If you were to observe Mars from Earth and follow its position every night, you might find that Mars follows a weird loop over the course during your daily observation like in the picture above. The planet, which would usually move eastward relative to the celestial sphere, would suddenly move westward for some time, creating a loop. This apparent backwards motion is known as the retrograde motion of planets. So, what is going on with the planet? Does it really move in the opposite direction?
As mentioned above, a retrograde motion is just the ‘apparent’ motion of a planet when seen from another planet. The reason why it appears to move backwards is that of the different orbital speeds of the two planets moving in the same direction in play with our eyes and nothing beyond that. In case of the Earth and Mars, the orbital speed of the Earth is faster than that of the Mars. Let’s break it down with a neat illustration to help us.
In the above illustration, the position of Earth and Mars over a period of time is marked from 1 to 5. For positions E1 and M1 up to E2 and M2, both the planets are moving eastward relative to the celestial background. So if Mars is observed from Earth during E1 and E2. The apparent motion of Mars, A1 and A2 on the celestial sphere would be eastward.
As Earth moves faster than Mars, as soon as it overtakes the red planet at E3 and E4, Mars’s position M3 and M4 lag a bit respectively. When observed, the apparent position of Mars on the celestial sphere would lag behind too, as marked by A3 and A4 respectively. This sudden westward motion (from A3 towards A4) is called as the apparent retrograde motion of Mars.
In case of the Earth and Mars, this type of retrograde motion happens once in every two years, as Mars takes approximately two Earth years to orbit the sun.