THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970

             "And that explains everything."

THE DAILY ASTRONOMER

Thursday, May 12, 2016

Mercury in Retrograde

Some philosophers believe that there are no coincidences.   Everything that occurs in life does so for a specific reason.   Coincidences that appear to emerge from randomness are merely illusions created by our finite minds.  Were we, like political candidates, endowed with infinite wisdom, we would perceive the greater order of things and, in so doing, likely lapse into despondency.     For instance, most of the time life in the Southworth Planetarium is a leisurely stroll through perfumed gardens.  This week has been more of an adventure, one that we believe will soon end in a resolution which will vanquish the wicked and elevate the benevolent.  (Yes, I know I'm stuffed with flannel.)

So, imagine our surprise when we dipped into Pandora's Jar this morning and withdrew a question about Mercury in retrograde.   Mercury's retrograde motion has often been  blamed for the myriad troubles with which life assails us.    It is difficult to believe that choosing such a query during such an interesting (Chinese definition) week is mere coincidence.  Instead, it is the cosmos informing us that "Ha, serves you right, fathead!"

Also, again, we have time for one Pandora.  In other words, we are too cowardly to pick another one.

"What does it mean with Mercury is in retrograde?  I know that it supposed to mean bad things are going to happen, but why does Mercury go into retrograde?"

-Mary W, Belgrade

When a planet's position shifts from west to east over the course of many nights, it is said to be in "prograde motion,"    However, at times, a planet can appear to halt (be stationary) and then move in a retrograde manner: east to west.   (Note:  One cannot directly observe this movement.  Instead, a planet's motion is perceived only by its position shifts from night to night.)    

Both inferior planets -those closer to the Sun than Earth- and superior planets -those farther away- can exhibit both prograde and retrograde motions.    A superior planet appears to move in a retrograde manner whenever the faster moving* Earth catches up to a planet, moves between the planet and the Sun (opposition) and then moves beyond it.   From our perspective, the superior planet appears to move backward against the stellar backdrop.   When passing slower moving cars on a freeway, passengers in the faster car perceive the slower vehicle appear to go backward against the more distant objects.   The car isn't actually moving backward, but merely seems to do so from your perspective.   We remember that when it comes to planets, we're watching moving objects from a moving platform.

Inferior planets, -i.e. Mercury and Venus- revolve around the Sun just as all the other planets do.  However, we watch their orbits from the outside, just as spectators as a car race see the automobiles moving rapidly along an oval track.    When the car moves along the same side of the track as a given spectator, it goes in one direction,  When it whips around the curve it might appear stationary for a moment before it races along the track's other side.      The car's direction changes as a consequence of its oval track.   The same is true for our view of inferior planets.    At times they move between Earth and the Sun and eventually will move along the far side of the Sun relative to Earth.  These direction changes result in a shift from prograde to retrograde and then back to prograde motion.    The one difference is that Earth is always moving, so the motions appear more complex.

Mercury is often in retrograde but, then again, is often in prograde motion too.     I suppose we have to try to make the best of it.

*We recall from Kepler's Second Planetary Law that the closer a planet is to the Sun, the faster it moves.