Retrograde motion is:
The apparent motion of the planets when they appear to move backward (from east to west) from the direction that they move ordinarily with respect to the stars.
Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?
Both Mercury and Venus undergo retrograde motion, but are visually close to the Sun when this happens, so we don't notice it.
Below is Mercury's right ascension from May 15 to July 15 of this year. Note that it starts off increasing, then decreases (retrograde motion) and then increases again.
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Ephemeris / WWW_USER Thu Feb 19 08:02:41 2015 Pasadena, USA / Horizons
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Target body name: Mercury (199) {source: DE-0431LE-0431}
Center body name: Earth (399) {source: DE-0431LE-0431}
Center-site name: GEOCENTRIC
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Start time : A.D. 2015-May-15 00:00:00.0000 UT
Stop time : A.D. 2015-Jul-15 00:00:00.0000 UT
Step-size : 1440 minutes
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Target pole/equ : IAU_MERCURY {East-longitude -}
Target radii : 2439.7 x 2439.7 x 2439.7 km {Equator, meridian, pole}
Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}
Center pole/equ : High-precision EOP model {East-longitude +}
Center radii : 6378.1 x 6378.1 x 6356.8 km {Equator, meridian, pole}
Target primary : Sun
Vis. interferer : MOON (R_eq= 1737.400) km {source: DE-0431LE-0431}
Rel. light bend : Sun, EARTH {source: DE-0431LE-0431}
Rel. lght bnd GM: 1.3271E+11, 3.9860E+05 km^3/s^2
Atmos refraction: NO (AIRLESS)
RA format : HMS
Time format : CAL
EOP file : eop.150218.p150512
EOP coverage : DATA-BASED 1962-JAN-20 TO 2015-FEB-18. PREDICTS-> 2015-MAY-11
Units conversion: 1 au= 149597870.700 km, c= 299792.458 km/s, 1 day= 86400.0 s
Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (>38.000=NO), Daylight (NO )
Table cut-offs 2: Solar Elongation ( 0.0,180.0=NO ),Local Hour Angle( 0.0=NO )
**************************************************************************************************************
Date__(UT)__HR:MN R.A._(ICRF/J2000.0)_DEC APmag S-brt delta deldot S-O-T /r S-T-O
**************************************************************************************************************
$$SOE
2015-May-15 00:00 04 42 01.78 +24 08 32.2 1.53 4.36 0.69142867651373 -28.9445515 18.6988 /T 129.3762
2015-May-16 00:00 04 43 23.87 +24 00 54.9 1.74 4.49 0.67512627148213 -27.4926615 18.0144 /T 132.4768
2015-May-17 00:00 04 44 24.75 +23 51 35.3 1.96 4.63 0.65967892310678 -25.9844651 17.2466 /T 135.5875
2015-May-18 00:00 04 45 04.61 +23 40 37.1 2.21 4.77 0.64511881456032 -24.4211059 16.3963 /T 138.7127
2015-May-19 00:00 04 45 23.80 +23 28 04.3 2.47 4.91 0.63147747327791 -22.8037380 15.4651 /T 141.8554
2015-May-20 00:00 04 45 22.86 +23 14 01.5 2.75 5.06 0.61878570842679 -21.1337209 14.4555 /T 145.0181
2015-May-21 00:00 04 45 02.49 +22 58 33.7 3.05 5.20 0.60707343838108 -19.4128031 13.3707 /T 148.2014
2015-May-22 00:00 04 44 23.61 +22 41 46.8 3.37 5.33 0.59636941752453 -17.6432855 12.2149 /T 151.4048
2015-May-23 00:00 04 43 27.36 +22 23 47.7 3.71 5.45 0.58670087720487 -15.8281545 10.9939 /T 154.6255
2015-May-24 00:00 04 42 15.08 +22 04 44.1 4.07 5.56 0.57809309959244 -13.9711803 9.7148 /T 157.8583
