NEW YORK – During World Space Week, October 4-11, The National Herald takes a look at a few of the famous ancient Greek astronomers who made remarkable discoveries and contributions that continue to inspire and astound us to the present day.

According to Space.com, at a time when most people believed the world was flat, the renowned Greek mathematician, astronomer and geographer Eratosthenes (276-195 BC) used the sun to measure the size of the round Earth. His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340 km) off the true measurement.

The NASA Goddard Space Flight Center website noted that “in 200 BC, the size of the Earth was actually calculated to within 1% accuracy!”

Eratosthenes “used Aristotle's idea that, if the Earth was round, distant stars in the night sky would appear at different positions to observers at different latitudes,” NASA GSFC reported, adding that “Eratosthenes knew that on the first day of summer, the Sun passed directly overhead at Syene, Egypt. At midday of the same day, he measured the angular displacement of the Sun from overhead at the city of Alexandria – 5000 stadia away from Syene. He found that the angular displacement was 7.2 degrees – there are 360 degrees in a circle, making 7.2 degrees equivalent to 1/50 of a circle. Geometry tells us that the ratio of 1/50 is the same as the ratio of the distance between Syene and Alexandria to the total circumference of the Earth. Thus the circumference can be estimated by multiplying the distance between the two cities, 5000 stadia, by 50, equaling 250,000 stadia.”

To convert stadia to kilometers, NASA GSFC noted that “we believe that the unit of the ‘stadium’ was about 0.15 km,” adding that “this means that Eratosthenes estimated the circumference of the Earth to be about 40,000 km. He also knew that the circumference of a circle was equal to 2 times π (3.1415…) times the radius of the circle. (C = 2πr) With this information, Eratosthenes inferred that the Earth's radius was 6366 km. Both of these values are very close to the accepted modern values for the Earth's circumference and radius, 40,070 km and 6378 km respectively, which have since been measured by orbiting spacecraft.”

“The diameter of a circle is twice the radius, giving us a diameter for Earth of 12,756 km,” NASA GSFC reported, noting that “the Earth is almost, but not quite, a perfect sphere. Its equatorial radius is 6378 km, but its polar radius is 6357 km – in other words, the Earth is slightly flattened. Eratosthenes was measuring the polar radius, and his value (using the 0.15 km/stadium conversion) lies between the polar and equatorial values.”

“During the 18th and 19th centuries, astronomers used the diameter of the Earth as the basic yardstick in determining the size of the solar system,” NASA GSFC reported, pointing out that “today's astronomers usually do not need to know the size of the Earth for their daily research activities. Nevertheless, the diameter of the Earth still is the first step for us, the residents of this planet, in our attempt to understand the cosmic distance scale.”

A crater on the moon is named after Eratosthenes.

The Eratosthenes crater on the moon named after the ancient Greek astronomer. Photo: Jstuby, CC0, via Wikimedia Commons

Hipparchus is considered the greatest astronomer of antiquity for his impressive contributions to the field. His quantitative and accurate models for the motion of the Sun and Moon are the earliest known to have survived. For those models, he likely made use of the observations and perhaps the mathematical techniques accumulated over centuries by the Babylonians and by Meton of Athens (fifth century BC), Timocharis, Aristyllus, Aristarchus of Samos, and Eratosthenes, among others. Hipparchus was dubbed “the father of astronomy” by the late 18th and early 19th century French astronomer Jean Baptiste Joseph Delambre. He also developed trigonometry, constructed trigonometric tables, and solved several problems of spherical trigonometry. Utilizing his solar and lunar theories and his trigonometry, Hipparchus may have been the first to develop a reliable method of predicting solar eclipses.

Among his other reputed achievements are the discovery and measurement of Earth's precession, the compilation of the first comprehensive star catalog in the western world, and possibly the invention of the astrolabe, and of the armillary sphere that he used to create the star catalog. Born in Nicaea, Bithynia, Hipparchus was active as an astronomer from 162-127 BC. He probably died on the island of Rhodes, where he is thought to have made most of his calculations. The lunar crater Hipparchus and the asteroid 4000 Hipparchus are named after him.

Aglaonice or Aganice of Thessaly was a Greek astronomer and thaumaturge of the 2nd or 1st century BC. Mentioned in the writings of Plutarch and in the scholia to Apollonius of Rhodes as a female astronomer and as the daughter of Hegetor (or Hegemon) of Thessaly, she was regarded as a sorceress for her ability to “make the moon disappear from the sky” (καθαιρεῖν τὴν σελήνην : kathaireĩn tìn selénen) which has been taken – first by Plutarch and subsequently by modern astronomers – to mean that she could predict the time and general area where a lunar eclipse would occur, according to the book Women in Science by Marilyn Bailey Ogilvie.

A Greek proverb makes reference to Aglaonice's alleged boasting: "Yes, as the Moon obeys Aglaonice," Sethanne Howard wrote in Hidden Giants, adding that a number of female astrologers, apparently regarded as sorcerers, were associated with Aglaonice, known as the "witches of Thessaly" who were active from the 3rd-1st centuries BC.

In Plato's Gorgias, Socrates mentions "the Thessalian enchantresses, who, as they say, bring down the moon from heaven at the risk of their own perdition."

Plutarch wrote in the Conjugalia Praecepta that she was "thoroughly acquainted with the periods of the full moon when it is subject to eclipse, and, knowing beforehand the time when the moon was due to be overtaken by the earth's shadow, imposed upon the women, and made them all believe that she was drawing down the moon."