2021, May 22: Saturn Retrogrades

May 22, 2021:  Saturn, the Ringed Wonder, begins its retrograde motion today.  The planet is in the southern sky to the upper right of bright Jupiter before sunrise.  The planet is at opposition on August 2, and ends its retrograde motion on October 10.

by Jeffrey L. Hunt

The Ringed Wonder Saturn begins its retrograde motion on May 22.  The planet stops its eastward motion on the ecliptic coordinate system and seems to start moving westward compared to the stellar background.

The planet has been inching toward Theta Capricorni.  The eastward motion stops and the planet seems to move away from the star.

Retrograde motion is an illusion from the faster moving Earth catching and passing between the sun and the planet.

This regular westward movement of the outer planets was the cosmological problem of the day.  What caused the outer planets to seem to back up and at some later date resume their eastward motion compared to the stars?

The answer depends on the placement of the earth in space.  Was it unmoving at the center of the cosmos or was it one of the known planets revolving around the central sun?

Ancestral astronomers who thought the Earth was central and unmoving used circles on circles to explain the strange westward motion.  The planet was on a small circle that rode on a larger circle that moved the planet backwards compared to the stars at the appropriate time.

Sun-centered astronomers thought retrograde motion was an optical illusion of our faster moving world – revolving around the sun on a circle – passing an outer planet.

Astronomers used their two models to predict the positions of the planets compared to the background stars. Because both used circles, one theory could not out-perform the other with the prediction of the planets’ locations in the sky,

Not until Johannes Kepler determined that the planets revolve on elliptical-shaped orbits, along with Newton’s principle of gravitation, did better models emerge to predict the locations of the planets in the sky.

Many “planetarium” computer programs use the mathematical equations that are derived from Newton’s explanation of gravity.  Lunar phases, tides, moon phases, and glamorous flights to the moon and planets are predicted very accurately from the principles of planetary motion.

Precise measurements of Earth’s revolution around the sun did not occur until 1838.  Earth is one of the planets in the gravitatioal grasp of an average star.

To find out more about the quest to measure the earth’s revolution around the sun, read Alan Hirshfeld’s “Parallax: The Race to Measure the Cosmos.”

2021, May 22: As seen from north of the solar system, planet Earth begins to catch Saturn. The Ringed Wonder begins to retrograde against the distant stars.
Chart Caption – 2021, May 22: As seen from north of the solar system, planet Earth begins to catch Saturn. The Ringed Wonder begins to retrograde against the distant stars.

The chart above shows the relative position of Earth and Saturn compared to the sun.  On May 22, Earth begins catching Saturn.  In the sky, the planet begins to retrograde.  The planet rises at about 1 a.m. CDT and appears in the southern sky near sunrise.

As May turns into June, Saturn is moving toward opposition, when our planet is between the sun and Saturn. By early July, the Ringed Wonder rises around 10 p.m.

2021, August 2: Still retrograding, Earth moves between the sun and Saturn. At opposition the planet rises as the sun sets.
Chart Caption – 2021, August 2: Still retrograding, Earth moves between the sun and Saturn. At opposition the planet rises as the sun sets.

Earth moves between the sun and Saturn on August 2.  As the sun sets, Saturn rises.  This is opposition.  Saturn continues to retrograde.

2021, October 10: Saturn’s retrograde ends and the planet resumes its eastward motion compared to the starry background.
Chart Caption – 2021, October 10: Saturn’s retrograde ends and the planet resumes its eastward motion compared to the starry background.

After opposition, Earth moves away from Saturn.  By early September, Saturn is in the southeastern sky after sunset. The planet’s westward motion continues until October 10.  The Ringed Wonder begins its eastward motion again after this date. The planet starts the night nearly in the south.

The pattern occurs every year as our planet catches up and passes between Saturn and the sun.

Animated Gif: This is an animated gif of all three charts together, showing the orbital directions of Earth and Saturn. The pictures change every 5 seconds and repeat.
Chart Caption – Animated Gif: This is an animated gif of all three charts together, showing the orbital directions of Earth and Saturn. The pictures change every 5 seconds and repeat.


Categories: Astronomy, Sky Watching

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