
In fact, before Hipparchus, many had attempted to calculate the length of a year. For instance, in 330 BC, Callippus, a Greek astronomer, stated that a year is 365 and 1/4 days long.
Hipparchus wanted to verify this. Therefore, he measured how long it took for the Sun to return to its previous position in the sky, similar to what many researchers had done before him.
However, Hipparchus’s results were not convincing and changed with each measurement—simply because he did not have enough data. Thus, he referred to the detailed astronomical records of the ancient Greeks, Egyptians, and Babylonians.
In fact, the Sun always changes position throughout the year. If you try to photograph the Sun in the same spot at the same time each day, you will notice that it does not appear in the same position as it did the day before.
The analemma is a collection of points describing the Sun’s relative motion to the same fixed position on Earth. (Photo: Matthew Chin).
These discrepancies gradually form an analemma – a diagram showing the Sun’s deviation from the same fixed position on Earth. When the Sun returns to its original position, it means a year has passed.
Through this, Hipparchus discovered that over approximately 300 years, the positional changes of celestial bodies occurred nearly on the same day. Therefore, he argued that Callippus’s calculations contained an error of 1/300 of a day.
Hipparchus took 365 1/4 and subtracted 1/300, arriving at 365 74/300 days as the length of a year. Or, in decimal form, 365.2467 days.
It wasn’t until more than 2,000 years later, when science and technology reached new heights, that we discovered Hipparchus’s result had an error margin of about 0.005. A year is accurately 365.2422 days long.
Hipparchus was a pioneer in astronomical devices, but he and his colleagues had far less technology than we do in the 21st century. They understood the universe, not with the aid of telescopes, computers, or satellites, but through observation, intellect, and especially a passionate fascination with astronomy.
