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Celestial navigation plus accurate clocks. I'd think this would be especially good on a planet without clouds (as long as your planet rotates fast enough, so I

Celestial navigation plus accurate clocks. I'd think this would be especially good on a planet without clouds (as long as your planet rotates fast enough, so I guess Mercury is out). Anyone know if Mars has any polar stars? https://en.wikipedia.org/wiki/Celestial_navigation https://en.wikipedia.org/wiki/Celestial_navigation

ars 11y ago
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.
- vilmosi 11y ago
  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
  - DanBC 11y ago
    I wonder if "giant head" enhanced depth perception works across planets? https://xkcd.com/941/ https://xkcd.com/941/
    - escherplex 11y ago
      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)
      - CapitalistCartr 11y ago
        IPD?
        Caer 11y ago
        Interpupillary distance, the distance between the centre of the pupil in one eye to the other.
    - yellowapple 11y ago
      Just a nitpick, but how could one see the tops of clouds when the two cameras are only negligibly above ground level?
      - escherplex 11y ago
        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.
    - kijin 11y ago
      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.
  - ars 11y ago
    > 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.
- jessaustin 11y ago
  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?
  - jfoutz 11y ago
    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.
  - ars 11y ago
    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.
    - gliese1337 11y ago
      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.
      - mikeash 11y ago
        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.
- jeza 11y ago
  > 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.
- gshubert17 11y ago
  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-different-planets.html 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.
jwhale 11y ago
An easier method than creating a star map is to use the brightest star in the sky (the sun) and an accelerometer. This is the method Opportunity uses to calibrate the gyroscopes/wheel encoders. [source pg 4] https://www-robotics.jpl.nasa.gov/publications/Reg_Willson/MER_Cameras.pdf https://www-robotics.jpl.nasa.gov/publications/Reg_Willson/M... "The attitude of the rover is based on measurements from two vector instruments: (1) accelerometers that determine the vector towards the center of gravity of Mars and (2) Pancam solar images that determine the vector to the Sun."
- contingencies 11y ago
  Star maps can be downloaded free. Because you generally only need the brightest subset of stars in any area, you don't even need a comprehensive one. A star map for Mars would functionally be pretty much equivalent to an earth one, I guess, given the distances involved.
  - aruggirello 11y ago
    The stars would be the same, of course. Out of curiosity, I launched Stellarium, moved to Mars, set myself at 90° latitude north, and watched directly overhead. Mars north pole seems to be aligned halfway between Cygnus and Perseus, so there is no "North Star" equivalent on Mars.
yellowapple 11y ago
Doesn't Mars have clouds, though?
- jameshart 11y ago
  Dust clouds, maybe. Water clouds, no, not at all.
  - yellowapple 11y ago
    That's actually not correct: http://mars.jpl.nasa.gov/MPF/science/clouds.html http://mars.jpl.nasa.gov/MPF/science/clouds.html They're pretty much all cirrus-equivalents, granted, but they're still water (more precisely, water ice) clouds. That is, unless NASA is misinterpreting them, but seeing as they're, like, the end-all-be-all experts on Martian climate, I reckon that's unlikely. Even barring that, however, dust clouds would also cause issues for celestial navigation.

ars 11y ago

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.

vilmosi 11y ago
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
- DanBC 11y ago
  I wonder if "giant head" enhanced depth perception works across planets? https://xkcd.com/941/ https://xkcd.com/941/
  - escherplex 11y ago
    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)
    - CapitalistCartr 11y ago
      IPD?
      - Caer 11y ago
        Interpupillary distance, the distance between the centre of the pupil in one eye to the other.
  - yellowapple 11y ago
    Just a nitpick, but how could one see the tops of clouds when the two cameras are only negligibly above ground level?
    - escherplex 11y ago
      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.
  - kijin 11y ago
    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.
- ars 11y ago
  > 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.
jessaustin 11y ago
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?
- jfoutz 11y ago
  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.
- ars 11y ago
  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.
  - gliese1337 11y ago
    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.
    - mikeash 11y ago
      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.
jeza 11y ago
> 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.
gshubert17 11y ago
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-different-planets.html 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.

jwhale 11y ago

An easier method than creating a star map is to use the brightest star in the sky (the sun) and an accelerometer. This is the method Opportunity uses to calibrate the gyroscopes/wheel encoders. [source pg 4] https://www-robotics.jpl.nasa.gov/publications/Reg_Willson/MER_Cameras.pdf https://www-robotics.jpl.nasa.gov/publications/Reg_Willson/M... "The attitude of the rover is based on measurements from two vector instruments: (1) accelerometers that determine the vector towards the center of gravity of Mars and (2) Pancam solar images that determine the vector to the Sun."

contingencies 11y ago
Star maps can be downloaded free. Because you generally only need the brightest subset of stars in any area, you don't even need a comprehensive one. A star map for Mars would functionally be pretty much equivalent to an earth one, I guess, given the distances involved.
- aruggirello 11y ago
  The stars would be the same, of course. Out of curiosity, I launched Stellarium, moved to Mars, set myself at 90° latitude north, and watched directly overhead. Mars north pole seems to be aligned halfway between Cygnus and Perseus, so there is no "North Star" equivalent on Mars.

yellowapple 11y ago

Doesn't Mars have clouds, though?

jameshart 11y ago
Dust clouds, maybe. Water clouds, no, not at all.
- yellowapple 11y ago
  That's actually not correct: http://mars.jpl.nasa.gov/MPF/science/clouds.html http://mars.jpl.nasa.gov/MPF/science/clouds.html They're pretty much all cirrus-equivalents, granted, but they're still water (more precisely, water ice) clouds. That is, unless NASA is misinterpreting them, but seeing as they're, like, the end-all-be-all experts on Martian climate, I reckon that's unlikely. Even barring that, however, dust clouds would also cause issues for celestial navigation.