# Thread: A little problem of geography

1. Hello!

Consider three locations A, B and C being aligned and being separated as indicated:

A ====100km===== B =====30km==== C

Let's suppose a guy doesn't have map, nor satellites, nor GPS,
and that he doesn't know the precise distances mentionned above but has only a approximation of them.
if he uses only a compass and stars, can he know these locations are aligned?

2.

3. the compass and the stars would be utterly useless in this scenario. reading what you have writte, he starts at point A and knows that to get to point B he must travel 100km - but - he has no map (you have not mentioned whether or not he has been given any mag bearings to each destination; so ill take that as a no - if thats the case then all he DOES know is that he has to travel 100Km from his present destination; as that could take him on a journey along a literally infinatew number of radials from his current position then he'd have no idea and has a high probability of never arriving at his destination.

if he IS given mag bearings then yes he would know that they are aligned and the mag bearings would be the giveaway:

Start point A
travel 100Km 45degrees (east)
end point B
travel 30Km 45 degrees (east)
end point C

so yes; as the second mag bearing from point B tells the traveller to continue on the same path and direction, then yes; he will now that they are aligned.

4. Originally Posted by termina
Hello!

Consider three locations A, B and C being aligned and being separated as indicated:

A ====100km===== B =====30km==== C

Let's suppose a guy doesn't have map, nor satellites, nor GPS,
and that he doesn't know the precise distances mentionned above but has only a approximation of them.
if he uses only a compass and stars, can he know these locations are aligned?
By aligned I assume you mean the same latitude... He could take a "fix" of the sun's maximum elevation within a day or two from the 3 sites and confirm they are at the same latitude. He just needs something to verify the elevation angle is the same, two sticks binded together would work as well as a sextent.

He could also use a polynesian navigation technique and find pairs of stars that cross the horizon (rising or falling) at the same time. Like pairs of stars cross the horizon at the same time at the same latitude and it works year round.

5. Originally Posted by termina
Hello!

Consider three locations A, B and C being aligned and being separated as indicated:

A ====100km===== B =====30km==== C

Let's suppose a guy doesn't have map, nor satellites, nor GPS,
and that he doesn't know the precise distances mentionned above but has only a approximation of them.
if he uses only a compass and stars, can he know these locations are aligned?
If he knew where on the Earth's surface these points were, he could walk them using his compass. If he needed to alter his heading between A to B and B to C in order to get there, then he knows they are not aligned.

6. Ok, if I understand the assumptions:
1/ The 3 points are aligned
2/ The person has no maps, no GPS
3/ The person has a compass (and "the stars" (I assume this includes a sextant))

In that case, the person will be able to know the location the place are aligned only if he knows that the bearing of the second point i.e. the direction of the alignement. Else he/she can walk in any direction between 0 and 359 deg !
If the person wants to know the exact distance between the 3 points, he/she will need an accurate chronometer to measure time. Measuring latitude is not such a problem but measuring longitude requires accurate time measurement. Without referring to scientific readings, I prefer to send you to Mr Umberto Eco's wonderful book "The Island of the Day Before"...

7. Originally Posted by termina
Hello!

Consider three locations A, B and C being aligned and being separated as indicated:

A ====100km===== B =====30km==== C

Let's suppose a guy doesn't have map, nor satellites, nor GPS,
and that he doesn't know the precise distances mentionned above but has only a approximation of them.
if he uses only a compass and stars, can he know these locations are aligned?
His compass will keep him on a more or less straight heading, so long as he does'nt take too long walking the line. If he happens to pass through three places called a,b and c then he knows they are alligned.
He could also follow a moving star to Bethlehem, for example.

8. Hi mise,

How does he know he is passing A, B and/or C?

Steve

9. Originally Posted by Steve Miller
Hi mise,

How does he know he is passing A, B and/or C?

Steve
Ah... silly me, I assumed ab&c were villages, towns or some other feature which would stick in his memory.

10. So does it work on a ship which is sailing on the sea? Yes, I guess it would, more or less.

Steve

11. Originally Posted by Steve Miller
So does it work on a ship which is sailing on the sea? Yes, I guess it would, more or less.

Steve
Hey Steve,
I think you've got me there. I hope we figure this one out.

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