Robert Maynard
Volunteer Moderator
.... or one point and a heading.
Charon Circumnavigation
- Thread starter Ian Phillips
- Start date
Ian Phillips
Volunteer Moderator
Well, that was my problem. I had a starting point, and therefore an antipodal point but didn't know the heading.
Robert Maynard
Volunteer Moderator
If you only have mutually antipodal points then the heading is indeterminate.
Then your great circle would run through the poles.
Your original question was simply what does one need to define a great circle. Now if you are wanting to define a SPECIFIC great circle, then different information would be needed in some circumstances. In any great circle other than a polar one, two points is fine, as is a point and a heading. In the case of a polar great circle, then one pole and a heading is needed, as by definition, both poles will be on the GS.
Robert Maynard
Volunteer Moderator
At a pole there is only one possible heading....
But once you left the starting point and went to a different point, a heading to the antipodal point was calculated...with the advice to recalculate regularly
Yes, I misused the term to convey that from the pole you pick your meridian angle and go down it.
Ian Phillips
Volunteer Moderator
Ok.
I'm not actually sure that's correct, as any two opposite points on a sphere (like the poles) will have an infinite number of great circles that pass through those two points.
So although I knew my two points I didn't know which circle I should follow to properly circumnavigate the moon, not having a heading from my starting point.
I did however mark my position as I crossed the equator, which gave me 3 points. Post #80. That's the idea I woke up with this morning. So now I know where I need to head for on the equator to be on my Great circle.
I'm not actually sure that's correct, as any two opposite points on a sphere (like the poles) will have an infinite number of great circles that pass through those two points.
So although I knew my two points I didn't know which circle I should follow to properly circumnavigate the moon, not having a heading from my starting point.
I did however mark my position as I crossed the equator, which gave me 3 points. Post #80. That's the idea I woke up with this morning. So now I know where I need to head for on the equator to be on my Great circle.
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Robert Maynard
Volunteer Moderator
But once you left the starting point and went to a different point, a heading to the antipodal point was calculated...with the advice to recalculate regularly
Tru dat...!
For the purposes of a circumnavigation from a specific starting point, there will indeed be an infinite number of great circle routes to choose. The other thing about these circumnavigations is that the actual route is largely immaterial so long as one crosses beyond the antipodal point and arrives back at the starting point along a path close to the opposite side of the planet. That will guarantee a circumnavigation. The first time I calculated a heading for you to go to your antipodal, you had already moved off of your starting point. The second you moved off of your starting point, you defined your own great circle.
Ian Phillips
Volunteer Moderator
Hmmm.
I've wobbled a bit on my way around
I've wobbled a bit on my way around
Robert Maynard
Volunteer Moderator
Hmmm.
I've wobbled a bit on my way around
Which means that you've travelled more than half a circumference just reaching the antipodal point in the first place!
You already had a few points. Start, antipodal, then ~ 60, 42 which is what I used to calculate your original heading.. So that means that -60, -138 will be pretty close to being on your great circle, so you could go to that point, and then calculate a heading towards your starting point from there
Haven't we all ... fear not, it won't jeopardise the status of your circumnavigation! I like how, while other circumnavigators have been wrestling with terrain or rocks or speed, you've run into a far more fundamental problem ... when every road leads South, which one goes the right way?Hmmm.
I've wobbled a bit on my way around
Robert Maynard
Volunteer Moderator
I've started working on a "track following" feature that will permit a player to enter start point, initial heading and track "width" (i.e. error) and then follow the corresponding great circle by way of a bearing to an ever receding target point. Early days yet, the spherical geometry has me a bit baffled at the moment (trying to work out distance of an arbitrary point from a great circle - without running into errors....).
Equator for the w... did we already say that? :x [big grin]
If you'll permit me a very small boast, I've done around 200 degrees and never deviated from the equator by more than 0.2 of a degree (there's been a deal of wobbling and wriggling, though). To misquote the great Tom Petty, I don't back down and don't go round (obstacles).
Hope you can make good progress, Ian, now you know where you're going.
If you'll permit me a very small boast, I've done around 200 degrees and never deviated from the equator by more than 0.2 of a degree (there's been a deal of wobbling and wriggling, though). To misquote the great Tom Petty, I don't back down and don't go round (obstacles).
Hope you can make good progress, Ian, now you know where you're going.
Robert Maynard
Volunteer Moderator
Did you end up freefallin' at any point on that journey?
Did you end up freefallin' at any point on that journey?
Well played, sir! I have, indeed, done a great deal of freefallin', and it would be entirely accurate to say I'm learning to flive. Perhaps we should leave it there...
I've blown one to smithereens from full health in the past* so if that's correct, they either changed it or it was bugged.
*Before the update where they normalised the terrain, I tried jumping across a canyon on a 3G world; it was 2km deep and I think I fell about 2,050m
Edit: Never mind, ninja'd about 15 times lol
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Robert Maynard
Volunteer Moderator
I've managed to get my head round the spherical geometry - here's an image of a working example of a great circle being followed (the equator, in this case):
