Sextant Navigation on Land

The sextant is an instrument that can be used on ships or in airplanes to calculate a geodetic position if the time is known, or to calculate the time if position is known. Unlike a theodolite, it doesn't normally provide any means of determining the horizontal plane. Instead, it requires a view of the geodetic horizon, which a ship has on a large body of water, or which an airplane has when it has reached an appreciable altitude.

In principle, a sextant can also be used on land if some additional means of finding the horizon is provided. Attachments to the instrument with level vials have been used, as well as dishes filled with the liquid metal mercury. Nowadays, mercury is hard to come by, and for good reasons; in addition, the mercury surface can be disturbed by air currents when you are working in the great outdoors. 

Here I show you an alternative that can give results as good as the mercury-filled dish, is completely safe, and requires no special skills or tools to construct.

Come back later and learn how it's done!

Leveling with a laser pointer

Orienting with a magnetic compass

Sighting via the artificial horizon

Timing coincidence

Here are results from some elevation measurements made in Constance, Germany, as compared to elevations predicted by the planetarium program KStars (Linux). Note that the program itself probably has uncertainties of a few arc seconds.

27.9.2009 Sextant observations in KN

Jupiter     Measd    KStars

21:28:25 24 49 50 24 46 15

22:31:40 25 37 28 25 38 08

22:55:55 25 09 05 25 08 21

23;03:53 24 55 05 24 50 28

23:09:18 24 41 55 24 40 19

23:22:20 24 06 38 24 05 19

Moon

23:33:20 9 10 55 9 08 22

Aldebaran

23:58:20 16 33 00 17 06 21

00:09:50 18 27 47 18 22 40

00:22:45 20 39 10 20 33 14

_________________________________

Observations using adjusted mirror-horizon. Positions are apparent positions from KStars.

K-Stars seems to give "apparent altitude" i.e., without atmospheric reduction.

"<" means mirror in position 0

">" means mirror rotated 180°

5 Sept 2010, Sonnenbühl 57

UT Ist-Alt Reduced Soll-Alt Mirror Err Err Avg.

JUPITER

20:56:31 21° 49.9' 21° 47.6'21° 45.4' < +2.2'

21:09:27 23° 38.9' 23° 36.8' 23° 37.8' < -1.0

6 Sept 2010, Sonnenbühl 57

JUPITER

[21:12:25 25° 11.8' 25° 9.8' 25° 14.5' < -4.7' ]

21:22:25 26° 39.7' 26° 37.8' 26° 41.5' > -3.7

21:29:33 27° 33.2' 27° 31.4' 27° 34.2' < -2.8

21:36:21 28° 29.6' 28° 27.9' 28° 29.5' > -1.6

21:45:06 29° 38.0' 29° 36.4' 29° 36.9' < -0.5 -2.15'

HAMAL

21:53:00 30° 39.1' 30° 37.5' 30° 40.1' < -2.6

21:59:22 31° 45.6' 31° 44.1' 31° 44.5' > -0.4

22:05:09 32° 42.6' 32° 41.1' 32° 42.9' < -3.4

22:09:46 33° 31.4' 33° 30.0' 33° 29.8' > 0.2 -1.55'

Since there are 360 arc degrees but only 24 hours in a circle, we have to divide angle measure by 15 to get time measure (hours, minutes, seconds).

An error of -2 arc minutes (or 120 arc seconds) would be equivalent to a clock error of -120"/15 or 8 seconds. Pretty good for a low-budget operation!