Talk:Retrograde and prograde motion

Archive

Old messages: /Archive 1. — Preceding unsigned comment added by 81.132.169.8 (talk) 02:54, October 19, 2009 (UTC)

This is not a busy page therefore there is no need for messages to be auto-archived. Auto-archiving is harmful - it removes messages which are still relevant. They should be manually archived only when they are no longer relevant. — Preceding unsigned comment added by 86.143.135.86 (talk) 13:40, November 4, 2009 (UTC)

Missing information

To make this article comprehensive there are still some more things to include:

• info about differences between prograde and retrograde orbits in general relativity.
• info on the orbits and rotations of stars in binary and multiple star systems 81.132.169.8 (talk) 08:57, 19 October 2009 (UTC)

Pluto

How come this page says there are 8 planets? What about Pluto? 86.24.147.63 (talk) 18:12, 4 September 2011 (UTC)

You didn't get the memo? —Tamfang (talk) 19:41, 10 October 2011 (UTC)

hahaha Pluto is out... ____Ἑλλαιβάριος Ellaivarios____ 01:18, 26 February 2012 (UTC)

the range of inclination

Recent changes:

... An object with an inclination between -90 and +90 degrees is orbiting or revolving in the same direction as the primary is rotating. ... An object with an inclination between 90 degrees and 270 degrees is in a retrograde orbit.

... An object with an axial tilt between 90 degrees and 270 degrees is rotating in the opposite direction to its orbital direction.

Inclination is the angle between two vectors in 3-space; it is not signed, nor can it exceed 180°.

Am I mistaken? Can someone illustrate for me the difference between negative and positive inclination, or between 120° tilt and 240° tilt? —Tamfang (talk) 19:32, 23 October 2011 (UTC)

There are multiple mathematically equivalent descriptions of the same situation. An inclination <0° is the same as a declination >0° with the line of nodes rotated 180°. An inclination >90° is the same as an inclination <90° with the period negated. Retrograde motion can be described as motion with an inclination >90° with a period >0, or as motion with an inclination <90° with a period <0. It doesn't matter. When solving different problems, different descriptions of the same situation may be more practical. PiusImpavidus (talk) 17:03, 17 December 2015 (UTC)

Still not clear on WHY/HOW retrogate planetary motion occurs

it seems MOST celestial bodies will follow a prograde motion, but I am not sure the article clearly answers or provides a clear explanation as to why/how retrograde rotation and retrograde orbit occur as well as whether there is any connection between retrograde orbit and retrograde rotation. Don't forget it has to be written in a away that even the amateur or idiot (that's me) should understand it.

____Ἑλλαιβάριος Ellaivarios____ 01:21, 26 February 2012 (UTC)

There are various possibilities. Simulations have shown that is is somewhat "easier" for an object to be captured into a retrograde orbit around a planet than a prograde one. Capture normally requires an interaction with a third body, usually the Sun, or occasionally a pre-existing satellite. It's easier for a body to be captured in the opposite direction than the planet's motion around the Sun or the satellite's motion around the planet. So captured satellites tend to be in retrograde orbits.

Another possibility involves impacts. A planet may have satellites revolving in the same direction as the planet's rotation, but then a collision may reverse the direction of the rotation, leaving the satellites in retrograde orbits. The planet's rotation also becomes retrograde.

Long-period comets have orbits that are aligned pretty randomly. When they fall in from the Oort Cloud, then may end up going around the Sun in any direction. So about half of these comets have retrograde orbits.

DOwenWilliams (talk) 18:34, 31 March 2013 (UTC)

Article scope

The article scope seems to only consider celestial bodies, yet the scope is not so limited by the article title. So I'm a bit surprised their is no mention of retrograde orbits for artificial satellites. One example of this from earlier this month was at the recent NASA workshop on the Global Exploration Roadmap on 10 April 2014, retrograde orbits are mentioned at 8:55 in the video of the workshop.

Is there any particular reason why? What do others think about expanding the article a bit to address retrograde orbits for artificial satellites? N2e (talk) 18:00, 28 April 2014 (UTC)

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Regular natural satellites

The section Retrograde and prograde motion#Natural satellites and rings says

With the exception of Hyperion all the known regular planetary natural satellites in the Solar System are tidally locked to their host planet, so they have zero rotation relative to their host planet, but have prograde rotation relative to the Sun because they have prograde orbits around their host planet.