2015-May-25 00:00 04 40 48.34 +21 44 44.8 4.46 5.63 0.57056894619759 -12.0769753 8.3871 /T 161.0937
2015-May-26 00:00 04 39 08.89 +21 23 59.8 4.87 5.66 0.56414836550867 -10.1510071 7.0239 /T 164.3154
2015-May-27 00:00 04 37 18.70 +21 02 40.1 5.29 5.62 0.55884790663755 -8.1995645 5.6456 /T 167.4924
2015-May-28 00:00 04 35 19.87 +20 40 57.9 n.a. n.a. 0.55468026744915 -6.2296724 4.2893 /T 170.5572
2015-May-29 00:00 04 33 14.63 +20 19 06.0 n.a. n.a. 0.55165390565664 -4.2489596 3.0439 /T 173.3313
2015-May-30 00:00 04 31 05.30 +19 57 18.0 n.a. n.a. 0.54977273920552 -2.2654833 2.1722 /T 175.2574
2015-May-31 00:00 04 28 54.24 +19 35 48.1 n.a. n.a. 0.54903595776444 -0.2875219 2.2105 /L 175.1837
2015-Jun-01 00:00 04 26 43.78 +19 14 50.1 n.a. n.a. 0.54943796056976 1.6766529 3.1266 /L 173.1900
2015-Jun-02 00:00 04 24 36.24 +18 54 38.3 n.a. n.a. 0.55096842790710 3.6190086 4.3897 /L 170.4268
2015-Jun-03 00:00 04 22 33.81 +18 35 25.8 5.29 5.65 0.55361252502737 5.5319758 5.7564 /L 167.4122
2015-Jun-04 00:00 04 20 38.57 +18 17 25.2 4.88 5.70 0.55735122910170 7.4086481 7.1443 /L 164.3125
2015-Jun-05 00:00 04 18 52.42 +18 00 48.1 4.49 5.68 0.56216176249744 9.2429487 8.5180 /L 161.1913
2015-Jun-06 00:00 04 17 17.09 +17 45 44.4 4.12 5.62 0.56801810956371 11.0297524 9.8584 /L 158.0785
2015-Jun-07 00:00 04 15 54.09 +17 32 22.7 3.77 5.53 0.57489158958468 12.7649573 11.1530 /L 154.9902
2015-Jun-08 00:00 04 14 44.76 +17 20 50.1 3.45 5.42 0.58275145605987 14.4455044 12.3929 /L 151.9356
2015-Jun-09 00:00 04 13 50.19 +17 11 11.8 3.15 5.31 0.59156549247635 16.0693469 13.5713 /L 148.9204
2015-Jun-10 00:00 04 13 11.32 +17 03 31.6 2.87 5.18 0.60130057734526 17.6353760 14.6833 /L 145.9474
2015-Jun-11 00:00 04 12 48.85 +16 57 51.6 2.61 5.06 0.61192319592983 19.1433125 15.7250 /L 143.0178
2015-Jun-12 00:00 04 12 43.37 +16 54 12.5 2.37 4.94 0.62339988172426 20.5935736 16.6938 /L 140.1313
2015-Jun-13 00:00 04 12 55.28 +16 52 33.7 2.15 4.81 0.63569757625371 21.9871244 17.5876 /L 137.2863
2015-Jun-14 00:00 04 13 24.86 +16 52 53.2 1.94 4.69 0.64878390050511 23.3253217 18.4055 /L 134.4805
2015-Jun-15 00:00 04 14 12.31 +16 55 08.3 1.75 4.58 0.66262733523404 24.6097559 19.1469 /L 131.7103
2015-Jun-16 00:00 04 15 17.69 +16 59 15.2 1.57 4.46 0.67719731085661 25.8420965 19.8117 /L 128.9719
2015-Jun-17 00:00 04 16 41.05 +17 05 09.2 1.40 4.35 0.69246421084326 27.0239479 20.4002 /L 126.2604
2015-Jun-18 00:00 04 18 22.35 +17 12 45.4 1.25 4.24 0.70839929524826 28.1567176 20.9133 /L 123.5707
2015-Jun-19 00:00 04 20 21.53 +17 21 57.8 1.10 4.14 0.72497455264057 29.2415000 21.3516 /L 120.8972
2015-Jun-20 00:00 04 22 38.49 +17 32 40.5 0.96 4.04 0.74216248879516 30.2789728 21.7164 /L 118.2337
2015-Jun-21 00:00 04 25 13.15 +17 44 46.7 0.83 3.94 0.75993585914434 31.2693054 22.0086 /L 115.5738
2015-Jun-22 00:00 04 28 05.39 +17 58 09.6 0.71 3.84 0.77826734979511 32.2120717 22.2296 /L 112.9109
2015-Jun-23 00:00 04 31 15.13 +18 12 42.2 0.59 3.75 0.79712920966069 33.1061670 22.3805 /L 110.2380
2015-Jun-24 00:00 04 34 42.27 +18 28 17.0 0.48 3.65 0.81649283449812 33.9497246 22.4625 /L 107.5478
2015-Jun-25 00:00 04 38 26.75 +18 44 46.2 0.37 3.56 0.83632830264123 34.7400316 22.4769 /L 104.8329
2015-Jun-26 00:00 04 42 28.49 +19 02 02.2 0.27 3.47 0.85660386201288 35.4734443 22.4247 /L 102.0858
