The term retrograde is from the Latin word retrogradus – "backward-step", the affix retro- meaning "backwards" and gradi to step or "to go". Retrograde is most commonly anadjective 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. In this context, the term refers to planets, as they appear from Earth, to stop briefly and reverse direction at certain times though in reality, of course, we now understand that they perpetually orbit in the same uniform direction."Mercury in retrograde" is an example of the term used as a noun for retrograde motion. Retrograde is also sometimes used as an intransitive verb meaning to become, to appear, to behave—or appear to move—in a retrograde fashion.Although 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. Retrograde (backward) and prograde (forward) are observed as though the stars revolve around the Earth. Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the solar system but still used the terms retrograde and prograde to describe the movement of the planets in relation to the stars. 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. Like the sun, the planets appear to rise in the East and set in the West. When a planet travels eastward in relation to the stars, it is called prograde. When the planet travels westward in relation to the stars (opposite path) it is called retrograde.