Compute and Visualize Retrograde Motion

Explore the apparent backwards motion of the outer planets in the sky

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Retrograde motion of the outer planets starts and ends near the time of opposition, when a planet is opposite from the Sun in the sky.

Set up a utility function to get the time of the next opposition of a planet:

In[1]:=

getNextOpposition[entity_Entity]:=entity[Dated["OppositionTimeNext",DateObject[{2022,7,1}]]]

Define a function to construct a list of positions of a planet in a range of dates surrounding its next opposition:

In[2]:=

planetPositionNearOpposition[entity_Entity,offset_Quantity,step_Quantity]:=Module[{opposition,daterange},opposition=getNextOpposition[entity];If[MissingQ[opposition],Return[Missing["NotApplicable"]]];daterange=DateRange[opposition-offset,opposition+offset,step];AstroPosition[entity,{"Equatorial",#}]&/@daterange]

Compute the position of Mars every 3 days, from 5 months before to 5 months after opposition:

In[3]:=

mars=planetPositionNearOpposition

Mars

PLANET

,Quantity[5,"Months"],Quantity[3,"Days"];

Visualize Mars moving from west to east (right to left) near and along the ecliptic plane, but then appearing to move backwards before resuming its typical westward motion:

In[4]:=

AstroGraphics[{White,Point[mars]},AstroReferenceFrame"Ecliptic"]

Out[4]=

All of the outer planets can exhibit retrograde motion, for example Neptune:

In[5]:=

neptune=planetPositionNearOpposition

Neptune

PLANET

,Quantity[8,"Months"],Quantity[3,"Days"];

In[6]:=

AstroGraphics[{White,Point[neptune]},AstroReferenceFrame"Ecliptic"]

Out[6]=

Planets inside the orbit of Earth, like Venus and Mercury, do not exhibit retrograde motion:

In[7]:=

planetPositionNearOpposition

Venus

PLANET

,Quantity[8,"Months"],Quantity[3,"Days"]

Out[7]=

Missing[NotApplicable]

Related Symbols

AstroGraphics
AstroPosition

Publisher Information

Contributed by: Jeff Bryant