It is interesting, on clear nights, to observe the sky and see the multitude of stars it has. When we look at the sky, we don't necessarily only see stars and galaxies, we may also be seeing some planets. Usually people believe that because they are so far away, we cannot see them with the naked eye. However, we can see, with the naked eye, five planets.
The planets we see are Mercury, whose observation is quite difficult; and Venus, which is also known to many as the morning star. Venus is usually visible because it has a very intense glow at dawn and dusk. The other planets we can see are Mars, Jupiter and Saturn, which, depending on the time of year, can be seen easily every night.
Through two basic tips we can find these planets in the starry sky. These planets look a lot like bright stars, but they hardly sparkle. Another tip is that the planets do not have a fixed position in the sky in relation to the other stars in the Universe, that is, while we always see a star in the same position, the planets, as the days (time) pass, change their position, traversing the sky more or less in the same range as the moon and the Sun.
Astronomers many centuries ago discovered that all planets that have orbits outside the Earth's orbit, in a certain time of the year, they retreat in their trajectory, describing a "noose" in the sky and then resuming the previous sense of their trajectory. Astronomers then realized that during this period these planets describe a retrograde movement, but actually this movement is an apparent movement due to the positions of the Earth and the planet.
In the figure below we can see the retrograde motion described by a planet that has an orbit outside the Earth's orbit. The figure depicts nine successive positions of the Earth and also of the planet at equal times. Each ray that connects the Earth to the planet indicates the position in which the planet is seen in the sky in relation to the scenery of the fixed stars. Following the end of these straight lines in this scenario, it's easy to see that from 1, whose end doesn't appear, up to end 4, the direction of movement of the planet is that of the arrow that indicates the “normal” direction of the trajectory.
At the end of ray 4, however, apparent movement ceases and the planet appears to retreat. See that the end of ray 5 moves back from the direction of the previous apparent movement. This apparent retreat continues to the end of ray 6, when it then ceases, and the planet returns to the previous direction of its apparent motion.
It is in the period of translation that there is greater proximity between the planet and the Earth, so it is also the a time when the planet shines more and its observation is more profitable, from the point of view astronomical.
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/movimento-retrogrado-um-planeta.htm