NavList:

The many threads here have got me to thinking (again) about land navigation and how it should be different than maritime or aviation navigation. The retro ocean race - no GPS or computers, etc. and a new thread about lunars has made me think how a 18th C. land explorer or even a Navy type who is on an island could have measured his longitude.

If you set up a telescope (transit, theodolite) looking 180 true or 360 if south and lock the azimuth, with only a hack watch needed you note the meridian passage of a star then the moon, making note of the time difference. Now the time difference is just difference in GHA.

It seems to me that it would be easy to figure out from the almanac data when the moon and that star would have the noted difference of GHA. Parallax and refraction would not be a factor, nor the distance between them - traditional lunar distance - only the GHA of meridian passage. Semi-diameter would be needed as a limb on the cross wire would be more accurate than center.

Now I know it would take some time to set up the telescope to look true north or south but it was done in the 1800s. Not something done on the move but say for winter quaters or building a new town on the island.

This seems too simple to be correct so I ask for comments good or ill.

John H.