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If Jupiter were as far from us as Mars, how big would it appear in Earth's sky? Would it appear huge, or would it be as small as a star?

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    $\begingroup$ The rewording of this question makes it a purely hypothetical. I'm voting to close this question as off-topic because purely hypothetical questions are off topic for this stack exchange. The previous version (How big does Jupiter appear from Mars) would be acceptable. $\endgroup$ – James K Aug 26 '16 at 16:57
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    $\begingroup$ Wow that really does change the question and answer a lot... but the same math applies, from below, just change the numbers accordingly. If you do that you'll find the size quite surprising! $\endgroup$ – LaserYeti Aug 26 '16 at 18:23
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If we were to view Jupiter from Mars's orbit with the the naked eye, how big would it appear to be, as compared with its appearance from the Earth? Would Jupiter appear as big as the moon does from Earth? Would it appear to be a disk to the naked eye, or would it be star-like?

Here are some back-of-the-envelope calculations.

The maximum apparent magnitude of Jupiter is -2.94, which is observed when it's closest to Earth (4.2 AU). Mars can get as close to Jupiter as 3.68 AU. Since the flux is inversely proportional to distance squared, Jupiter will be 30% brighter on Mars than it is on Earth, giving it an apparent magnitude of -3.23. It will still be a bright star-like dot outshined by both Martian moons, Phobos and Deimos with magnitudes of -9 and -5 respectively. It will certainly not be as bright as Earth's Moon seen from Earth, which can be as bright as −12.9.

Here is a detailed article which describes how various celestial bodies look from Mars. They have a picture of Earth, Moon and Jupiter taken by Mars Orbiter Camera (MOC) in 2003, when both Earth and Jupiter were close to Mars:

enter image description here

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    $\begingroup$ To put that in perspective: Jupiter would be less bright than Venus is from Earth, and no disk would be visible: It would look like an exceptionally bright star. $\endgroup$ – James K Aug 25 '16 at 18:35
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    $\begingroup$ The OP asks how big Jupiter "would" appear as seen from Mars. Only one sentence here addresses that. $\endgroup$ – Rob Jeffries Aug 26 '16 at 6:11
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    $\begingroup$ @RobJeffries: The original question asked how Jupiter would appear from Earth if Jupiter occupied Mars's current orbit. It was (inappropriately) edited to ask instead how Jupiter appears from Mars, then edited back to more or less the original version. $\endgroup$ – Keith Thompson Aug 26 '16 at 18:50
  • $\begingroup$ My comment would apply in that case also. Only one sentence vaguely refers to the apparent size. Besides, the answer is clearly talking about how things appear from Mars, so I don't understand your comment @KeithThompson $\endgroup$ – Rob Jeffries Aug 26 '16 at 22:39
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    $\begingroup$ @RobJeffries: Just summarizing the history, which isn't clear if you just read the current version of the question. $\endgroup$ – Keith Thompson Aug 26 '16 at 22:46
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Since the question is actually asking for a size of Jupiter as seen from Mars, there is a very simple equation that will give you a good idea.

$$\frac{\alpha}{360^{\circ}}=\frac{\ell}{2\pi r} $$ $\alpha$ is the angle that Jupiter would subtend in the sky, or the angular size, and $\ell$ is the actual diameter, or linear size, of Jupiter. $2\pi r$ is the circumference of Jupiter's orbit about Mars. Now I know what you're thinking, that this doesn't make any sense because of the vast differences of Jupiter's orbit when it is close compared to when it is at opposition. It'll work out...

The following are all approximations.

Mars orbits the Sun at $\sim1.53AU$ and Jupiter at $\sim5.2AU$. Because $1AU \approx 150Mkm$, The difference in their orbits are roughly $550.5Mkm$.

$$\alpha=\frac{140000km}{2\pi(550.5\times10^6km)}\times360^{\circ}=0.015^{\circ} $$

That is just the average angular size, at the average closest distance, but given that the only real tool to measure angular size with the naked eye (that isn't a sextant) is your hand, and your smallest digit (the pinky) at arms length marks out $1^{\circ}$ you'll never be able to tell the difference in the variation of size due to the change in distance.

Now answering your other questions directly:

  • Jupiter would look larger, but not enough to actually tell a difference in size.
  • As shown above, Jupiter would be much smaller than the moon appears to us from Earth, $0.5^{\circ}$.
  • No disk, as this is very much a point-like size as far as human eyes are concerned.
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  • $\begingroup$ i want to clear something people.. i'm trying to ask here that if jupiter replace mars planet in mars orbit how big it will shown to earth.. $\endgroup$ – RANSARA009 Aug 26 '16 at 8:19
  • $\begingroup$ @RANSARA009 In that case, the wording of your post and its title are both very misleading, as that's not what they say you're asking about at all. $\endgroup$ – zibadawa timmy Aug 26 '16 at 13:33
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    $\begingroup$ 0.015 deg is almost 1 arcmin - the resolution limit for the human eye. At closest approach it would be even larger. People with good eyesight would be able to tell it's not a star-like object anymore, but more like a "fat dot". $\endgroup$ – Florin Andrei Aug 26 '16 at 17:24
  • $\begingroup$ @FlorinAndrei, yes 0.015 deg is 54 acrsec, but at their closest approach to one another, the angle is only 0.016 deg or 58 arcsec, soo... you still won't be able to resolve that given your argument that the defraction limit for a human eye is 1 arcmin or 60 arcsec. $\endgroup$ – LaserYeti Aug 26 '16 at 18:31

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