Combine the results of your observations over several weeks, and you’ll see a planet outlining a loop. To observe the retrograde motion, you’ll need to mark the position of a planet in the sky every day at the same time. By the way, in the middle of this period, Mars will be at opposition and shine brighter than usual. It will begin on December 6 and end on February 24, 2025. To see the retrograde motion of Mars, we’ll have to wait until 2024. The following Solar System planets also go retrograde in 2023: The elusive planet Mercury will be moving “backwards” in the sky from August 23 to September 15, and from December 13 to January 1, 2024. Only in the 16th century, when Nicolaus Copernicus introduced his heliocentric model, scientists understood that retrograde motion was an illusion. However, their retrograde motion is hard to observe: when an inferior planet passes us by, it is positioned between the Earth and the Sun, so the Sun’s glare hides it from view.Īncient astronomers were rather puzzled by the retrograde motion phenomenon – especially those who thought that the Earth was at the center of the Universe. The inferior planets, Venus and Mercury, that orbit the Sun faster than the Earth, also periodically appear to move “backward”. This mechanism works for all the superior planets. The same thing happens when you pass by a slower-moving car on the highway – for a moment, it appears to move in the opposite direction. When we pass Mars, it seems to be moving “backward” because we're moving faster than it is. This superior planet moves slower in its orbit than the Earth. The apparent retrograde motion is caused by differences in the planets’ orbital speed. What causes the apparent retrograde motion of the planets? This westward movement is called apparent retrograde motion. This motion shouldn’t be confused with the daily motion of the planets and the Sun in the sky, which goes from east to west and is caused by the Earth’s rotation on its axis.Īt specific periods of time, a planet can start moving “backward” – from east to west. Astronomers call it direct or prograde motion.
When a planet seems to reverse its direction in the sky, it’s called retrograde motion (from the Latin word retrogradus – "going backward").ĭay to day and week to week, as the Earth revolves around the Sun, the planets in the sky typically move in the same direction as the Sun – from west to east. See Infographic What does it mean when a planet is retrograde? Want to know what causes the apparent retrograde motion of the planets? Check out this infographic to learn how retrograde motion works.
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