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  1. #1
    Tom Hise's Avatar
    Tom Hise Guest

    Default Mercury in retrograde



    I have a friend who frequently talks about "Mercury in retrograde". She
    is, I believe, referring to some astrological mumbo-jumbo but it got me
    wondering. Can an inferior planet such as Mercury or Venus ever exhibit
    retrograde motion? I know the exterior planets can appear to reverse
    direction relative to the background stars but I am not sure it that can
    happen for Mercury.

    TIA,

    Tom Hise



  2. #2
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Mercury in retrograde

    On Mon, 06 Oct 2008 16:16:01 -0500, Tom Hise <nc0o@yahoo.com> wrote:


    Inferior planets also exhibit retrograde motion.
    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  3. #3
    MAT's Avatar
    MAT Guest

    Default Mercury in retrograde


    "Charlie Siegrist" <owl tuna at hot mail dot com> wrote in message
    news:b09le41r2rq6sof2qr0f4sfqen8e3q89vf@4ax.com...


    But Mercury is the FTD florist.



  4. #4
    Charlie Siegrist's Avatar
    Charlie Siegrist Guest

    Default Mercury in retrograde

    On Mon, 06 Oct 2008 23:01:41 GMT, "MAT" <MAT@telstrat.com> wrote:


    Hermes no doubt, don't know about Mercury.


  5. #5
    Quadibloc's Avatar
    Quadibloc Guest

    Default Mercury in retrograde

    On Oct 6, 3:16 pm, Tom Hise <n...@yahoo.com> wrote:

    Oh, most certainly it can. Mercury orbits the Sun in the same counter-
    clockwise direction as the Earth, and it has a shorter orbital period.
    The direction from the Earth to the Sun, being 180 degrees from the
    direction from the Sun to the Earth, moves in the same counter-
    clockwise direction as the Earth does, with the same period, and this
    movement is added to that of Mercury.

    When Mercury is at superior conjunction with the Sun, being behind the
    Sun as viewed from Earth, then Mercury's real motion and the Sun's
    apparent motion are both in the same direction, and so Mercury's
    apparent motion is direct.

    What about when Mercury is at inferior conjunction? Will its motion be
    retrogade?

    Its angular motion is faster than that of the Earth. But what matters
    is its linear motion. That is slower than the apparent linear motion
    of the Sun. So, if that were not outweighed by the fact that Mercury,
    at inferior conjunction, is closer to us than the Sun, indeed Mercury
    never would exhibit retrogade motion. Thus, a planet *very* close to
    the Sun would never be retrogade.

    John Savard

  6. #6
    canopus56's Avatar
    canopus56 Guest

    Default Mercury in retrograde

    On Oct 6, 3:16 pm, Tom Hise <n...@yahoo.com> wrote:

    A visible portion of retrograde for an inferior planet occurs during
    the inferior planet's 1/4 orbit before each inferior conjunction with
    the Earth. During that period of an inferior planet's orbit, they
    appear to move towards the Sun, approaching it from the east. Because
    it is to the east of the Sun, the planet still is visible for a short
    time after the Sun sets. The apparent retrograde motion of Mercury
    continues for a period after inferior conjunction, but, since the
    planet is to the west of the Sun, it sets first and is not visible.

    See the two top diagrams on this page:

    http://www.synapses.co.uk/astro/planets2.html

    You can look at this apparent effect using the NASA JPL Planet
    Simulator at:

    http://space.jpl.nasa.gov/

    Set the simulator to look at "Mercury" from the "Earth" beginning at
    let's say Sep. 21 and looking at each day through Oct. 8, 2008.

    Any basic desktop planetarium program, used with respect to the dates
    discussed below, is also illustrative.

    Another useful simulator to fix the concept in your mind is an old
    David Chandler program done in DOS called "Planet Tracker". You can
    download it from SEDS at:

    ftp://ftp.seds.org/pub/software/pc/solar/pltver42.zip

    See also Chandler's page at http://www.davidchandler.com/pltrack.htm

    For Mercury, its synodic period is about 115 days, so you are talking
    about 115/4 or about 28-29 days of retrograde motion. All of that will
    not be visible from Earth, since you cannot see Mercury as it gets to
    close to the Sun.

