3. Astronomical Determinations of Latitude

Observations for latitude by altitudes of the pole-star were carried out on thirty-five nights at nineteen principal camps, using the same three-inch theodolite as was employed for the time-observations. Three readings of altitude were made on each face, using each of the three horizontal stadia-wires in turn, the corresponding times being noted on a half-chronometer watch whose error on local time was accurately known from sun or star-observations carried out just before the latitude-observation. Particular care was taken with the level-adjustment, and the air-pressure and temperature at the time of the observation were recorded.

The following table gives the results of the observations:

Astronomical Latitudes

				°	′	″
Sollum	4	nights	Lat.	31	35	9	N.
Siwa	1	“	“	29	12	41	“
Jaghbub	5	“	“	29	44	26	“
Camp near Jalo	1	“	“	29	11	56	“
Jalo (El Erg)	1	“	“	29	2	33	“
Buttafal	1	“	“	28	54	26	“
El Harrash	1	“	“	25	26	29	“
Taj	6	“	“	24	13	47	“
Arkenu	2	“	“	22	12	32	“
Ouenat	1	“	“	21	52	29	“
Erdi	1	“	“	18	35	39	“
Agah	1	“	“	17	52	38	“
Enebah	1	“	“	17	21	24	“
Bao	1	“	“	16	28	24	“
Furawia	2	“	“	15	21	51	“
Um Buru	2	“	“	15	3	57	“
Kuttum	1	“	“	14	12	15	“
El Fasher	2	“	“	13	38	3	“
El Obeid	1	“	“	13	10	51	“

Of six of the above places (Sollum, Siwa, Jaghbub, Kuttum, El Fasher, and El Obeid) the latitudes are accurately known from the Egyptian and Sudan official surveys, and the agreement in these cases is very satisfactory, though a very close comparison is not generally possible owing to uncertainty as to Hassanein Bey’s precise observation-spot. At Jaghbub, Hassanein Bey records that his observation-spot was 200 meters S.S.W. of the dome of the mosque. Applying the corresponding difference of latitude (− 6″) to my own determination of the latitude of the dome in 1917 (29° 44′ 41″) we obtain 29° 44′ 35″, showing a difference of only 9″ from Hassanein Bey’s observed latitude.

A further test of the degree of precision of the latitude-observations can be made by comparison of the latitudes found for the same camp by observations taken on different nights. The following gives the average deviation of a single observed latitude from the mean at all the camps where two or more observations for latitude were made:

				″
Sollum	4	nights	Av. dev.	8
Jaghbub	5	“	“	40
Taj	6	“	“	12
Arkenu	2	“	“	6
Furawia	2	“	“	8
Um Buru	2	“	“	23
El Fasher	2	“	“	6

It thus appears unlikely that any observed latitude can be as much as 1′ in error, and consequently in the preparation of the map, Hassanein Bey’s observed latitudes have been accepted as fundamental data for all the places for which no previous determinations are known to exist, viz., El Harrash, Taj, Arkenu, Ouenat, Erdi, Agah, Enebah, and Bao. His latitudes for Jalo (El Erg), Bir Buttafal, and Furawia have also been adopted for the map, the first because it is possibly better than that of Rohlfs, with whose mapped position it is however in very close agreement; the second because while differing by about 2′ from Rohlfs’s value (28° 36′ 22″) it is doubtless more exact, because it checks admirably well with Hassanein Bey’s dead reckoning; and the third because, although the position of Furawia is shown on the Sudan maps, it is outside the present limits of the Sudan triangulation and is possibly liable to some slight error.[1]