The Red Planet’s retrograde motion ends during mid-November. The planet slowly resumes its eastward direction among the dim stars of Pisces. Bright Mars is visible in the east-southeastern sky after sunset during November.
by Jeffrey L. Hunt
Use a binocular to track the motion of the planet against the dim starry background of Pisces. Observations each clear night reveal the changing position of the planet.
(On the chart above Greek letters and numbers designate the star names along with the abbreviation for Pisces.)
Astronomers use several coordinate systems to locate celestial objects. One system’s foundation is the celestial equator. It is an imaginary great circle in the sky that is directly above the earth’s equator. Equatorial latitude is known as declination, measured in degrees. Longitude – known as right ascension – is measured in hours and minutes. One hour of equatorial longitude is 15°.
A second system uses the plane of the solar system, known as the ecliptic. Degree measurements are used for celestial longitude and latitude. The sun appears to move along the ecliptic, as well as most of the other planets and the moon, although they are inclined by a few degrees.
The origin (0h right ascension, 0° declination and 0° celestial longitude, 0° celestial latitude) of both systems is the Vernal Equinox – a place in the sky and a calendar event. When the sun has the coordinates of the origin point, that is the first day of spring in the northern hemisphere. The sun’s light is directed toward the Earth’s equatorial regions.
When tracking the motions of the planets compared to other worlds or with the stars, a conjunction occurs in either system when the planet and the second object has either the same right ascension or the same celestial longitude.
The two coordinate systems are angled with respect to each other by 23.5°, the earth’s tilt. A conjunction can occur in one system, but not in the other. Additionally, the conjunction can occur on different dates in the two systems.
The ecliptic system measurements generally reveal the closest separations. The articles here typically reference the ecliptic system, because of the shortest gaps this system measures.
On the chart above, Mars moves westward compared to the stars (to the right on the chart) and then stops and begins to move eastward. In the ecliptic coordinates, the retrograde motion ends November 13. In equatorial coordinates, the motion ends on November 14.
Each night Mars appears in the eastern sky after sunset, moves across the sky during the night, and sets in the west after midnight. This occurs from our planet’s rotation.
The motion demonstrated on the chart, is from the combined orbital motions of Earth and Mars. As our planet approaches and passes any object that is farther away from the sun, those objects seem to stop moving eastward compared to the starry background and seem to back up or retrograde. As Earth moves away, they resume their eastward direction.
The reason for this motion was one of the great cosmological problems of astronomy. What caused this, the earth’s motion or some strange mechanism manipulating the planets’ motions while the earth was stationary?
Mercury and Venus retrograde as well. Their retrograde is not easily observed, although Mercury demonstrates some of that early this month as the planet’s retrograde motion carries it to the Spica region and then the speedy planet resumes its eastward motion. (See our article about Mercury and Venus during November.)
The waxing gibbous moon joins Mars on November 25. The Red Planet is 5.2° above Mars.
Watch the Red Planet reverse its westward motion and resume its normal eastward direction compared to the starry background.
Read more about the planets during November.
May 28, 2021: This evening Mercury passes brilliant Venus for the second of three conjunctions during this evening apparition of the second planet from the sun. Use a binocular about 45 minutes after sunset to see the speedy planet 0.4° to the lower left of Venus. This is the closest visible conjunction until 2033.
May 24, 2021: Morning planets Jupiter and Saturn are in the southeast before sunrise. In the evening sky, brilliant Evening Star Venus, Mercury, and Mars line up along the solar system’s plane. The bright moon is in the southeast near Zubenelgenubi, “the southern claw.”
May 23, 2021: Five bright planets parade across the sky. Jupiter and Saturn are visible before sunrise in the southeastern sky. The star Fomalhaut is becoming visible below bright Jupiter and near the horizon. After sundown, Evening Star Venus, Mercury, and Mars are in the western sky. The bright moon is in the southeastern sky during the nighttime hours.
May 22, 2021: Five planets parade across the sky. Jupiter and Saturn are in the southeast before sunrise. Evening Star Venus, Mercury and Mars are in the western sky after sunset. A bright moon is in the southeastern sky.
May 21, 2021: Three bright planets are dancing in the western sky after sundown. Evening Star Venus is entering the sky for a months-long residency after its solar conjunction two months ago. Mercury is heading for a conjunction with Venus after its best evening appearance of the year. Mars continues its eastward march in Gemini, but time is running out on its appearance as it approaches brighter evening twilight and a conjunction with Venus.