<p>You would need to have a planet orbit very slowly around the star. A planet moves retrograde because the planet is passing the other in its orbit so that the slower planet appears to be going backwards.  </p>

<p>So in order for this to happen you would need a planet having an orbit such that the outer planet passes the inner planet.  But I think that this would result in an unstable orbit for one of the two planets.  Either the inner one would fall into the orbiting star or the outer one would escape.</p>

<p>The inner planets have a smaller orbit so they move "faster" in order to not fall into the sun.  So they are always "passing" the planets in orbits that are outside of their orbit.</p>

CC BY-SA 3.0
2015-02-19T14:12:00.960Z
2015-02-19T14:30:12.853Z
2015-02-19T14:30:12.853Z
Schleis
<p><strong>TL;DR Not for planets, but for other stuff yes.</strong></p>

<p>As explained in an earlier answer, planets are on circular orbits (quasi-circular actually but that's a detail). The closer to the star, the faster the planet is on its orbit. So, the retrograde motion observed for Mars cannot happen for Venus for instance. At most you could see Venus going retrograde when it actually turns around the sun, but you would clearly understand that the planet is just on the other side of the solar system -- which I think is not what you were wondering about.</p>

<p>But there's another type of situation where you see a retrograde orbit for an inner body: if the orbit is not circular but a nice ellipse! The orbit would have to be very eccentric though (very narrow). Planets do not have eccentric orbits (in the solar system at least) because, in short, the formation of planets favors circular orbits in the long term. But some asteroids are in this situation (for example many in the Aten family).</p>

<p>If the perihelion (closest point) is very close to the sun and the aphelion (farthest point) is much further, then even if the asteroid remains in the interior of the Earth's orbit you would see it going backwards near aphelion. What happens is that speed on a eccentric (elliptic) orbit is slower at the apoapse. For instance, if your perihelion was grazing the surface of the sun, you would have a speed of almost 0 at your aphelion. And you would see it drifting backward because of Earth's motion.</p>

CC BY-SA 3.0
2015-02-19T16:29:55.520Z
2015-02-19T16:29:55.520Z
Godzy
<p>Unsure where the original poster obtained an idea that objects inside the Earth’s orbit, as seen from Earth, “move <strong>ordinarily</strong> with respect to the stars” from west to east.</p>

<p>First of all, of which “west” and “east” do we speak? To be specific, let’s understand these for the region of sky not far from the meridian. That is, when the object <em>X</em> culminates, then nearby objects having the right ascension greater than RA(<em>X</em>) are on the east from the meridian (not yet culminated), whereas those having RA &lt; RA(<em>X</em>) are on the west from the meridian (already culminated).</p>

<p>For the Sun itself, every educated person can remember that its diurnal motion is slightly <em>slower</em> than diurnal motion of the inertial celestial sphere (fixed stars) – that is, with respect to those stars Sun goes from west to east. Its right ascension increases monotonically. For other objects inside the Earth’s orbit, their right ascension is within 90° of the Sun’s, hence <em>in average</em> they move “from west to east” as well, and quicker do it when are behind the Sun.</p>

<p>But definitely the terrestrial picture is opposite for e.g. Venus when it overtakes Earth in their orbital motions near the inferior conjunction.</p>

<p><img src="https://upload.wikimedia.org/wikipedia/commons/b/b7/Venusorbitsolarsystem.gif" alt="Venus and Earth orbit the Sun"></p>

<p>During this part of the orbit the right ascension of Venus decreases – seen on this animation near the end of cycle. “Experts” asserting that it’s impossible to see mistake – Venus is not very close to Sun during it, and anyway perfectly observable (albeit rare) during Venus’s transits before the Sun.</p>

CC BY-SA 4.0
2019-05-29T13:13:13.207Z
Sorry, but you made a simple answer ("Yes, inferior planets also show retrograde motion") very complicated with extraneous information. About the animation, I do not understand how it demonstrates the retrograde motion. To me, it shows all of the planets continuing to move in the same direction. A figure or animation of Venus' position on a sky chart would be infinitely more useful.
JohnHoltz
CC BY-SA 4.0
2019-05-29T17:30:54.173Z
@JohnHoltz: the picture requires a mild geometric imagination (namely, ability to construct segments in mind) to understand. RA increases counter-cw here, but one sees that the Earth–Venus vector rotates clockwise near the inf.conj. JohnHoltz thinks one way but more mathematical users, probably, think other way. As for *extraneous* information, Ī̲ deem it useful to disprove “a planet moves retrograde because is passing the other in its orbit so that the slower planet appears to be going backwards” bollocks where phenomena of outer planets are thoughtlessly extrapolated to interior planets.
Incnis Mrsi
CC BY-SA 4.0
2019-05-29T10:03:49.177Z
2019-05-29T10:03:49.177Z
Incnis Mrsi
<p>Retrograde motion is:</p>

<blockquote>
  <p>The apparent motion of the planets when they appear to move backward (from east to west) from the direction that they move ordinarily with respect to the stars.</p>
</blockquote>

<p>Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?</p>

<orbit>
Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?
user5532
# Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?

