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
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.
Mars retrogrades every 25.5
months when the process is repeated against the background of different
Also, Mars is not the
only planet that retrogrades. Even distant Neptune and Pluto retrograde.
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
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.
Water = 1
24 % helium
3.5 % nitrogen
20 % oxygen
10 % helium
3 % helium
15 % helium
19 % helium
24 % sodium
22 % hydrogen
Original published by the Astronomical Society of the Pacific
(1983) - addition in yellow by Rolf Witzsche 2011