If Jupiter was placed in Mars' orbit, how would Jupiter appear from the naked eye? [closed]

Question

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?

Answer 1

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.

Answer 2

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: