Four amazing astronomical discoveries from ancient Greece

Incredibly, scientists and mathematicians of ancient Greece made some of the most significant and amazing astronomical discoveries centuries ago.

“Stories” Herodotuswho lived from 484 BC. to 425 BC, open a remarkable window into the world as it was known to the ancient Greeks in the middle of the fifth century BC.

However, it is almost as interesting as what these people knew what they did not know. This sets the stage for stunning advances in their understanding over the next several centuries – simply by relying on what they could observe with their own eyes.

Herodotus argued that Africa is almost completely surrounded by the sea. But how did he know about such a huge land mass? It tells the story of the Phoenician sailors who were sent by King Neko II of Egypt (circa 600 BC) to sail clockwise around mainland Africa, starting at the Red Sea.

This story, if true, not only tells of the earliest known circumnavigation of the world in Africa, but also contains interesting information about the astronomical knowledge of the ancient world.

The journey, of course, lasted for several years. Circling the southern tip of Africa and heading west, the sailors saw the sun to their right above the northern horizon.

At the time, this observation simply did not make sense, because they did not yet know that the Earth was spherical and that there was a southern hemisphere.

The planets revolve around the sun

Illustration of the heliocentric system of the world from the atlas of Andrei Cellarius Harmonia Macrocosmica (1708)

Cosmology had already made significant progress a few centuries later. Aristarchus of Samos (310–230 BC) claimed that the Sun was the “central luminary” of the cosmos, and he even arranged all the planets known then in the correct order of distance around it.

This is the earliest known heliocentric theory of the solar system.

Unfortunately, the original text in which he makes this argument has been lost to history, so we cannot know exactly how the genius thinker developed it.

Aristarchus knew that the Sun is much larger than the Earth or the Moon, and he may have assumed that therefore it must be centrally located in the solar system.

Nonetheless, this is a startling discovery, especially when you consider that it was not rediscovered until the 16th century by Nicolaus Copernicus, who even recognized Aristarchus during the development of his own works.

One of the greatest astronomical discoveries of ancient Greece: the size of the moon.


In one of the surviving books of Aristarchus, the dimensions and distances to the Sun and Moon are considered. In this remarkable treatise, Aristarchus outlined the earliest known attempts to calculate the relative sizes and distances of the Sun and Moon.

It has long been noted that the Sun and the Moon appear to be the same apparent size in the sky, and the Sun is farther away. They figured this out from solar eclipses caused by the passage of the Moon in front of the Sun at a certain distance from the Earth.

In addition, at the moment when the Moon is in the first or third quarter, Aristarchus reasoned that the Sun, Earth and the Moon form a right-angled triangle.

Insofar as Pythagoras determined how the lengths of the sides of a triangle are related several centuries earlier, Aristarchus used a triangle to estimate that the distance to the Sun is 18-20 times the distance to the Moon.

He also calculated that the Moon was about three times smaller than Earth, based on accurate timing of lunar eclipses.

Although its estimated distance to the Sun was too low (actual ratio is 390), due to the lack of telescopic accuracy available at the time, the value for the ratio of the sizes of the Earth to the Moon is surprisingly accurate (the diameter of the Moon is 0.27 times that of the Earth).

Today, we know the exact size and distance of the Moon, as determined by various means, including precision telescopes, radar observations, and laser reflectors left on the surface by Apollo astronauts.

Circumference of the earth


Eratosthenes (276–1995 BC) was the chief librarian Alexandria Library and a great experimenter. Among his many accomplishments was the earliest known calculation of the circumference of the Earth.

Pythagoras usually considered one of the earliest proponents of a spherical earth, although apparently not its size. The famous and at the same time simple method of Eratosthenes was based on measuring the different lengths of shadows cast by pillars vertically inserted into the ground at noon, during the summer solstice, at different latitudes.

The sun is far enough away, therefore, wherever its rays come to Earth, they are actually parallel, as previously shown by Aristarchus. Thus, the difference in shadows showed how much the Earth’s surface is curved.

Eratosthenes used this to estimate the circumference of the Earth at about 40,000 km. This is within a couple of percent of the actual value established by modern geodesy (the science of the shape of the earth).

Later, another scientist named Posidonius (135-51 BC) used a slightly different method and came to almost exactly the same answer. Posidonius has lived on the island of Rhodes for most of his life. There he noticed the bright star Canopus, which should have been very close to the horizon. However, while in Alexandria, Egypt, he noted that Canopus would rise about 7.5 degrees above the horizon.

Considering that 7.5 degrees is 1/48 of the circumference, he multiplied the distance from Rhodes to Alexandria by 48 and obtained a value of approximately 40,000 km.

The first astronomical calculator from Ancient Greece

The oldest surviving mechanical calculator in the world is “Antikythera mechanism“. An amazing device was discovered at the sunken ancient ship off the Greek island of Antikythera in 1900.

The device has been damaged over time, but intact it would have looked like a box containing dozens of finely crafted bronze gears. When manually rotated by the crank, the gears wrap around the dials on the outside, showing the phases of the moon, the times of lunar eclipses and the positions of the five planets known at that time (Mercury, Venus, Mars, Jupiter and Saturn) at different times of the year. This even explains their retrograde motion – an illusory change in the motion of the planets across the sky.

We don’t know who created it, but it dates back to between the 3rd and 1st centuries BC and may even have been the work of Archimedes. A technology of such the highest level of complexity as the Antikythera Mechanism, mankind has not been able to recreate for over 2000 years. Unfortunately, the vast majority of this work has been lost to history, and our scientific awakening has been delayed for millennia.

As a tool for introducing scientific measurements, the methods of Eratosthenes are relatively easy to use and do not require special equipment, which allows those just starting to take an interest in science to understand them by performing, experimenting and ultimately following the steps on foot of some of the early scientists. …

One can only assume where our civilization could be now if this ancient science had not been destroyed by wars, natural and man-made disasters and, alas, early Christianity.

Gareth Dorrian is a Space Research Fellow at the University of Birmingham.

Ian Whittaker is professor of physics at Nottingham Trent University. The article first appeared in The Conversation and is reissued under a Creative Commons license.

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