User:Guy vandegrift/draft/Apparent retrograde motion

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Retrograde motion of the outer planets as seen from Earth
from https://en.wikipedia.org/w/index.php?title=Apparent_retrograde_motion&oldid=614364803

Retrograde motion is the apparent motion of a planet to move in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as other bodies.

When we observe the sky, the Sun, Moon, and stars appear to move from east to west because of the rotation of Earth. Most objects in the Solar system are situated near the plane of the ecliptic, and for that reason seem to rise and set, just as the Sun does (provided the observer is near the equator or the mid-latitudes). All these objects make a complete revolution within a few minutes of 24 hours, so that to the casual observer, the Sun, planets, and stars all seem to be fixed to the celestial sphere. But careful observations taken over many days show that planets slowly drift relative to the stars. The motion is usually to the east for the outer planets as well as for Asteroids and Kuiper Belt Objects (including Pluto). Since this drift to the east is normal for these planets, the easterly drift considered direct motion.

But, these outer planets appear to periodically switch direction and and travel west relative to the stars. This motion is called retrograde motion.

To understand this motion, note that Earth completes its orbit in a shorter period of time than the planets outside its orbit. This causes Earth to periodically overtakes the outer planets, like a faster car on a multi-lane highway. When this occurs, the planet being passed will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east.

Comparison of retrogrades versus distance from the Sun
The more distant planets retrograde more frequently, as they don't move as far in their orbits while Earth completes an orbit itself: The retrogradation of a hypothetical extremely distant (and non moving) planet would take place during a half-year, with the planet's apparent yearly motion being reduced to a parallax ellipse. This hypothetical planet would move east for six months and west for six months, as shown in the figure to the right. Note that Neptune's 367 day cycle is not far from 365, which is the number of days in a year. And, Neptune's 158 days of retrograde motion is not far from six months, or 180 days. At the opposite extreme, we have Mars, which repeats its cycle over a time much longer than 365 days. This is because Mars revolves around the Sun in only 697 days. But to repeat the cycle of retrograde motion, it is necessary for Earth to spend extra days to "chase" the moving planet Mars.
 * Mars retrogrades for 72 days every 780 days (25.6 sidereal lunar months).
 * Jupiter for 121 days every 399 days (13.1 months).
 * Saturn for 138 days every 378 days (12.4 months).
 * Uranus for 151 days every 370 days (12.15 months) and
 * Neptune for 158 days every 367 days (12.07 months).



Historical significance of retrograde motion
This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'. In the geocentric model of the solar system proposed by Appolonius in the third century BCE, retrograde motion was explained by having the planets travel in deferents and epicycles. It was not understood to be an illusion until the time of Copernicus, although the Greek astronomer Aristarchus in 240 BCE proposed a heliocentric model for the solar system.

Interestingly, Galileo's drawings show that he first observed Neptune on December 28, 1612, and again on January 27, 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to Jupiter in the night sky, hence, he is not credited with Neptune's discovery. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that very day. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.

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