The retrograde mystery

Does Mars Retrograde?

No, it is the Earth, the platform of observation, 
that catches up and moves ahead, which causes the
apparent retrogression

See an animation of a Mars Retrograde

Starting in May 2003, Mars moved eastward through the constellation of Capricornus. At the end of June, Mars seemed to come to a halt and began moving west-to-east in the constellation of Aquarius. It continues its backward, or "retrograde," motion until the end of September, when it resumes its normal west-to-east motion.

Notice that Venus starts out to the east of Mars in May, and ends up to the west of Mars at the end of December. The little dots on Mars’ path in the video mark the planet’s position every 10 days, and yes, that is the Moon you see, apparently speeding by about once every month.


Retrograding Mars

As the Earth moves in its orbit from its starting position on May 2, 2003, an observer would see Mars far to the right against the starry background, while at the opposite side of the visual field Venus would appear near the constellation Pices. While the Earth moves from its starting position (to position 1), the apparent position of Mars, when seen against the starry background, is now much farther to the East. This continues in the same manner with the Earth moving faster than Mars, catching up with Mars. By the difference in speed that causes the visual angle to change the previously accelerated apparent progression of Mars slows to a halt at position 3.   

With the visual angle now becoming ever steeper as the Earth gains on Mars the apparent position of Mars is drawn backwards towards point 5 and slightly past it. Mars 'retrogrades' for 72 days (2 months and 12 days ,from position 3 to just past position 5). 

Just past point 5 on the viewing able becomes steeper again as the Earth moves away from Mars, so that the apparent position begins to move forward once more. As the viewing angle becomes ever steeper, the apparent movement of Mars accelerates. The entire process ends 8 months after it started. In mid-December, as the movie closes, Venus comes into view again, this time in the West. 

The movie's visual orientation is keyed to the celestial reference points of the constellation Capricornus and is tracking it for the 8 months while the apparent position is platted against it along the line of the ecliptic. 

The puzzle that remains, is the loop of the retrograde. There should be no loop apparent if the Earth and the planet are on the same plane of the ecliptic.  In reality the alignment of the planets to the solar-system's ecliptic is less than perfect, with a difference in orbital inclination between the Earth and Mars of nearly two degrees, as the table below indicates. The difference in the inclination is naturally most apparent during the time of the closest encounter. It reflects the slightly changing visual perspective that results.

The following table (by Wikipedia) shows the orbital inclination differences.

 

Inclination

Name

Inclination
to ecliptic

Terrestrials

Mercury

7.01°

Venus

3.39°

Earth

Mars

1.85°

Gas giants

Jupiter

1.31°

Saturn

2.49°

Uranus

0.77°

Neptune

1.77°

 

Mars retrogrades every 25.5 months when the process is repeated against the background of different constellations.

Also, Mars is not the only planet that retrogrades. Even distant Neptune and Pluto retrograde. 

Retrograding Neptune

The planet Neptune is 30 times as distant from the Sun than the Earth, which makes the viewing angles extremely steep all the time, almost parallel (the illustration to the left is exaggerated). The movement of Neptune is minuscule. It takes Neptune 165 years to complete an orbit around the Sun. Because of its minuscule movement it retrogrades almost the entire 6 months period of the Earth moving from position 1 to 2. In practice it retrogrades 22 days less (158 days instead of the maximum 180 days), for reasons that the planet does move slightly.

  • Mars retrogrades for 72 days every 25.6 months.

  • Jupiter for 121 days every 13.1 months.

  • Saturn for 138 days every 12.4 months.

  • Uranus for 151 days every 12.15 months and

  • Neptune for 158 days every 12.07 months.

When the Earth moves from point A to slightly past point 1 (East to West), the planet appears to move forward (West to East). From this point on, and all the way to close to point 2, (as the Earth orbits from 'West' to' East,' during almost that entire period), the planet appears to move backwards against the background of the fixed stars (retrograding East to West). As the Earth's orbit now moves the Earth in the opposite  relative direction, to point B, the planet appears to move forward again in the normal West-to-East direction.

The entire process is repeated every year, plus the tiny bit that Neptune has advanced during that year. The  other planets take slightly longer to repeat the process as their faster orbiting advances them a greater distance. For every consecutive retrogression cycle the starry background will be different as the orbit of the Earth around the Sun exposes the night sky to different areas of the celestial sphere, a sphere seen from the inside bringing into view different constellations according to the season . 

The best constellation maps are divided into four zones of orientation according to the seasons. Here is a nicely-made example of the seasonal star maps.

 



Planet
Distance
(AU)1
Rotation2

Period
Orbital

Period

Orbital
Speed
km/sec
Axis3

Tilt

Radius4

(x RE)

Mass5

(x ME)

Atmosphere

composition

Dipole7

Tilt

Density

Water = 1

Sun 0.000 25 days# N/A N/A 7.25o& 109+ 332,000 73 % hydrogen
24 % helium
X$ 1.4
Mercury 0.387 58.7 days 88 days 47.9 ~0o 0.382 0.055   ~10o 5.6
Venus 0.723 243 days 225 days 35.0 -2o 0.949 0.815 95 % CO2
3.5 % nitrogen
N/A 5.2
Earth 1.000 23.9 hrs 365 days 29.8 23.5o 1.000 1.000 78 % nitrogen
20 % oxygen
11.5o 5.52
Mars 1.520 24.6 hrs 1.88 yrs 24.1 24o 0.533 0.107 100 % CO2 ? 3.95
Jupiter 5.200 9.92 hrs 11.9 yrs 13.1 3.1o 11.2 318 89 % hydrogen
10 % helium
9.6o 1.3
Saturn 9.540 10.7 hrs 29.5 yrs 9.64 29o 9.45 95.2 96 % hydrogen
3 % helium
0.7o 0.704
Uranus 19.200 -17.29hrs 84.0 yrs 6.81 -82.1o 4.1 14.6 83 % hydrogen
15 % helium
59o 1.21
Neptune 30.100 16.11 hrs 165 yrs 5.43 28.8o 3.88 17.2 80 % hydrogen
19 % helium
47o 1.67
Pluto 39.400 6.39 days 248 yrs 4.74 >50o? ~0.24 0.002 42 % oxygen
24 % sodium
22 % hydrogen
N.K.*  

Original published by the Astronomical Society of the Pacific (1983) - addition in yellow by Rolf Witzsche 2011

 

 

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