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Ancient Greek astronomy has baffled experts for centuries – how did they know what they knew so long ago? These four discoveries epitomize this. Pictured: view of the Earth, Moon and Sun from the space elements (original image from NASA). Source: vovan / Adobe stock

Four Discoveries of Ancient Greek Astronomy That Still Baffle Experts

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The Histories by Herodotus (484BC to 425BC) offers a remarkable window into the world as it was known by ancient Greek astronomy in the mid fifth century BC. Almost as interesting as what they knew, however, is what they did not know. This sets the baseline for the remarkable advances in their understanding over the next few centuries – simply relying on what they could observe with their own eyes.

Herodotus claimed that Africa was surrounded almost entirely by sea. How did he know this? He recounts the story of Phoenician sailors who were dispatched by King Neco II of Egypt (about 600BC), to sail around continental Africa, in a clockwise fashion, starting in the Red Sea. This story, if true, recounts the earliest known circumnavigation of Africa, but also contains an interesting insight into the astronomical knowledge of the ancient world.

The voyage took several years. Having rounded the southern tip of Africa, and following a westerly course, the sailors observed the Sun as being on their right-hand side, above the northern horizon. This observation simply did not make sense at the time because they didn’t yet know that the Earth has a spherical shape, and that there is a southern hemisphere.

1. The Planets Orbit the Sun

A few centuries later, there had been a lot of progress. Aristarchus of Samos (310BC to 230BC) argued that the Sun was the “central fire” of the cosmos and he placed all of 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 argument has been lost to history, so we cannot know for certain how he worked it out. Aristarchus knew the Sun was much bigger than the Earth or the Moon, and he may have surmised that it should therefore have the central position in the Solar System .

Nevertheless, it is a jaw dropping finding, especially when you consider that it wasn’t rediscovered until the 16th century, by Nicolaus Copernicus , who even acknowledged Aristarchus during the development of his own work.

Portrait of Nicolaus Copernicus. The first man to rediscover Aristarchus’ findings from ancient Greek astronomy.  (Toruń Regional Museum / Public domain)

Portrait of Nicolaus Copernicus. The first man to rediscover Aristarchus’ findings from ancient Greek astronomy.  (Toruń Regional Museum / Public domain )

2. The Size of the Moon

One of Aristarchus’ books that did survive is about the  sizes and distances of the Sun and Moon . In this remarkable treatise, Aristarchus laid out the earliest known attempted calculations of the relative sizes and distances to the Sun and Moon.

It had long been observed that the Sun and Moon appeared to be of the same apparent size in the sky, and that the Sun was further away. They realized this from solar eclipses, caused by the Moon passing in front of the Sun at a certain distance from Earth.

Also, at the instant when the Moon is at  first or third quarter , Aristarchus reasoned that the Sun, Earth, and Moon would form a right-angled triangle.

As Pythagoras had determined how the lengths of triangle’s sides were related a couple of centuries 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 approximately one-third that of Earth, based on  careful timing of lunar eclipses .

A 10th century reproduction of a diagram by Aristarchus showing some of the geometry he used in his calculations, demonstrating advancements made in ancient Greek astronomy. (Public domain)

A 10th century reproduction of a diagram by Aristarchus showing some of the geometry he used in his calculations, demonstrating advancements made in ancient Greek astronomy. ( Public domain )

While his estimated distance to the Sun was too low (the actual ratio is 390), on account of the lack of telescopic precision available 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 0.27 times that of Earth).

Today, we know the size and distance to the moon accurately by a variety of means, including precise telescopes,  radar observations  and laser reflectors  left on the surface by Apollo astronauts.

3. The Earth’s circumference

Eratosthenes (276BC to 195 BC) was chief librarian at the Great Library of Alexandria , and a keen experimentalist. Among his many achievements was the earliest known calculation of the circumference of the Earth. Pythagoras is generally regarded as the  earliest proponent  of a spherical Earth, although apparently not its size. Eratosthenes’ famous and yet simple method relied on measuring the different lengths of shadows cast by poles stuck vertically into the ground, at midday on the summer solstice, at different latitudes.

