The celestial sphere model at that time was one of the best methods to study the motion of planets, stars, and Sun. From the above figure, it is clear that the celestial sphere consists of Celestial poles and celestial equator. Let us know about each.

• Celestial Poles:- When we extend the imaginary line connecting the north and south pole of Earth, it intersects the Celestial sphere at two points called as Celestial poles. Celestial pole along the north pole of Earth is called as North Celestial pole whereas who aligned along the south pole of Earth is termed as South Celestial pole.

• Celestial Equator:- The extended equator of Earth when intersects the celestial sphere is termed as celestial equator. It is at 90 degrees to North and South celestial poles.

Now if you are at the north pole of Earth, you will only see the north celestial pole and if you are at the south pole, the south celestial pole will be in your sight.

Breaking the myth of the flat Earth

The most common myth among the ancient people was that Earth is flat. It was Pythagoras who first proposed the Earth is round instead of flat around 500 BC. But as an explanation for it, he simply said that as Moon and Sun both are round, then the Earth must also be round.

But later on the great Greek philosopher Aristotle (384-322 BC) gave two main observations which support the shape of Earth as round one.

• The very first observation of Aristotle was based upon the study of a lunar eclipse. He noted that during a Lunar eclipse, the shadow of Earth on the moon was a curved one. As only round object can create a curved shadow, so it is clear that Earth is round. He further added that if the Earth was flat, it would cast a shadow in form of a line, instead of curved shadow.

• His second observation was based upon the vision of different stars, as one move from north to south or from south to north. He observed that when a traveler travels from north to south pole of Earth, he hot familiar with new stars which are not visible from the north. So, if the Earth was flat, everyone would see every star in the sky. From this Aristotle concluded that all this happens because of the curvature of Earth, proving Earth as round as a sphere. To better understanding see above image. When one is on North pole, it can only see the North celestial pole and hence only stars of North celestial pole and vice-versa. This is all happening because of the curvature of Earth.

Proposal of Sun as a center, instead of Earth

To be the first time around 260 BC, a great Greek astronomer, and mathematician Aristarchus(310-230BC) suggested that Earth is moving around the Sun, not Sun around Earth. But Aristotle and most of the other astronomers rejected the idea of Aristarchus, as all of them were also believe that Earth is at center and all other objects like Sun, moon revolve around it. Against the idea of Aristarchus, Aristotle, and other astronomers gave two points. According to them, if Earth is moving around the Sun, we would observe the stars at different position daily. They thought that if Earth is moving, the position of stars must also differ. Also, no one on Earth is feeling its motion. Both these two points lead to the myth of Earth as a center.

First ever measurement of the size of Earth

Greek astronomers did not only measure the shape of Earth but also able to measure its size.

The first fairly accurate determination of Earth's circumference was made by Eratosthenes (276-194 BC) in about 200BC.

Observations made by Eratosthenes for measurement of Earth's circumference:-According to Eratosthenes:-

• The light rays coming from Sun are parallel.

• Distance between two cities Alexandria and Syene is 5000 stadia.

• Considered Earth as a perfect sphere.

• Syene is on the tropic of cancer.

For measurement of the circumference of Earth, he considered two cities Syene and Alexandria having a distance of 5000 stadia. Eratosthenes knew that at normal noon on the summer solstice in Syene, the Sun was directly overhead. So, there was no shadow at that time on Syene. Now he took a rod and hold it vertically straight in Alexandria. This time he got a little shadow. Then by knowing the length of the rod and length of its shadow, who was making a triangle, he calculated the angle between surface and shadow of the rod. The angle turned out to be 7°. As Earth makes a circle of 360°, so we can say that he got th or around th of circumference of Earth.

Now he simply multiplied the distance between Syene and Alexandria i.e.5000stadia by 50 and got the circumference of Earth that was 250000 stadia. Here stadia is an ancient unit of distance and most accepted value of 1 stade=184.4 meter.

So, Eratosthenes measured circumference of Earth in Kilometer comes out to be:-

Today we know, the actual value of circumference of Earth = 40075 Km

Therefore,

error in percentage=%

How interesting, the astronomer around 2218 years back without the use of any instrument was able to measure the circumference of Earth with only 15.2% of error.

Ptoelmy's geocentric model

Although many Greek scientists including Aristotle, Plato and other believed Earth as at the center. But the first model showing Earth at the center was given by an astrologer Ptolemy around 200 A.D.This model is called as Ptolemaic model or geocentric model. See below image of the geocentric model, in which Earth is at center and all other celestial objects revolve around it:-

One of the great thing about this model was that it can predict the position of planets much more accurately than any traditional method. Before this model, astronomers have thought that all the planets revolve around the Earth with constant speed. But sometimes it was observed that planets misbehave their motion around Earth. For example:- Sometimes they appeared to move backward from their normal position. Why this happens nobody, at that time was able to explain. But Ptolemy in his model gave the reason behind it by considering planets to move in two orbits. According to him, planets revolve around Earth using two orbits- one is small orbit called epicycle and center of this epicycle revolve around Earth in an orbit called deferent as shown below:-

He said that planet moves on epicycle and center of epicycle orbit around Earth by following a path known as deferent. Now when both planet and center of the epicycle are in the same direction, the planet follows the normal path. But when planet on epicycle moves backward with respect to the center of epicycle, planet appear us to move backward.

Conclusions:-

So it is clear that ancient astronomers not only predicted the shape and size of Earth but also gave us clue about planet's orbit and retrograde motion. Moreover, today's heliocentric model of the solar system is also modified form of Ptolemaic's model.

References:-

Want to read and know more, follow the following links:-

• (https://en.m.wikipedia.org/wiki/Celestial_sphere)

• (https://oneminuteastronomer.com/9905/celestial-equator-poles/)

• (https://en.m.wikipedia.org/wiki/Spherical_Earth)

• (https://www.mrdowling.com/601-aristotle.html)

• (https://en.m.wikipedia.org/wiki/Aristarchus_of_Samos)

• (https://en.m.wikipedia.org/wiki/Eratosthenes)

• (http://www.polaris.iastate.edu/EveningStar/Unit2/unit2_sub1.htm)

• (https://en.m.wikipedia.org/wiki/Deferent_and_epicycle)

Thanks For Reading!