    Inferior conjunctions between Mercury and the Earth occur through 2009
    on:

    10-06-2008 (today)
    01-09-2009
    05-18-2009
    09-20-2009

    The greatest eastern enlongations for Mercury corresponding a few
    weeks before the remaining inferior conjunction are:

    01-04-2009
    04-26-2009
    08-24-2009

    and for a 2010 inferior conjunction on - :

    12-18-2009

    Beginning on those greatest eastern enlongation dates for 2009 is when
    I would check your 2009 RASC Handbook for good dates to watch Mercury
    in retrograde motion in the setting western sky.

    You can also see some of Mercury's retrograde motion in the rising sky
    between the weeks after the inferior conjunctions but before the date
    Mercury's greatest western enlongation. Here, Mercury is visible
    before the sun rises. During this period, Mercury is still moving in
    a retrograde apparent motion relative to the motion of the Sun, but it
    gets progressively farther from the Sun each day.

    The dates of the remaining 2008 and 2009 greatest western enlongations
    for Mercury with respect to the inferior conjuctions listed above
    are:

    10-22-2008
    02-13-2009
    06-13-2009
    10-06-2009

    About a week to 10 days before each of the above dates, I would check
    your 2008-2009 RASC Handbook for good times to look for retrograde
    motion of Mecury just before sunrise.

    It is probably better to practice observing retrograde motion on Venus
    because it is further from the Sun and has a larger window to observe
    the daily changes in retrograde motion. Venus is presently rising in
    the western sky at sunset. It will move progressively further from
    the Sun until its greatest eastern enlongation on January 14, 2009.
    Then Venus begins its apparent retrograde motion through and beyond
    the next inferior conjunction on March 27, 2009. The really big daily
    changes in Venus's apparent retrograde motion do not begin until the
    end of February and the beginning of March, 2009.

    Hope that helps and isn't too confusing.

    - Canopus56


  7. #7
    MAT's Avatar
    MAT Guest

    Default Mercury in retrograde


    Get oriel36 on the job.



  8. #8
    Paul Schlyter's Avatar
    Paul Schlyter Guest

    Default Mercury in retrograde

    In article <nd-dnXz3GeiMHXfVnZ2dnUVZ_sTinZ2d@supernews.com>,
    Tom Hise <nc0o@yahoo.com> wrote:

    Yes, it happens for the inferior planets too.

    Superior planets have their retrogade motion around opposition.
    Inferior planets can never be seen in opposition of course, instead
    they exhibit their retrogade motion around inferior conjunction when,
    as seen from the inferior planet, the Earth will be in opposition and
    appear to have retrogade motion too.



    --
    ----------------------------------------------------------------
    Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
    e-mail: pausch at stjarnhimlen dot se
    WWW: http://stjarnhimlen.se/

  9. #9
    Paul Schlyter's Avatar
    Paul Schlyter Guest

    Default Mercury in retrograde

    In article <a9c15e9a-8feb-4d38-be5b-f9bfac40b893@k7g2000hsd.googlegroups.com>,
    Quadibloc <jsavard@ecn.ab.ca> wrote:

    FYI: Mercury is in fact closer to the Sun than to us when in inferior
    conjunction. In addition, *any* planet in a direct orbit around the Sun,
    and with an orbit which isn't too eccentric or too inclined, will regularly
    appear to have retrogade motion as seen from the Earth, or from any other
    planet. The only way for a celestial body to avoid exhihiting retrogade
    motion is to have an unusual orbit with steep inclination or a very eccentric
    orbit. A celestial body in a sufficiently strongly hyperbolic orbit for
    instance could avoid exhibiting retrogade motion.



    --
    ----------------------------------------------------------------
    Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
    e-mail: pausch at stjarnhimlen dot se
    WWW: http://stjarnhimlen.se/

  10. #10
    Quadibloc's Avatar
    Quadibloc Guest

    Default Mercury in retrograde

    On Oct 7, 2:14 am, pau...@saaf.se (Paul Schlyter) wrote:



    Apologies for writing a sentence that parses ambiguously.

    Yes, the distance from Mercury to the Sun is 0.4 AU, which is less
    than 0.5 AU.

    What I was saying was that at inferior conjunction, the distance
    (Mercury-Earth) is less than the distance (Sun-Earth); that is,
    Mercury, at inferior conjunction, is closer to us than the Sun [is to
    us], not the incorrect statement which you corrected, that Mercury, at
    inferior conjunction, is closer to us than [it is to] the Sun.

    John Savard

 

 
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