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For example, because the Sun is the center of the solar system; when we think that the Sun is on the constellation of Aries, in actuality, isn't Earth lining up with the constellation opposite Aries: Libra (while the sun stays in the same place)?

Similarly, this would apply to any planet that is closer to the sun than the Earth is: when we see that Mercury is lining up with the constellation Gemini, wouldn't it be lining up with a different constellation in actuality, aka from the perspective of the Sun?

However, there would be no such problem with planets past Earth, because we're looking outward to observe those planets.

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    $\begingroup$ I made a mistake on my wording here, i meant the center of the solar system -- then what would be the answer to this question? $\endgroup$
    – Atlas
    Mar 17 at 16:26
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    $\begingroup$ Yes. The angular distance between an inner planet and Sun cannot be arbitrary, unlike the outer planets. You cannot have Sun in Taurus and Mercury oř Venus in Scorpius. $\endgroup$
    – Leos Ondra
    Mar 17 at 17:47
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    $\begingroup$ Mars is close enough to Earth that it can appear in a significantly distant constellation from the viewpoint of the Sun than from Earth. It's not really any kind of problem, though. $\endgroup$
    – notovny
    Mar 17 at 21:23

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Yes. There is nothing magical here. If two people look at the same object from two different viewpoints, then the background they see behind the object will be different. When I'm sitting at my dining table, I might see a salt-shaker in front of my wife. My wife will see the salt shaker in front of me. My daughter will see the salt shaker too, but from her point of view it might be in front of my son. All these points of view are valid.

When we are talking about the position of planets in the night sky, it is convenient to describe them relative to the background of the stars, as we see them from Earth. It would not be convenient to describe them relative to the background seen from Mars or from the centre of the solar system, because there is nobody there. It's not that those viewpoints are invalid, they just aren't much use to someone on Earth.

Although we might say "Mars is in Aries", this is just a figure of speech, which means "From Earth, the position of Mars is such that it is in front of the region of the sky that we call Aries." It doesn't mean there is any particular physical connection between those stars and the planet at that time or any other time.

It isn't more accurate, nor more correct to describe the background stars of a planet from a point of view close to the sun. This isn't a "problem".

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You’re partly right. The imaginary line from the Sun to a planet, when extended, almost always points towards a different than as seen from the Earth. For example, in the image below (not to scale!), Venus appears to be between Virgo and Libra as seen from the Earth, but in Cancer as seen from the Sun—as you suspected.

But look at the situation for Mars: it’s an exterior planet, but it doesn’t seem to be in the same constellation as seen from the Earth (beginning of Aries) or as seen from the Sun (beginning of Taurus).

Sometimes, though, exterior planets are in the same constellation as viewed from both the Sun and the Earth: this is near their opposition, as is the case here for Jupiter.

(Circles represent orbits and are for illustrative purposes only.)

enter image description here

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  • $\begingroup$ related, and it seems not yet completely answered: How will planets behave in the night sky as seen by Mars colonists? $\endgroup$
    – uhoh
    Mar 18 at 4:34
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    $\begingroup$ "It does happen, though, that exterior planets are in the same constellation as viewed from both the Sun and the Earth: this is near their opposition, as is the case here for Jupiter." The phrase "it does happen that" is generally understood to mean that it is always true. I think you mean "Sometimes". $\endgroup$
    – Acccumulation
    Mar 19 at 0:33
  • $\begingroup$ @Acccumulation: Thank you; I have made the change. $\endgroup$
    – Pierre Paquette
    Mar 20 at 5:26
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Similarly, this would apply to any planet that is closer to the sun than the Earth is: when we see that Mercury is lining up with the constellation Gemini, wouldn't it be lining up with a different constellation in actuality, aka from the perspective of the Sun?

Yes, the only way that the Earth and the Sun would see a planet as being in the same "constellation" (note that the concept of a constellation in the first place is simply how things look from out perspective) would be if the planet is on the line between the Earth and the Sun, but not between them. That's true for both inner and outer planets.

For inner planets, that would mean that it's behind the Sun from out perspective, so we wouldn't be able to see it (although we could infer that it's in that constellation).

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