Four amazing astronomical discoveries from ancient Greece

Africa is surrounded by the sea, the earth is a sphere, the diameter of the moon is almost a third of that of the earth, and the sun is large and far away and the earth surrounds it, this was already known about 2,500 years ago, but a large part of the wisdom of ancient Greece was lost and it took more than a thousand years to discover the The same facts again

By Gareth Dorian, Postdoctoral Research Fellow in Space Sciences, University of Birmingham, and Ian Whittaker, Lecturer in Physics, Nottingham Trent University. Translation: Avi Blizovsky

The Greek historian Herodotus (484 to 425 BC) opens an amazing window into the world as it was known to the ancient Greeks in the fifth century BC. Although they knew little, and relied only on observations with their own eyes, they formed the basis for the amazing advances in understanding the universe over the next millennia.

Herodotus claimed that Africa is almost entirely surrounded by sea. How did he know that? It tells the story of Phoenician sailors sent by King Necho II from Egypt (around 600 BC) to sail around Africa clockwise, with their point of departure being the Red Sea. This story, if true, predates the most familiar circumnavigation of the African continent but it also contains an interesting insight into the astronomical knowledge of the ancient world.

The voyage took several years. After rounding the southern tip of Africa, during which they turned west, the sailors noticed that the sun was on the right, above the northern horizon. This observation simply did not make sense at the time because they did not yet know that the earth was a sphere and that there was a southern hemisphere.

1. The planets revolve around the sun
A few hundred years later, much progress was made. Aristarchus of Samos (310 BC to 230 BC) claimed that the sun was the "central fire" of the cosmos and he placed all the then known planets in their 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 claim has been lost, so we cannot know for sure how he developed it. Aristarchus knew that the Sun was much larger than the Earth or the Moon, and he may have hypothesized that it should therefore be the center of the system. Nevertheless, this is a remarkable find, especially when you know that it was not rediscovered until the 16th century by Nicolaus Copernicus, who even quoted Aristarchus during the development of his own work.

2. The size of the moon
One of the treatises of Aristarchus that did survive included the calculation of the sizes and distances of the sun and moon. In this remarkable treatise, Aristarchus laid out the earliest known attempts to calculate the relative sizes and distances from the Earth to the Sun and Moon.
Scientists at the time hypothesized that although the Sun and Moon appeared to be similar in size in the sky, the Sun was further away. They understood this from solar eclipses, caused by the moon passing in front of the sun at a certain distance from the earth.
Also, at the moment when the moon is in the first or third quarter, Aristarchus stated that the sun, the earth and the moon will form a right triangle. Pythagoras proved the relationship between the sides of the right triangle several hundred years earlier, Aristarchus used the triangle to estimate that the distance to the sun was between 18 and 20 times the distance to the moon. He also estimated that the size of the moon was about one-third the size of the earth, based on careful timing of lunar eclipses.

While its estimated distance to the Sun was too short (the actual ratio is 390), due to the lack of telescopic precision at the time, the value for the ratio of the size of the Earth to the Moon is surprisingly accurate (the Moon has a diameter of 0.27 the diameter of the Earth).

Today, we know the distance to the Moon precisely by a variety of means, including precision telescopes, radar observations, and laser reflectors left on the surface by the Apollo astronauts.

3. The circumference of the earth
Artosthenes (276 BC to 195 BC) was chief librarian in the Great Library of Alexandria, and an enthusiastic experimenter. Among his many achievements was the earliest known calculation of the circumference of the Earth. Pythagoras is generally considered to be the earliest proponent of the spherical earth, though probably not his size. The famous and simple method of Artosthenes relied on measuring the lengths of the different shadows cast by poles planted vertically in the ground, at noon on the longest day of the year at different latitudes.
The sun is far enough away that its rays reach the earth in essentially parallel lines, as previously demonstrated by Aristarchus. Thus the difference between the two shadows illustrated the curvature of the earth's surface. Artosthenes calculated the circumference of the earth and arrived at 40 thousand km, a deviation of a few percent from the true value determined in modern geodesy science.

Later, another scientist named Posidonius (135 BC to 51 BC) used a slightly different method and arrived at almost the same answer. Posidonius lived on the island of Rhodes for a large part of his life. There he noticed that the bright star Canopus was very close to the horizon. However, when 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 a circle, he doubled the distance from Rhodes to Alexandria by 48 and arrived at a value of about 40,000 km.

4. The first astronomical calculator
The oldest surviving mechanical calculator is the Antikythera Mechanism. The amazing device was discovered in an ancient ship near the Greek island of Antikythera in 1900.
The device disintegrated over time, but when it was complete it was a box containing dozens of thin gears made of bronze. When turned by a handle, the outer wheels show the phases of the moon, the timing of the lunar eclipses, the orbits of the five planets then known (Mercury, Venus, Mars Jupiter and Saturn) at different times of the year. The device even calculated their retrograde motion - an illusory change in the movement of planets in the sky.

We do not know who built it, but it dates back to some time between the first third century BC, and may even have been the work of Archimedes. Gear technology as sophisticated as the Antikythera Mechanism was not seen again for a thousand years.

Unfortunately, the vast majority of these works have been lost and our scientific awakening has been delayed by thousands of years. As a tool for understanding the process of scientific measurement, Artosthenes' techniques are relatively easy to perform and do not require special equipment, allowing those just beginning their interest in science to understand through experience and eventually follow in the footsteps of the first scientists.
One can only speculate where our culture would be now if this ancient science had continued unabated.

For an article in The Conversation