For those of you who missed the AGM, Themos Tsikas tried to squeeze a full length talk into an after tea space. Sadly, time dilation did not work, even though he blue shifted near the end. Perhaps we should try to list the variety of hobbies our members have, besides astronomy. We have now found out that Themos is a very keen sailor.
So he approached his talk from a sailing point of view, and really only hit on the astronomical side at the end.
So, basically you are on a boat on a globe, much of it sea. How do you keep your bearings? Easy if you know what time it is and you can see how high the Sun, Moon, or certain stars are. But that only gives the latitude, not the longitude.
The simplest way is to think of yourself looking up at the sky, hopefully clear(ish) of clouds. At night you should see lots of stars, and these will all be overhead somewhere as seen from Earth. You use a sextant to work out the angle of altitude of certain known stars and imagine a line starting from your position, going round their zenith point and coming back to you. You then wait ten minutes and remeasure. The circle will have changed, unless you happened to pick on Polaris…. If you use the Sun or the Moon, it is not so easy, because of their larger proper motion. You can then work out the circles’ intersection points and that’s where you are. You must also make sure not to crouch or climb onto a higher point to take readings, as that alters the object’s height above the horizon.
Themos did not get as far as the list of navigational stars, of which some have intriguing names (Needless to say, Polaris is not among them.). He did have two sextants, and showed us how to line up the moving mirror (which is linked to the scale via a sliding arm) so that the horizon is lined up with a star, Moon or Sun, thereby giving the altitude.
Please Themos, do bring those lovely gadgets again so we can have another play with them during the tea break. Time ran out on you too soon. At least you had time to show us a quick video of how to use a sextant.