Attic white-ground lekythos depicting a fallow deer. Artist unknown; ca. 540-525 BCE. Found at the ancient city of Ialysos, Rhodes;
The Histories by Herodotus (484BC
to 425BC) offers a remarkable window into the world as it was known to the
ancient Greeks 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 jawdropping
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.
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 realised 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.
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. 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.
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 some time 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 foot steps some of the first
scientists.
One can but speculate where our civilization
might be now if this ancient science had continued unabated.