Is this valid for Uranus' regular moons? It seems to me that viewed from above the Sun's north pole, since Uranus is rotating clockwise and since a regular moon orbits in the same direction as its host planet rotates, its regular moons must orbit clockwise and hence retrograde relative to the Sun. Loraof (talk) 19:22, 28 August 2017 (UTC)

You are correct, the moons of Uranus are exceptions. WolfmanSF (talk) 00:52, 29 August 2017 (UTC)

Mercury near perihelion

As per Mercury (planet)#Orbit, rotation, and longitude, at perihelion ± 4 Earth days Mercury's angular orbital velocity exceeds its angular rotational velocity. So while the planet is rotating prograde relative to the distant stars, it has retrograde rotation relative to the Sun. Would this be worthy of being in the article? Loraof (talk) 00:12, 30 August 2017 (UTC)

I think so. WolfmanSF (talk) 00:47, 30 August 2017 (UTC)

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All of these orbits are highly inclined

@WolfmanSF: this is a very good point. "retrograde" is per definition an inclination > 90°, but this is not straight forward visible in the text. Still I think this further explanation of the situation added by you should be added at another, more prominent place and not in the section of examples. -- Ralfkannenberg (talk) 10:49, 24 October 2018 (UTC)

I added the range of inclinations for these examples, plus a link to the paragraph discussing inclination. WolfmanSF (talk) 00:13, 25 October 2018 (UTC)

Variation in length of Venusian day

Venus' present slow retrograde rotation is in equilibrium balance between gravitational tides trying to tidally lock Venus to the Sun and atmospheric tides trying to spin Venus in a retrograde direction.

The wikipage for Venus mentions that over only 16 years (between Magellan in 1990 and Venus Express in 2006) the length of the Venusian day has slowed by six and a half minutes. Looking at some sources, that might be an oversimplification: Nevertheless, dynamic behavior appears to challenge the assertion that Venus's rotation is in equilibrium. I think a mention of this merits inclusion here, and possibly further elaboration in the Venus article. 71.168.173.2 (talk) 16:47, 27 August 2019 (UTC)

That difference represents about a 0.002% change. It probably does not merit mentioning here if it is simply part of some random variation or a cyclical process. It seems unlikely it represents a long term secular trend, given how rapidly that would change the Venusian day (i.e., double it in a million years or so). WolfmanSF (talk) 17:39, 27 August 2019 (UTC)

References

Proportion of satellites in retrograde orbits

Regarding this edit: According to Celestrak's list (data is in TLE format) of all 2359 currently active satellites (probably excluding some classified payloads not officially tracked), 1012 are in retrograde orbits with i>90 and 1347 in prograde orbits with i<90. The claim that Almost all artificial satellites of Earth have been placed in a prograde orbit, because less propellant is required to reach orbit when launching in a prograde direction. directly conflicts with these numbers, as 43 % hardly means "almost all".

The source [1] WolfmanSF linked falsely claims that Only a small fraction of operational satellites fall into this category [containing SSOs]. The data from Celestrak clearly shows SSOs as by far the most popular orbits, followed by GSOs.

The second part of my edit concerned this false statement: Artificial satellites are usually launched in the prograde direction, since this minimizes the amount of propellant required to reach orbit by taking advantage of the Earth's rotation (an equatorial launch site is optimal for this effect). When launching into a high inclination orbit starting at higher latitudes is more efficient and thus uses less fuel than starting from a more equatorial location. The reason for that is the lateral velocity imparted from the surface of the Earth that needs to be removed for polar orbits or SSOs. My clarification changed that to low inclination orbits where that statement holds true.

Any objections to reinstating my edits after clarifying the issues? --Sense Amid Madness, Wit Amidst Folly (talk) 20:09, 28 November 2019 (UTC)

See my suggested alternate edits. WolfmanSF (talk) 21:58, 28 November 2019 (UTC)

Works for me! -Sense Amid Madness, Wit Amidst Folly (talk) 22:09, 28 November 2019 (UTC)

Unclear Text

This text is fairly convoluted:

"In the Solar System, the orbits around the Sun of all planets and most other objects, except many comets, are prograde. They orbit around the Sun in the same direction as the sun rotates about its axis, which is counterclockwise when observed from above the Sun's north pole. Except for Venus and Uranus, planetary rotations are also prograde."

The rest of the paragraph is fine, and I think these 3 sentences should be written in a similar fashion.

I think this text should be altered to something like:

"In the Solar System, the Sun rotates around its axis in a counterclockwise direction when observed from above the Sun's north pole. All planets in the Solar System and most other objects are prograde, orbiting the Sun in the same direction as the Sun rotates (counterclockwise). Most comets are exceptions that have retrograde orbits. Planetary rotations are also prograde except for Venus and Uranus." Cowgod14 (talk) 02:48, 14 October 2022 (UTC)