The Sun is sufficiently far away that, wherever its rays arrive at Earth, they are effectively parallel, as had previously been shown by Aristarchus. So, the difference in the shadows demonstrated how much the Earth’s surface curved. Eratosthenes used this to estimate the Earth’s circumference as approximately 40,000km (24,856 mi). This is within a couple of percent of the actual value, as established by  modern geodesy  (the science of the Earth’s shape).

Later, another scientist called Posidonius (135BC to 51BC) used a slightly different method and arrived at almost exactly the same answer. Posidonius lived on the island of Rhodes for much of his life. There he observed the bright star Canopus would lie very close to the horizon. However, when in Alexandria, in Egypt, he noted Canopus would ascend to some 7.5 degrees above the horizon.

Given that 7.5 degrees is 1/48th of a circle, he multiplied the distance from Rhodes to Alexandria by 48, and arrived at a value also of approximately 40,000km.

4. The First Astronomical Calculator

The world’s oldest surviving mechanical calculator is the  Antikythera Mechanism . The amazing device was discovered in an ancient shipwreck off the Greek island of Antikythera in 1900.

This device, the Antikythera Mechanism, is considered the oldest computer in the world. Through a series of gears, it was used to predict astronomical positions and eclipses for calendrical and astrological purposes. (Weekend Wayfarers / CC BY 2.0)

This device, the Antikythera Mechanism, is considered the oldest computer in the world. Through a series of gears, it was used to predict astronomical positions and eclipses for calendrical and astrological purposes. (Weekend Wayfarers / CC BY 2.0 )

The device is now fragmented by the passage of time, but when intact it would have appeared as a box housing dozens of finely machined bronze gear wheels. When manually rotated by a handle, the gears’ span dials on the exterior showing the phases of the Moon, the timing of lunar eclipses, and the positions of the five planets then known (Mercury, Venus, Mars, Jupiter, and Saturn) at different times of the year. This even accounted for their  retrograde motion  – an illusionary change in the movement of planets through the sky.

We don’t know who built it, but it dates to sometime between the 3rd and 1st centuries BC, and may even have been the work of  Archimedes. Gearing technology with the sophistication of the Antikythera Mechanism was not seen again for a thousand years.

Sadly, the vast majority of these works were lost to history and our scientific awakening was delayed by millennia. As a tool for introducing scientific measurement, the techniques of Eratosthenes are relatively easy to perform and require no special equipment, allowing those just beginning their interest in science to understand by  doing, experimenting  and, ultimately, following in the footsteps of some of the first scientists.

One can but speculate where our civilization might be now if this ancient science had continued unabated.

Top image: Ancient Greek astronomy has baffled experts for centuries – how did they know what they knew so long ago? These four discoveries epitomize this. Pictured: view of the Earth, Moon and Sun from the space elements (original image from NASA). Source: vovan / Adobe stock

The article ‘Four Discoveries of Ancient Greek Astronomy That Still Baffle Experts’ by Gareth Dorain and Ian Whittaker was originally published as ‘Four amazing astronomical discoveries from ancient Greece’ on The Conversation and has been republished under a Creative Commons license.

Comments

 A brief and compehensive article. One small clarification though. The important error in calculating the Earth – Sun distance by Aristarchus was due to an expected lack of precision in angle measuring instruments at his time. When Moon is exactly half illuminated, the Sun should be in an almost 90 degrees angle to the Moon, as observed from the Earth. The precision of this measurment is very critical, because as the angle approaches 90 degrees the distance is changing a lot and very rapidly, with the slightest change of the angle. Aristarchus measured 87 degrees, while modern measurements give 89 degrees 40 minutes 2 seconds. For comparison, if this measurement was 90 degrees exactly, then the distance should have been (almost) infinite.

"One can but speculate where our civilization might be now if this ancient science had continued unabated."

One can only imaging where our civilisation would be if this ancient science of Astrology had continued to be acceptable to us all.

Hi Thoth 101,

I've been reading Enoch's writing's online for free. It appears that there maybe 6 parts to Enoch's writings.

As of right now I'm reading Enoch 1, and Enoch 2 The Book of Secrets of Enoch.

