Post 2 :: #astronomyatschool #astronomywithrahul #astronomy #sciencepopularization #popularscience

Men have been looking up at the skies ever since they roamed the earth. Intelligent ancient civilizations like the Indians, Egyptians, Arabs, Mayans and others noticed certain ‘stars’ were brighter than the others and that they moved across the sky night after night. These moving stars were called planets. But something was very puzzling about them. At times these planets seemed to slow down, then move backwards and then would go forwards again! This apparent backward motion is called retrograde. Mercury is a good candidate for retrograde motion.
The scientific explanation for the phenomenon did not come until Nicolaus Copernicus came up with his heliocentric theory. (which says that the sun is at the centre of the solar system, with the planets going around it). To understand retrograde motion, let’s start with the “Right Hand Rule”.
Imagine that you hold your right hand out and make a fist with your thumb pointing upwards. The direction in which the thumb points, represents the direction of the Sun’s north pole and the direction in which the fingers wrap around, represents the direction in which the planets move around the sun. (Have you ever wondered why all the planets revolve around the sun in the same direction? The laws of physics would not be violated even if a planet revolved the other way round!)
The explanation to retrograde motion lies in the understanding of relative motion. When your car is overtaking another car, doesn’t the other car seem to be moving backwards? And if the other car speeds up, then you see it as going forwards. The same is true for Earth and Mercury. The average orbital speed of Mercury is much higher than that of the earth, and the shape of the orbit is quite elliptical. Most of the time Mercury is travelling much faster than the Earth, except when Mercury is near the extreme end of its elliptical orbit, i.e., when it is farthest from the sun. That’s when Mercury slows down a bit and the earth kind of catches up. So, this is the time when the Earth is overtaking Mercury and Mercury seems to move backwards. But is it really moving backwards? No ! It just appears that way because of our relative motions. This apparent backward motion is referred to as retrograde motion. So, will other planets also display retrograde motion?

Mercury will be in retrograde in 2020 between the following dates:
February 17 to March 10; June 18 to July 12; October 14 to November 3

Astrologers, (not astronomers) ascribe such periods to periods of confusion simply because there was confusion in the understanding of the phenomenon at one point of time. They would advise you not to take major decisions in these periods. Now that you know the science, you take your own decision! 🙂 Think if there is any real logical connection between superstitious beliefs and the real science.

Watch the clip I’ve attached and the picture to understand better.
The clip has been edited from https://www.youtube.com/watch?v=FtV0PV9MF88 and the picture taken from https://astrobob.areavoices.com/…/mars-is-close-bright-and…/