2015-Jun-27 00:00 04 46 47.47 +19 19 56.7 0.17 3.38 0.87728536859778 36.1453054 22.3070 /L 99.2988
2015-Jun-28 00:00 04 51 23.65 +19 38 21.2 0.07 3.30 0.89833567803594 36.7498651 22.1247 /L 96.4642
2015-Jun-29 00:00 04 56 17.02 +19 57 07.2 -0.02 3.21 0.91971399456822 37.2802130 21.8789 /L 93.5742
2015-Jun-30 00:00 05 01 27.57 +20 16 05.6 -0.12 3.13 0.94137518555319 37.7282290 21.5704 /L 90.6210
2015-Jul-01 00:00 05 06 55.30 +20 35 07.1 -0.20 3.05 0.96326907552925 38.0845640 21.2001 /L 87.5974
2015-Jul-02 00:00 05 12 40.17 +20 54 01.8 -0.29 2.96 0.98533974152847 38.3386660 20.7688 /L 84.4961
2015-Jul-03 00:00 05 18 42.16 +21 12 39.8 -0.38 2.88 1.00752484088954 38.4788698 20.2775 /L 81.3104
2015-Jul-04 00:00 05 25 01.19 +21 30 50.5 -0.46 2.80 1.02975501346900 38.4925685 19.7269 /L 78.0344
2015-Jul-05 00:00 05 31 37.13 +21 48 23.0 -0.54 2.73 1.05195341081967 38.3664848 19.1182 /L 74.6631
2015-Jul-06 00:00 05 38 29.80 +22 05 05.9 -0.63 2.65 1.07403541451894 38.0870574 18.4526 /L 71.1926
2015-Jul-07 00:00 05 45 38.91 +22 20 47.8 -0.71 2.57 1.09590861358459 37.6409536 17.7315 /L 67.6206
2015-Jul-08 00:00 05 53 04.08 +22 35 16.7 -0.79 2.49 1.11747311584451 37.0157141 16.9564 /L 63.9467
2015-Jul-09 00:00 06 00 44.78 +22 48 20.6 -0.87 2.41 1.13862226817234 36.2005225 16.1294 /L 60.1725
2015-Jul-10 00:00 06 08 40.36 +22 59 47.7 -0.95 2.34 1.15924385212199 35.1870766 15.2529 /L 56.3023
2015-Jul-11 00:00 06 16 49.99 +23 09 26.1 -1.03 2.26 1.17922180051076 33.9705111 14.3295 /L 52.3431
2015-Jul-12 00:00 06 25 12.69 +23 17 04.6 -1.11 2.17 1.19843844390623 32.5502953 13.3626 /L 48.3050
2015-Jul-13 00:00 06 33 47.27 +23 22 32.8 -1.19 2.09 1.21677724378686 30.9310023 12.3560 /L 44.2011
2015-Jul-14 00:00 06 42 32.40 +23 25 41.0 -1.28 2.00 1.23412590569533 29.1228358 11.3140 /L 40.0476
2015-Jul-15 00:00 06 51 26.58 +23 26 21.1 -1.37 1.91 1.25037969990154 27.1418048 10.2415 /L 35.8640
$$EOE
**************************************************************************************************************
Column meaning:
TIME
Prior to 1962, times are UT1. Dates thereafter are UTC. Any 'b' symbol in
the 1st-column denotes a B.C. date. First-column blank (" ") denotes an A.D.
date. Calendar dates prior to 1582-Oct-15 are in the Julian calendar system.
Later calendar dates are in the Gregorian system.
Time tags refer to the same instant throughout the universe, regardless of
where the observer is located.
The dynamical Coordinate Time scale is used internally. It is equivalent to
the current IAU definition of "TDB". Conversion between CT and the selected
non-uniform UT output scale has not been determined for UTC times after the
next July or January 1st. The last known leap-second is used over any future
interval.
NOTE: "n.a." in output means quantity "not available" at the print-time.
R.A._(ICRF/J2000.0)_DEC =
J2000.0 astrometric right ascension and declination of target center.
Adjusted for light-time. Units: HMS (HH MM SS.ff) and DMS (DD MM SS.f)
APmag S-brt =
Target's approximate apparent visual magnitude & surface brightness. For
planets and satellites, values are available only for solar phase angles in the
range generally visible from Earth. This is to avoid extrapolation of models
beyond their valid (data-based) limits.
