Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?

  • 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?


  • barrycarter

    barrycarter Correct answer

    7 years ago

    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.



    *******************************************************************************
    Ephemeris / WWW_USER Thu Feb 19 08:02:41 2015 Pasadena, USA / Horizons
    *******************************************************************************
    Target body name: Mercury (199) {source: DE-0431LE-0431}
    Center body name: Earth (399) {source: DE-0431LE-0431}
    Center-site name: GEOCENTRIC
    *******************************************************************************
    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
    *******************************************************************************
    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]

    **************************************************************************************************************

    By "visually close to the Sun" do you mean we can't see it because the Sun is too bright, and not that Mercury/Venus are behind the Sun?

License under CC-BY-SA with attribution


Content dated before 7/24/2021 11:53 AM

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