When Nicholas Copernicus developed his Sun-centred model of the solar system, he thought that the planets orbited the Sun in circles. In the early 1600s, German mathematician Johannes Kepler began studying the orbits of the planets. He discovered the shapes of the orbits are not circular. They are oval shaped, or elliptical. His calculations further showed that the Sun is not at the centre of the orbits but is slightly offset. Kepler also discovered that the planets travel at different speeds in their orbits around the Sun, as shown in Table 1.

Table 1: AVERAGE ORBITAL SPEED
Planet	Average Orbital Speed (km/s)
Mercury	48
Venus	35
Earth	30
Mars	24
Jupiter	13
Saturn	9.7
Uranus	6.8
Neptune	5.4
Pluto	4.7

By studying these speeds, u can see that the planets closer to the Sun travel faster than planets farther away from the Sun. Because of their slower speeds and the longer distance they must travel, the outer planets take much longer to orbit the Sun than the inner planets do.

Copernicus�s ideas, considered radical at the time, led to the birth of modern astronomy. Early scientists didn�t have technology such as computers and space probes to perform rapid calculations and learn about the planets. Nevertheless, they developed theories about the solar system that still are used today.