4 ms·

List of selected stars for navigation

76 points by bcaa7f3a8bbc 7y ago

asdfman123 7y ago
That's a lot of charts to be tattooed onto my forearm.
- gmiller123456 7y ago
  Such a tattoo would be pretty useless without a sextant, a good watch, and a nautical almanac. And since any almanac would likely have the star charts too, you can save your skin for something else. You can't use static charts because the Earth wobbles a bit (called precession and nutation), and doesn't rotate at a constant speed. This effect can make a difference of being off my many miles just over a year, and 10's of miles over a few years.
  - UI_at_80x24 7y ago
    This is good to hear so thank-you for writing that. I have a plan to do a bicycle tour from Canada to Mexico sometime in my retirement years (or sooner). Part of that included a 'Wouldn't it be quaint/interesting' desire to bring along an old sextant I inherited and see how close I could chart my progress to my actual location. If the charts change that much over time, then this might work the way I was expecting.
    - gmiller123456 7y ago
      Sextants aren't really made for use on land, you need a very flat horizon to use one, you can use an "artificial horizon" but you'll loose some accuracy. A sextant is made for observations at sea, where a ship would be bobbing up and down. On steady land, it's much easier to just measure the angle a star makes with a level base. I'm not sure of the exact name, I've seen old telescopes with a protractor on the side, and a level and compass on the base. But, so long as it's just for fun, you can rely on your phone for updated star information and time.
      - jdc 7y ago
        A combination compass - inclinometer would probably be good for his purposes.
- 4A414B56 7y ago
  All you really need is an accurate watch at the local solar noon and the time of Greenwich's solar noon that day for longitude. Latitude is gained my measuring the angle of Polaris from the horizon (northern hemisphere only).
  - gmiller123456 7y ago
    You need at least one more piece of information, otherwise how would you know it's solar noon? You either need to know where due South is, or you need to make multiple measurements of the sun at different times to determine when it reached its lowest point. The good thing about using two stars is you don't need that information and you didn't need to wait around in the same spot from solar noon until nighttime.
    - 4A414B56 7y ago
      put a stick in the ground and note the time at narrowing intervals until the shadow starts getting longer, taking the shortest shadow as solar noon??? seems like a non-issue since you've already got an accurate watch. if you're lost without a gps and have no idea what your long. is, you've probably got a lot of spare time to figure it out.
      - gmiller123456 7y ago
        That's one way, and it also tells you where due South is too. The watch is only one piece of information, solar noon changes from day to day (called the "Equation of Time"), so you need a way to correlate the two. An almanac would have that info, or the equation could be memorized and computed. This isn't just recreational fun, most (maybe all) Navys still require having people on board each major ship that can perform sight reductions and wind up watches. The assumption would be that during a war GPS and electronic devices would be destroyed. The "stick in the ground" method doesn't work if you're moving, and takes a while, and speed matters when you're being hunted.
        4A414B56 7y ago
tjohns 7y ago
This got me reading up on the topic, and apparently you can download a free book from the FAA that covers how to do celestial navigation: Flight Navigator Handbook, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-18.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a... (Apparently early models of the Boeing 747 even had a sextant port built into the roof of the cockpit.)

76 points by bcaa7f3a8bbc 7y ago

asdfman123 7y ago

That's a lot of charts to be tattooed onto my forearm.

gmiller123456 7y ago
Such a tattoo would be pretty useless without a sextant, a good watch, and a nautical almanac. And since any almanac would likely have the star charts too, you can save your skin for something else. You can't use static charts because the Earth wobbles a bit (called precession and nutation), and doesn't rotate at a constant speed. This effect can make a difference of being off my many miles just over a year, and 10's of miles over a few years.
- UI_at_80x24 7y ago
  This is good to hear so thank-you for writing that. I have a plan to do a bicycle tour from Canada to Mexico sometime in my retirement years (or sooner). Part of that included a 'Wouldn't it be quaint/interesting' desire to bring along an old sextant I inherited and see how close I could chart my progress to my actual location. If the charts change that much over time, then this might work the way I was expecting.
  - gmiller123456 7y ago
    Sextants aren't really made for use on land, you need a very flat horizon to use one, you can use an "artificial horizon" but you'll loose some accuracy. A sextant is made for observations at sea, where a ship would be bobbing up and down. On steady land, it's much easier to just measure the angle a star makes with a level base. I'm not sure of the exact name, I've seen old telescopes with a protractor on the side, and a level and compass on the base. But, so long as it's just for fun, you can rely on your phone for updated star information and time.
    - jdc 7y ago
      A combination compass - inclinometer would probably be good for his purposes.
4A414B56 7y ago
All you really need is an accurate watch at the local solar noon and the time of Greenwich's solar noon that day for longitude. Latitude is gained my measuring the angle of Polaris from the horizon (northern hemisphere only).
- gmiller123456 7y ago
  You need at least one more piece of information, otherwise how would you know it's solar noon? You either need to know where due South is, or you need to make multiple measurements of the sun at different times to determine when it reached its lowest point. The good thing about using two stars is you don't need that information and you didn't need to wait around in the same spot from solar noon until nighttime.
  - 4A414B56 7y ago
    put a stick in the ground and note the time at narrowing intervals until the shadow starts getting longer, taking the shortest shadow as solar noon??? seems like a non-issue since you've already got an accurate watch. if you're lost without a gps and have no idea what your long. is, you've probably got a lot of spare time to figure it out.
    - gmiller123456 7y ago
      That's one way, and it also tells you where due South is too. The watch is only one piece of information, solar noon changes from day to day (called the "Equation of Time"), so you need a way to correlate the two. An almanac would have that info, or the equation could be memorized and computed. This isn't just recreational fun, most (maybe all) Navys still require having people on board each major ship that can perform sight reductions and wind up watches. The assumption would be that during a war GPS and electronic devices would be destroyed. The "stick in the ground" method doesn't work if you're moving, and takes a while, and speed matters when you're being hunted.
      - 4A414B56 7y ago

tjohns 7y ago

This got me reading up on the topic, and apparently you can download a free book from the FAA that covers how to do celestial navigation: Flight Navigator Handbook, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-18.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a... (Apparently early models of the Boeing 747 even had a sextant port built into the roof of the cockpit.)