<p>Welcome!</p>
<p>So, the way we define retrograde motion for celestial objects isn't the same way we define it, or even demonstrate it here on Earth, as in, the car next to you appears to be moving backwards if your car is moving faster.</p>
<p>First, all planets appear to be moving across the sky in an eastward direction.</p>
<p>But for Mercury and Venus, these are the only 2 planets closer to the Sun than our Earth, and therefore you're correct: Earth will <em>never</em> move faster than either of those planets, due to our larger orbital period. Which introduces a new definition for retrograde. We (some astronomers) define it as planets that appear to move <em>westward</em> through the sky. For those that agree with that definition, even the inner planets retrograde as they move farther from the Sun (or the horizon) in the eastern morning sky or approach the Sun (horizon) in the western evening sky.</p>
<p>It's all still <em><strong>apparent</strong></em> motion, or the motion <em>as we see it from Earth</em>.</p>
<p>Cheers.</p>

CC BY-SA 4.0
2022-07-18T13:13:49.757Z
Please excuse my ignorance, I 'd like to clarify one point. You state, "all planets appear to be moving across the sky in an eastward direction". Do you mean relative to the constellations? For the past number of weeks I've noticed Jupiter is currently in Pisces. I've seen it most most nights. In 2017 it was in Virgo, this time of the year. When looking at the sky, Pisces can be considered to be eastwards of Virgo. Is this what you meant?
Fred
CC BY-SA 4.0
2022-07-18T19:24:33.993Z
I understand its apparent. I dont understand how it could even appear to move westward if it is moving faster than earth in the same tangential direction
Dylan VanAllen
CC BY-SA 4.0
2022-07-18T20:40:38.003Z
@Fred, relative to the ecliptic.
ChairmanMeow
CC BY-SA 4.0
2022-07-18T20:41:36.243Z
@Dylan VanAllen, see novotny's comment on the other answer to you. He's correct.
ChairmanMeow
CC BY-SA 4.0
2022-07-18T02:42:07.237Z
2022-07-18T02:42:07.237Z
ChairmanMeow
<p>Apparent retrograde motion for the inferior planets occurs when the planet is moving between the Earth and the Sun.</p>
<p>Because the inferior planet is moving <em>faster</em> than the Earth around the sun, it overtakes the apparent relative motion of the Sun dragging the inferior planets along, and the planet appears to move westward along the ecliptic as it passes between the Sun and the Earth, then slows down, and returns to prograde motion as it heads back to the far side of its orbit, relative to Earth.</p>

CC BY-SA 4.0
2022-07-18T19:22:10.890Z
I must be confused. If it is moving faster than earth in the same tangential direction how could it possibly appear to move westward?
Dylan VanAllen
CC BY-SA 4.0
2022-07-18T19:49:36.897Z
@DylanVanAllen The Sun is moving Eastward along the ecliptic from day to day, and carrying Mercury and Venus across the sky also eastward along the plane of the Ecliptic. When Mercury and Venus are on  the same side of the Sun as the Earth, though, they're moving _westward_ relative to the Sun's ecliptic motion,  and if that westward motion is fast enough, and in the proper direction, it exceeds the effect of the Sun's eastward motion along the ecliptic, and the interior planets appear to move westward along the ecliptic.
notovny
CC BY-SA 4.0
2022-07-18T14:42:53.917Z
2022-07-18T14:42:53.917Z
notovny
<p>I was having a discussion about Mercury being in retrograde, and I had explained it as Earth moving faster tangentially than Mercury so it looks like it’s moving backwards when we compare it with the Sun’s backdrop during an inferior conjunction. However, I realized that Earth is never moving faster than Mercury. So, why is it that it appears to move backward?</p>

CC BY-SA 4.0
2022-07-17T21:50:22.520Z
It actually does move backwards as it orbits around the other side of the Sun, same with Venus.  Mars' retrograde is due to our perspective.
Greg Miller
<orbital-mechanics><parallax><conjunction><retrograde-orbit>
What is Mercury and Venus’ apparent retrograde motion?
Dylan VanAllen
# What is Mercury and Venus’ apparent retrograde motion?

(Asked by: username_0 on 2022-07-17)

I was having a discussion about Mercury being in retrograde, and I had explained it as Earth moving faster tangentially than Mercury so it looks like it’s moving backwards when we compare it with the Sun’s backdrop during an inferior conjunction. However, I realized that Earth is never moving faster than Mercury. So, why is it that it appears to move backward?

**Tags:** orbital-mechanics, parallax, conjunction, retrograde-orbit

> **Comments:**
> **username_1**: It actually does move backwards as it orbits around the other side of the Sun, same with Venus.  Mars' retrograde is due to our perspective.  

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## 2 answers

### Answer by username_2 on 2022-07-18. Score: 6

Welcome!

So, the way we define retrograde motion for celestial objects isn't the same way we define it, or even demonstrate it here on Earth, as in, the car next to you appears to be moving backwards if your car is moving faster.

First, all planets appear to be moving across the sky in an eastward direction.

But for Mercury and Venus, these are the only 2 planets closer to the Sun than our Earth, and therefore you're correct: Earth will *never* move faster than either of those planets, due to our larger orbital period. Which introduces a new definition for retrograde. We (some astronomers) define it as planets that appear to move *westward* through the sky. For those that agree with that definition, even the inner planets retrograde as they move farther from the Sun (or the horizon) in the eastern morning sky or approach the Sun (horizon) in the western evening sky.

It's all still ***apparent*** motion, or the motion *as we see it from Earth*.

Cheers.

> **Comments:**
> **username_3**: Please excuse my ignorance, I 'd like to clarify one point. You state, "all planets appear to be moving across the sky in an eastward direction". Do you mean relative to the constellations? For the past number of weeks I've noticed Jupiter is currently in Pisces. I've seen it most most nights. In 2017 it was in Virgo, this time of the year. When looking at the sky, Pisces can be considered to be eastwards of Virgo. Is this what you meant?  
> **username_0**: I understand its apparent. I dont understand how it could even appear to move westward if it is moving faster than earth in the same tangential direction  
> **username_2**: @username_3, relative to the ecliptic.  
> **username_2**: @username_0, see novotny's comment on the other answer to you. He's correct.  

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### Answer by username_4 on 2022-07-18. Score: 5

Apparent retrograde motion for the inferior planets occurs when the planet is moving between the Earth and the Sun.

Because the inferior planet is moving *faster* than the Earth around the sun, it overtakes the apparent relative motion of the Sun dragging the inferior planets along, and the planet appears to move westward along the ecliptic as it passes between the Sun and the Earth, then slows down, and returns to prograde motion as it heads back to the far side of its orbit, relative to Earth.

> **Comments:**
> **username_0**: I must be confused. If it is moving faster than earth in the same tangential direction how could it possibly appear to move westward?  
> **username_4**: @DylanVanAllen The Sun is moving Eastward along the ecliptic from day to day, and carrying Mercury and Venus across the sky also eastward along the plane of the Ecliptic. When Mercury and Venus are on  the same side of the Sun as the Earth, though, they're moving _westward_ relative to the Sun's ecliptic motion,  and if that westward motion is fast enough, and in the proper direction, it exceeds the effect of the Sun's eastward motion along the ecliptic, and the interior planets appear to move westward along the ecliptic.  

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