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

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?

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  : SunVis. 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       : HMSTime format     : CAL EOP file        : eop.150218.p150512                                           EOP coverage    : DATA-BASED 1962-JAN-20 TO 2015-FEB-18. PREDICTS-> 2015-MAY-11Units 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 inthe 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 ofwhere the observer is located.  The dynamical Coordinate Time scale is used internally. It is equivalent tothe current IAU definition of "TDB". Conversion between CT and the selectednon-uniform UT output scale has not been determined for UTC times after thenext July or January 1st.  The last known leap-second is used over any futureinterval.  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. Forplanets and satellites, values are available only for solar phase angles in therange generally visible from Earth. This is to avoid extrapolation of modelsbeyond 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 respectto the observer at the instant light seen by the observer at print-time wouldhave left the target center (print-time minus down-leg light-time); thedistance traveled by a light ray emanating from the center of the target andrecorded by the observer at print-time. "deldot" is a projection of thevelocity vector along this ray, the light-time-corrected line-of-sight from thecoordinate center, and indicates relative motion. A positive "deldot" means thetarget 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 fromobserver location at print-time. If negative, the target center is behindthe Sun. Angular units: DEGREES.    The '/r' column is a Sun-relative code, output for observing siteswith 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 anydirection. 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 attarget center formed by a vector to the apparent center of the Sun atreflection time on the target and the apparent vector to the observer atprint-time. Slightly different from true PHASE ANGLE (requestable separately)at the few arcsecond level in that it includes stellar aberration on thedown-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?