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fagricipni
2016-Aug-15, 04:11 AM
I understand that the superior planets (the ones farther away from the sun than Earth) usually appear to travel from west to east in the sky relative to the stars (as seen from Earth); ie, in the direction of increasing right ascension, but near opposition they appear to reverse direction with respect to the stars and travel from east to west; This reversal is called (apparent) retrograde motion What I am not so clear on is the inferior planets (closer to the sun than Earth). If I understand correctly they also usually appear to travel from west to east in the sky relative to the stars, but near inferior conjunction (when passing between the sun and Earth) they also appear to reverse direction and travel retrograde. Is my understanding correct?

Jens
2016-Aug-15, 10:47 AM
I don't think your explanation is correct but I could be wrong. For the outer planets the apparent reversal happens when they are not in opposition, when the earth overtakes them when they are on the same side of the sun. The inner planets also would appear to reverse direction (at opposition, in that case) but we can't see this obviously since it's daytime.

fagricipni
2016-Aug-15, 06:34 PM
I don't think your explanation is correct but I could be wrong. For the outer planets the apparent reversal happens when they are not in opposition, when the earth overtakes them when they are on the same side of the sun.

You appear to agree with me with the outer planets; the reversal occurs when both planets are on the same side of the sun.


The inner planets also would appear to reverse direction (at opposition, in that case) but we can't see this obviously since it's daytime.

Inner planets can't be in opposition.

This seems to be a case of my not defining terms -- though I was under the impression that they were standard.

This diagram at Wikipedia (http://commons.wikimedia.org/wiki/File:Positional_astronomy.png) shows how the terms are used.

Jens
2016-Aug-16, 01:53 AM
Sorry, your understanding is correct. I had the terms backward. So yes, in inferior conjunction the inferior planets would appear to go into retrograde motion if we could see them.

tony873004
2016-Aug-17, 02:01 PM
Here's a simulation where you can select a planet and watch its motion against the background stars as it orbits the Sun.
http://orbitsimulator.com/gravitySimulatorCloud/simulations/1471441987223_retrograde.html

Robert Tulip
2016-Aug-19, 07:40 AM
http://cosmoquest.org/forum/showthread.php?159639-Planet-Positions-2016 shows forward and retrograde motion for all planets for 2016. When the lines are going up the planet appears to be moving forward - west to east, and when the line goes down it appears to move retrograde - east to west.

If you look at Mercury next to Jupiter in the evening sky this week, you will see it station around the time of the solar eclipse on 1 September and then move rapidly westward to appear in the morning in a few weeks.

The term "opposition" is explained at https://en.wikipedia.org/wiki/Opposition_(planets) The opposition point when an outer planet rises at dusk is the inflection point of the retrograde curve.

fagricipni
2016-Aug-20, 06:57 AM
I found http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78780/Retro_Nav.swf::Retrograde%20Motion which shows retrograde motion well. It doesn't show the phases of the planets, though.

My problem is that all of my astronomy books and all the sites I could find before asking the question here explained the retrograde motion of the superior planets, but not of the inferior planets. I hadn't thought of Jens' idea: that the retrograde motion of the inner planets occurs mostly when they are lost in the glare of the sun; which could explain why most writers use Mars as the example in their diagram of retrograde motion.

Actually, I first found out about the inferior planets going retrograde when my astrology-minded sibling spoke of things going wrong because "Mercury is in retrograde". Unfortunately, I doubt if ey understands what "being in retrograde" means in terms of observational astronomy. So I tried to find out when the inferior planets did go in to retrograde motion; and failed to find a site that didn't use Mars as its example and discussed the other superior planets as undergoing the same phenomenon. So that was why I understood the conditions for retrograde motion for the superior planets, but not the inferior planets.

wd40
2016-Aug-20, 07:33 PM
Actually, I first found out about the inferior planets going retrograde when my astrology-minded sibling spoke of things going wrong because "Mercury is in retrograde". Unfortunately, I doubt if ey understands what "being in retrograde" means in terms of observational astronomy.

Astrologers use the Ptolemaic model. In terms of observational equality of representing the proportions of retrograde motion, it should be identical with the Copernican model.

fagricipni
2016-Aug-20, 10:31 PM
Actually, I first found out about the inferior planets going retrograde when my astrology-minded sibling spoke of things going wrong because "Mercury is in retrograde". Unfortunately, I doubt if ey understands what "being in retrograde" means in terms of observational astronomy.

Astrologers use the Ptolemaic model. In terms of observational equality of representing the proportions of retrograde motion, it should be identical with the Copernican model.

Um, your comment is difficult to decipher; perhaps mine was unclear. I wasn't speaking of "professional astrologers"; I was speaking of my sibling as not knowing that "being in retrograde" refers to a reversal in the motion of the planets relative to the star background. It would be interesting to ask my sibling what it means the next time ey speaks of "Mercury in retrograde".

As far as the "equivalence" between the geocentric and heliocentric models, it's possible to make the geocentric model fit the observations with a complicated set of epicycles; however, the mechanism behind retrograde motion is obviously different between those models; so in that sense, the models are not equivalent. However, if you simply mean that the astronomical events referred to by astrologers do occur at the same time at the actual true astronomically observed events, that's nice to know -- though not something I'm likely to use.

Hornblower
2016-Aug-21, 01:45 AM
It has always been my understanding that astrologers are concerned only with the angular positions and motions of the celestial objects, not the causative mechanisms. In that case, I would consider the Ptolemaic and Copernican models, suitably tweaked for accuracy, to be equivalent. The tweaking, to correct for the eccentricity of the actual orbits, could be by means of equants (Ptolemy), epicycles (Copernicus) or Keplerian ellipses.