This means that the Earth completes one sidereal rotation before the Moon is able to complete one orbit. The apparent westward motion of the Moon from the Earth's surface is actually an artifact of its being in a supersynchronous orbit. In fact, the Moon orbits the Earth from west to east, as do the vast majority of manmade satellites such as the International Space Station. Day after day however, the Moon appears to move to the east with respect to the stars. When standing on the Earth looking up at the sky, it would appear that the Moon travels from east to west, just as the Sun and the stars do. , A5 – projection to celestial sphere From Earth When the planet travels westward in relation to the stars (opposite path) it is called retrograde. When a planet travels eastward in relation to the stars, it is called prograde. Like the sun, the planets appear to rise in the East and set in the West. Although it is known today that the planets revolve around the sun, the same terms continue to be used in order to describe the movement of the planets in relation to the stars as they are observed from Earth.

Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the Solar System and therefore used the terms retrograde and prograde to describe the movement of the planets in relation to the stars. Retrograde (backward) and prograde (forward) are observed as though the stars revolve around the Earth. Īlthough planets can sometimes be mistaken for stars as one observes the night sky, the planets actually change position from night to night in relation to the stars. In this context, the term refers to planets, as they appear from Earth, stopping briefly and reversing direction at certain times, though in reality, of course, we now understand that they perpetually orbit in the same uniform direction. Retrograde is most commonly an adjective used to describe the path of a planet as it travels through the night sky, with respect to the zodiac, stars, and other bodies of the celestial canopy. The term retrograde is from the Latin word retrogradus – 'backward-step', the affix retro- meaning 'backwards' and gradus 'step'.

Planets in retrograde 2020