(Asked by: username_0 on 2015-02-19)

Retrograde motion is:

> The apparent motion of the planets when they appear to move backward (from east to west) from the direction that they move ordinarily with respect to the stars.

Can planets inside the Earth's orbit around the Sun appear to undergo retrograde motion?

**Tags:** orbit


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## 3 answers

### Answer by username_1 on 2015-02-19. Score: 0

You would need to have a planet orbit very slowly around the star. A planet moves retrograde because the planet is passing the other in its orbit so that the slower planet appears to be going backwards.

So in order for this to happen you would need a planet having an orbit such that the outer planet passes the inner planet. But I think that this would result in an unstable orbit for one of the two planets. Either the inner one would fall into the orbiting star or the outer one would escape.

The inner planets have a smaller orbit so they move "faster" in order to not fall into the sun. So they are always "passing" the planets in orbits that are outside of their orbit.


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### Answer by username_2 on 2015-02-19. Score: 0

**TL;DR Not for planets, but for other stuff yes.**

As explained in an earlier answer, planets are on circular orbits (quasi-circular actually but that's a detail). The closer to the star, the faster the planet is on its orbit. So, the retrograde motion observed for Mars cannot happen for Venus for instance. At most you could see Venus going retrograde when it actually turns around the sun, but you would clearly understand that the planet is just on the other side of the solar system -- which I think is not what you were wondering about.

But there's another type of situation where you see a retrograde orbit for an inner body: if the orbit is not circular but a nice ellipse! The orbit would have to be very eccentric though (very narrow). Planets do not have eccentric orbits (in the solar system at least) because, in short, the formation of planets favors circular orbits in the long term. But some asteroids are in this situation (for example many in the Aten family).

If the perihelion (closest point) is very close to the sun and the aphelion (farthest point) is much further, then even if the asteroid remains in the interior of the Earth's orbit you would see it going backwards near aphelion. What happens is that speed on a eccentric (elliptic) orbit is slower at the apoapse. For instance, if your perihelion was grazing the surface of the sun, you would have a speed of almost 0 at your aphelion. And you would see it drifting backward because of Earth's motion.


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### Answer by username_3 on 2019-05-29. Score: -1

Unsure where the original poster obtained an idea that objects inside the Earth’s orbit, as seen from Earth, “move **ordinarily** with respect to the stars” from west to east.

First of all, of which “west” and “east” do we speak? To be specific, let’s understand these for the region of sky not far from the meridian. That is, when the object *X* culminates, then nearby objects having the right ascension greater than RA(*X*) are on the east from the meridian (not yet culminated), whereas those having RA < RA(*X*) are on the west from the meridian (already culminated).

For the Sun itself, every educated person can remember that its diurnal motion is slightly *slower* than diurnal motion of the inertial celestial sphere (fixed stars) – that is, with respect to those stars Sun goes from west to east. Its right ascension increases monotonically. For other objects inside the Earth’s orbit, their right ascension is within 90° of the Sun’s, hence *in average* they move “from west to east” as well, and quicker do it when are behind the Sun.

But definitely the terrestrial picture is opposite for e.g. Venus when it overtakes Earth in their orbital motions near the inferior conjunction.

![Venus and Earth orbit the Sun](https://upload.wikimedia.org/wikipedia/commons/b/b7/Venusorbitsolarsystem.gif)

During this part of the orbit the right ascension of Venus decreases – seen on this animation near the end of cycle. “Experts” asserting that it’s impossible to see mistake – Venus is not very close to Sun during it, and anyway perfectly observable (albeit rare) during Venus’s transits before the Sun.

> **Comments:**
> **username_4**: Sorry, but you made a simple answer ("Yes, inferior planets also show retrograde motion") very complicated with extraneous information. About the animation, I do not understand how it demonstrates the retrograde motion. To me, it shows all of the planets continuing to move in the same direction. A figure or animation of Venus' position on a sky chart would be infinitely more useful.  
> **username_3**: @username_4: the picture requires a mild geometric imagination (namely, ability to construct segments in mind) to understand. RA increases counter-cw here, but one sees that the Earth–Venus vector rotates clockwise near the inf.conj. username_4 thinks one way but more mathematical users, probably, think other way. As for *extraneous* information, Ī̲ deem it useful to disprove “a planet moves retrograde because is passing the other in its orbit so that the slower planet appears to be going backwards” bollocks where phenomena of outer planets are thoughtlessly extrapolated to interior planets.  

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