Units: MAGNITUDE & VISUAL MAGNITUDES PER SQUARE ARCSECOND
delta deldot =
Range ("delta") and range-rate ("delta-dot") of target center with respect
to the observer at the instant light seen by the observer at print-time would
have left the target center (print-time minus down-leg light-time); the
distance traveled by a light ray emanating from the center of the target and
recorded by the observer at print-time. "deldot" is a projection of the
velocity vector along this ray, the light-time-corrected line-of-sight from the
coordinate center, and indicates relative motion. A positive "deldot" means the
target center is moving away from the observer (coordinate center). A negative
"deldot" means the target center is moving toward the observer.
Units: AU and KM/S
S-O-T /r =
Sun-Observer-Target angle; target's apparent solar elongation seen from
observer location at print-time. If negative, the target center is behind
the Sun. Angular units: DEGREES.
The '/r' column is a Sun-relative code, output for observing sites
with defined rotation models only.
/T indicates target trails Sun (evening sky)
/L indicates target leads Sun (morning sky)
NOTE: The S-O-T solar elongation angle is the total separation in any
direction. It does not indicate the angle of Sun leading or trailing.
S-T-O =
"S-T-O" is the Sun->Target->Observer angle; the interior vertex angle at
target center formed by a vector to the apparent center of the Sun at
reflection time on the target and the apparent vector to the observer at
print-time. Slightly different from true PHASE ANGLE (requestable separately)
at the few arcsecond level in that it includes stellar aberration on the
down-leg from target to observer. Units: DEGREES
Computations by ...
Solar System Dynamics Group, Horizons On-Line Ephemeris System
4800 Oak Grove Drive, Jet Propulsion Laboratory
Pasadena, CA 91109 USA
Information: http://ssd.jpl.nasa.gov/
Connect : telnet://ssd.jpl.nasa.gov:6775 (via browser)
telnet ssd.jpl.nasa.gov 6775 (via command-line)
Author : [email protected]
**************************************************************************************************************
You would need to have a planet orbit very slowly around the star. A planet moves retrograde because the planet is passing the other in its orbit so that the slower planet appears to be going backwards.
So in order for this to happen you would need a planet having an orbit such that the outer planet passes the inner planet. But I think that this would result in an unstable orbit for one of the two planets. Either the inner one would fall into the orbiting star or the outer one would escape.
The inner planets have a smaller orbit so they move "faster" in order to not fall into the sun. So they are always "passing" the planets in orbits that are outside of their orbit.
TL;DR Not for planets, but for other stuff yes.
As explained in an earlier answer, planets are on circular orbits (quasi-circular actually but that's a detail). The closer to the star, the faster the planet is on its orbit. So, the retrograde motion observed for Mars cannot happen for Venus for instance. At most you could see Venus going retrograde when it actually turns around the sun, but you would clearly understand that the planet is just on the other side of the solar system -- which I think is not what you were wondering about.
But there's another type of situation where you see a retrograde orbit for an inner body: if the orbit is not circular but a nice ellipse! The orbit would have to be very eccentric though (very narrow). Planets do not have eccentric orbits (in the solar system at least) because, in short, the formation of planets favors circular orbits in the long term. But some asteroids are in this situation (for example many in the Aten family).
If the perihelion (closest point) is very close to the sun and the aphelion (farthest point) is much further, then even if the asteroid remains in the interior of the Earth's orbit you would see it going backwards near aphelion. What happens is that speed on a eccentric (elliptic) orbit is slower at the apoapse. For instance, if your perihelion was grazing the surface of the sun, you would have a speed of almost 0 at your aphelion. And you would see it drifting backward because of Earth's motion.
Unsure where the original poster obtained an idea that objects inside the Earth’s orbit, as seen from Earth, “move ordinarily with respect to the stars” from west to east.
First of all, of which “west” and “east” do we speak? To be specific, let’s understand these for the region of sky not far from the meridian. That is, when the object X culminates, then nearby objects having the right ascension greater than RA(X) are on the east from the meridian (not yet culminated), whereas those having RA < RA(X) are on the west from the meridian (already culminated).
For the Sun itself, every educated person can remember that its diurnal motion is slightly slower than diurnal motion of the inertial celestial sphere (fixed stars) – that is, with respect to those stars Sun goes from west to east. Its right ascension increases monotonically. For other objects inside the Earth’s orbit, their right ascension is within 90° of the Sun’s, hence in average they move “from west to east” as well, and quicker do it when are behind the Sun.
But definitely the terrestrial picture is opposite for e.g. Venus when it overtakes Earth in their orbital motions near the inferior conjunction.
During this part of the orbit the right ascension of Venus decreases – seen on this animation near the end of cycle. “Experts” asserting that it’s impossible to see mistake – Venus is not very close to Sun during it, and anyway perfectly observable (albeit rare) during Venus’s transits before the Sun.
