Planet retrograde

Most of the time, Mars moves eastward in the sky. However, every two years or so, the path of Mars' eastward movement suddenly slows down and turns westward for dozens of days, with its trajectory tracing in a zigzag shape in the sky. This phenomenon is called planetary retrograde motion.

The unit is designed for unsupervised public use: controls are recessed or shielded, moving parts are enclosed, and the interface is operable by a child of about six with no instruction. Where a screen is used, the language can be switched without changing the hardware, and the content can be replaced on site.

Planets are not truly 'retrograde', but visually appear to be receding, a phenomenon that once plagued astronomers in the ancient world. In fact, the Sunday motion speed of Earth and retrograde planets (taking Mars as an example) is different. Earth orbits once every 365 days, while Mars orbits once every 687 days, which means that every 26 months, Earth will surpass Mars once. During the period of catching up with and surpassing Mars, from the perspective of Earth, Mars' movement speed slows down and is moving backwards. Due to the angle between the Earth's orbit and Mars' orbit, the retrograde trajectory of a planet does not follow the same line segment, but rather presents an S-shape or shape. The exploration of the causes of planetary retrograde motion has become an internal driving force throughout the development of astronomy. Understanding this phenomenon and its causes is an important gateway to understanding ancient and modern astronomical views.