The Ethiopian Enoch identified as Enoch 1. It was translated from Hebrew and is apart of The Ethiopian Bible, I checked for it online it's not up their what have learned is that this Bible in particular is The Oldest Bible in The World and that it has over 86, Bible Book's between it's scriptures.

Thank God for this but, I did find Enoch's goodbye to The World while God translated Him to Heaven so He would not taste death, his goodbye is in The Book of Secrets of Enoch free online. Check it out read the book!

These Specific Bible Book's Enoch and Ezras this would be Ezra that's not in The Current Bible, honestly I can't figure out why, in The World did Papal Rome and The Protestant Reformation Movement kick these Bible Book's out... When they have even more details in The Creation of The Universe by God.

Ethiopian Enoch Bible Book is referred to as Enoch 1, basically covers the crimes committed by The 200 Watcher Angels led by Azazel Ningirsu to all living thing's this would include people that has the Breath of Life, and God's judgement against these angels and their children, because of their abominable behavior.

Enoch 2 is called The Book of The Secret's of Enoch.

This part of Enoch 2 deals with the fact that Enoch is to be translated in to Heaven so that He would not taste Death this is crucial because right before God Takes Enoch God tells him to warn Earth of the impending Flood, the great deluge.

Remember that time in 2012, that a Voyager at least I think it's name was Voyager was reported as no longer being within our galaxy?

See that's not true in terms of how God measures space, technically Voyager is still in our galaxy.

So In a sense we're identified as the 1st Heaven, in Enoch 2.

What I have come to understand from both Enoch and Jubilees is that God hasn't stopped Creation, He is still creating object's and thing's.

Enoch 2, 2nd Heaven after ours, is well it's an prison for very bad angels now these angels are bound in chains; the entire heaven is described as dark & foreboding rather cold think colder than Antarctica or the Arctic

I think that those bound angels are the one's who fought in The War, that was in Heaven, told in Revelation chapter 12. I think God sent those other 200 angels who polluted creation on Earth, got sent elsewhere in The Heaven's.

Enoch 1 God tells the 200 angels again called the watchers that what they believed about the mysteries of the universe they've barely even scratch the surface.

The same is with me in relation to The Bible, it is through God's Prophet Enoch, that I am re-learning the thing's of which The Bible has spoken about.

Soon as I find me and English translation of Ezra (since apparently it is only found in The Latin Vulgate), that mentions The number of Heaven's and places that are throughout the Universe my Biblical lesson will be up to date.

Goodbye for now and thank you for reminding me, I forgot to say all of the multiple universes that's out there.

Thoth101's picture

Hey Zucchini,

Do you realize how many suns and moons in the Universe there are? Not to mention this may not even be the only universe and that there are multiple if not an infinite number of universes. Your Bible doesn't explain that.

So in one sentence you are saying God created one Sun and one moon and the planets. You do realize a star is a sun don't you? So if your God only made one Star the Sun then it wouldn't be hard to know it by name.

Hello Ancient World,

Astronomy is all throughout The Bible from Genesis to Revelation. So on the 4th day God creates The Sun & Moon, God creates The Planets, The Constellations the Star's.

In fact in Psalms It is Written God knows every Star and He calls them by Name.

I read in Enoch and Jubilees wow both Books speak about Astronomy. Enoch 1 talks about The Circumference of Earth revolving around The Sun and The relation to The Moon and Earth.

Enoch makes it known that Earth is Female so is referred to as Sheoul okay the name is in Hebrew so I've probably spelled it wrong but, that is what I read Sheoul.

Enoch in addition too makes known that The Sun is Male, and the Moon is Female.

I think it is interesting regarding the Greek Astronomer that it wasn't until, the 16th Century when Nicolaus Copernicus gave the Greek Astronomer Credit on how accurate this Scientist was.

Greece had Ancient libraries every Civilization had one and every library had copies or versions of The Book of Enoch.

Enoch's writings about the Solar System would have been read and quite possible The Ancient World was conducting experiments to verify Enoch's statements about The Universe itself.

This was a great and exciting article to read, thank you for posting article Ancient World.

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