Ancient Astronomy
 It was in Greece that the Golden Age of early astronomy was centered. Being philosophers, the Greeks used
philosophical arguments to explain the natural events happening around them including the movements of the
stars and other heavenly bodies. But they were also observers. They made use of their observational data to
explain certain events. They were the ones who measured the sizes and the distances of the sun and the moon
using the basics of geometry and trigonometry which they also developed.
 The early Greeks had a geocentric view of the earth. For them, it was the center of the universe; hence, a
motionless sphere. The sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn orbited the Earth.
 The Greeks also believed that stars traveled daily around the earth. However, they all stayed in a transparent,
hollow sphere located beyond the planets. They called this sphere as the celestial sphere.
Key Terms
● Oblate spheroid: the shape of the Earth. It has bulging equator and squeezed poles.
● Solstice: either of the two times in the year, the summer solstice and the winter solstice, when the sun reaches its
highest or lowest point in the sky at noon, marked by the longest and shortest days.
● Eclipse: an obscuring of the light from one celestial body by the passage of another between it and the observer or
between it and its source of illumination.
● Heliocentrism: the astronomical model in which the Earth and planets revolve around the Sun.
● Geocentrism: any theory of the structure of the solar system (or the universe) in which Earth is assumed to be at the
center of it all.
 Around 500 B.C., most Greeks believed that the Earth was round, not flat. It was Pythagoras and his pupils who
were first to propose a spherical Earth.
 In 500 to 430 B.C., Anaxagoras further supported Pythagoras' proposal through his observations of the shadows
that the Earth cast on the Moon during a lunar eclipse. He observed that during a lunar eclipse, the Earth's shadow
was reflected on the Moon's surface. The shadow reflected was circular.
 Around 340 B.C., Aristotle listed several arguments for a spherical Earth which included the positions of the North
Star, the shape of the Moon and the Sun, and the disappearance of the ships when they sail over the horizon.
North Star
The North Star was believed to be at a fixed position in the sky. However, when the Greeks traveled to places nearer the
equator, like Egypt, they noticed that the North Star is closer to the horizon.
The Shape of the Sun and the Moon
Aristotle argued that if the Moon and the Sun were both spherical, then perhaps, the Earth was also spherical.
Disappearing Ships
If the Earth was flat, then a ship traveling away from an observer should become smaller and smaller until it disappeared.
However, the Greeks observed that the ship became smaller and then its hull disappeared first before the sail as if it was
being enveloped by the water until it completely disappeared.
The Size of the Spherical Earth
Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations. It was Eratosthenes
who gave the most accurate size during their time. While he was working at the Library of Alexandria in Northern Egypt,
he received correspondence from Syene in Southern Egypt which stated that a vertical object did not cast any shadow at
noontime during the summer solstice. But this was not the case in Alexandria where, at noon time during the summer
solstice, a vertical object still casts a shadow. These observations could only mean that the Sun, during this time in
Alexandria, was not directly overhead.
Eratosthenes then determined the angle the Sun made with the vertical direction by measuring the shadow that a vertical
stick cast. He found out that in Alexandria, the Sun makes an angle of 7.2° from the vertical while 0° in Syene.
To explain the difference, he hypothesized that the light rays coming from the sun are parallel, and the Earth is curved.
From his measurements, he computed the circumference of the Earth to be approximately 250 000 stadia (a stadium is a
unit of measurement used to describe the size of a typical stadium at the time), about 40 000 kilometers.
Our understanding about the different heavenly bodies can be credited to the important findings of the following Greek
astronomers:
a. Anaxagoras
Anaxagoras was able to explain what causes the phases of the moon. According to him, the moon shone only by reflected
sunlight. Since it is a sphere, only half of it illuminated at a time. This illuminated part that is visible from the earth changes
periodically.
b. Eudoxus
Eudoxus proposed a system of fixed spheres. He believed that the Sun, the moon, the five known planets and the stars
were attached to these spheres which carried the heavenly bodies while they revolved around the stationary Earth.
c. Aristotle
Aristotle was a student of Plato. For him, the earth is spherical in shape since it always casts a curved shadow when it
eclipses the moon. He also believed that the earth was the center of the universe. The planets and stars were concentric,
crystalline spheres centered on the earth.
d. Aristarchus
Aristarchus is the very first Greek to profess the heliocentric view. The word helios means sun; centric means centered.
This heliocentric view considered the sun as the center of the universe. He learned that the sun was many times farther
than the moon and that it was much larger than the earth. He also made an attempt to calculate the distance of the sun
and the moon by using geometric principles. He based his calculations on his estimated diameters of the earth and moon
and expressed distance in terms of diameter. However, the measurements he got were very small and there were a lot of
observational errors.
e. Eratosthenes
The first successful attempt to determine the size of the earth was made by him. He did this by applying geometric
principles. He observed the angles of the noonday sun in two Egyptian cities that were almost opposite each otherSyene
(now Aswan) in the south and Alexandria in the north. He assumed they were in the same longitude.
f. Hipparchus
Hipparchus is considered as the greatest of the early Greek astronomers. He observed and compared the brightness of
850 stars and arranged them into order of brightness or magnitude. He developed a method for predicting the times of
lunar eclipses to within a few hours. Aside from this, he also measured the length of the year to within minutes of the
modern value.
g. Claudius Ptolemy
He believed that the earth was the center of the universe. His Ptolemic Model claimed that the planets moved in a
complicated system of circles. This geocentric model also became known as the Ptolemic System.
The Ptolemic Model
Claudius Ptolemy developed a model that was able to explain the observable motions of the planets.
According to the Ptolemic Mode, the sun, the moon, and the other planets move in circular orbits around the earth.
However, if observed night after night, these planets move slightly eastward among the stars. At a certain point, the planet
appears to stop then moves in the opposite direction for some time; after which it will resume its eartward motion. This
westward drift of the planets is called retrograde motion.
To justify his earth-centered model using retrograde motion, he further explained that the planets orbited on small circles,
called epicycles, revolving around large circles called deferents.