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Comment by ars | Hacker News Reader

The planet doesn't have to rotate to use celestial navigation. As long as you can see the stars you are fine.

So the dark side of mercury would work fine. On the hot side as long as you are not exactly on the equator the sun acts as a fixed direction and you can use that.

> Anyone know if Mars has any polar stars?

The orbital tilt of Mars is 1.85 degrees (vs 0 for earth) and the Axial tilt is 25.19 vs 23.44 for Earth. So Polaris would be pretty close to being a polar star for Mars as well.

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The pole star of a planet is in the direction of its axis of rotation. The orbits of Earth and Mars around the sun are in similar planes, and their axial tilts are similar, but they point in different directions. According to this

http://abaaonline.blogspot.com/2013/07/pole-star-on-differen...

Mars north pole points near Deneb. The reference also gives pole stars for other planets.

That's still like a couple of degrees (4 full moons) away so not sure how accurate it would be.

Still the thought of seeing literally the same stars on an alien world is blowing my mind. Just imagine the same sky, but different planet... wow

I wonder if "giant head" enhanced depth perception works across planets?

https://xkcd.com/941/

Fun link. I was planning on similar setup using a couple of remote linked Celestron C8s separated by 5 miles to observe thunderhead cloud formation in SW Florida. Average natural IPD is around 64mm so it should be interesting to see what the occipital lobe does when this is expanded to over 8km. (IPD = InterPupillary Distance; eyeball separation)

IPD?

Interpupillary distance, the distance between the centre of the pupil in one eye to the other.

Just a nitpick, but how could one see the tops of clouds when the two cameras are only negligibly above ground level?

In the flat FL interior during hot summer days with no wind it's common to observe distinct long wide columns of clouds form anvil tops over the space of a few hours in the early afternoon. Sometimes, if you're lucky, a cauldron shape will form which puts on an impressive light show at night. Rather than risking a light Cessna it might be interesting to view formation in exaggerated 3D from the ground.

Stellar parallax is virtually unnoticeable even when the Earth is on opposite sides of the Sun (~300M km apart), so I don't think the stars would look much differet even if you doubled or tripled that distance.

Objects inside of our own solar system, on the other hand, would be really interesting to look at through such a setup.

> That's still like a couple of degrees (4 full moons) away so not sure how accurate it would be.

It depends on how the two tilts interact - do they cancel or add? I don't know.

> So Polaris would be pretty close to being a polar star for Mars as well.

Poris is only useful if you're in the northern hemisphere. In the southern hemisphere on earth we use the southern cross. It's a bit trickier if you're using a quadrant because you have to align it with a blank area of sky. Though the 17th century explorers managed it.

On the hot side as long as you are not exactly on the equator the sun acts as a fixed direction and you can use that.

ISTM this would work on the equator too?

it would work anywhere you could figure out the direction of the light. An example might be a tree on top of a hill, compare that to that tree's shadow on some high cliffs. That's not going to be particularly accurate, but it would get you going in the right general direction.

If you can figure out where the sun is, and you know what time it is, you're in good shape.

On the equator the sun could be directly overhead - so you have no idea what any directions are, the sun is equally far from all horizons. (Remember the sun does not move on mercury.)

If you are on the equator, but not directly under the sun then you can figure things out.

I guess I should have said equator on the central meridian.

Edit: I was under the mistaken assumption that Mercury is tidally locked.

So it would sort of work on the equator as well, but you might have to wait a bit for the sun to move enough to tell where you are.

The sun does move on Mercury. Mercury is not tidally locked- it has a 3:2 spin-orbit resonance. I.e., it rotates 3 times for every two times it goes around the sun. This means that the two sides of the planet alternate facing the sun at perihelion.

Interesting tidbit about Mercury's rotation: although it's clearly visible to the naked eye and thus has been known since before the dawn of recorded history, and has been observed through telescopes since telescopes were invented, the fact that it is not tidally locked has only been known since 1965.

Mercury is pretty featureless, right? It's not like Jupiter where you could see the rotation just by keeping track of the Great Red Spot.

It has lots of craters and such. I don't know what it looks like through an older telescope. According to Wikipedia, the problem was that because the rotation is in a resonance with its orbit, and because it's so close to the sun and thus can only be observed at certain times, it was always observed when in just one orientation.

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