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Retrograde Motion

In Indi­an astron­o­my the move­ment of sev­er­al celes­tial objects are described from the per­spec­tive of the observ­er on earth. Be it the path of the sun, moon or the posi­tion of the plan­ets on the zodi­ac, the visu­al­iza­tions are based on what they look from a par­tic­u­lar point (lat­i­tude and lon­gi­tude) on earth.

Ret­ro­grade motion has cap­tured the imag­i­na­tion and fas­ci­na­tion of Indi­an astrologers and astronomers. The san­skrit name for ret­ro­grade is vakra (mean­ing reverse or crooked).

The Mahab­hara­ta has sev­er­al astro­nom­i­cal ref­er­ences and ret­ro­grade motion is a com­mon men­tion in these vers­es. Here are a few vers­es talk­ing about ret­ro­grade motion

Astronomical Aspects of Retrograde Motion

Ret­ro­grade motion is an appar­ent reverse move­ment of a plan­et for an observ­er on earth due to vari­a­tions in the speed of move­ment of earth and the plan­et under obser­va­tion. So plan­ets seem to move east­ward, stop at a point and then start mov­ing west­ward and then stop and move east­ward. This is a repeat­ing pat­tern.

While infe­ri­or plan­ets (plan­et in between the sun and earth i.e mer­cury and venus) seem to make a zig zag motion in the sky, supe­ri­or plan­ets (beyond the earth) seem to form a loop dur­ing this ret­ro­grade motion.

When the earth is mov­ing around the sun, so are oth­er plan­ets. At some point in time it appears that the earth is mov­ing ahead of the oth­er plan­ets and the plan­et appears to be mov­ing back­ward.

To elab­o­rate this, let us take the exam­ple of mer­cury. It takes only 88 days to go around the sun. While it appears that it is going ahead of the earth, at some point when it is com­ing back on its next rev­o­lu­tion it appears to be behind the earth. Hence for an observ­er on earth, it would appear that mer­cury was mov­ing from east to west and now from west to east. It is pure­ly based on per­spec­tive change.

For supe­ri­or plan­ets the pat­tern appears slight­ly dif­fer­ent­ly. Take Mars for instance. It takes 687 days to go around the sun. The earth takes 365 days. Every 26 months, the Earth catch­es up with Mars and moves ahead of Mars. It now looks like Mars is mov­ing back­ward.

The syn­od­ic peri­od is the peri­od between the suc­ces­sive con­junc­tions of the plan­et with the Sun. When a plan­et is seen in the same line as the sun from the earth, it is said to be a con­junc­tion. The moon is in con­junc­tion with the Sun dur­ing Amavasya (new moon).

Here is a video explain­ing ret­ro­grade motion

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It is very inter­est­ing to note that

Here are the ret­ro­grade motions for 2020. It is inter­est­ing to note that sev­er­al plan­ets seem to to be in ret­ro­grade dur­ing a cer­tain months. We leave it to the astrologers to inter­pret.

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