Report of a Mission to Yarkund in 1873 : vol.1 / Page 463 (Color Image)

0463	Report of a Mission to Yarkund in 1873 : vol.1

rate may be duly ascertained and allowed for. The effect of instrumental errors
will be materially reduced when the stars and the moon are on the same side
of the meridian and at nearly the same zenith distance; if time permits,
observations should be taken both east and west of the meridian, and both before
and after full moon. In north latitudes, when the moon is going from south
to north in declination on any day, she is most favorably situated for observ-
ing when west of the meridian; if moving in declination from north to south,
she should be observed east of the meridian. The best time for observation
is when the direction of the proper motion of the moon is towards the zenith
of the observer. The sidereal time when this occurs may be readily found,
graphically, by drawing on a chart of the heavens a tangent to the moon's
orbit, at some point near the mean position of the moon on the day of
observing, and producing it to cut the declination circle passing through the
observer's zenith; then the hour circle passing through the point of intersec-
tion gives the sidereal time of observation. For practical purposes it will suffice
to drop a perpendicular from the point indicating the moon's mean position on
to the ecliptic, and drawing through that point a line at right angles with the
perpendicular, and prolonging it to cut the declination circle. It will be found
that the most favorable times occur when the moon is on the observer's prime
vertical, and the least favorable when she is on the meridian. Whenever
possible a few observations should be taken daily on several days rather than
a large number on a single day."

An examination of the results of the observations now published shows, at
a glance, that those at Kashghar are both much more complete and satisfactory
in every way than those taken at Yarkand and elsewhere. This is easily
accounted for by several reasons:—

My stay at Yarkand was limited to twenty days in all, many of which
were cloudy and unfavorable for observing; whereas I was at Kashghar on and
off for more than two months, during which time I was enabled to select the
most favorable days for observing; I was at Yarkand during the early portion
of our stay in the country, and not knowing what opportunities I should have,
if any, for further observations, there or elsewhere, I observed the moon when-
ever I could get an opportunity quite irrespective of its position being favour-
able or otherwise. The observations were taken in a small court-yard, where
the paved flooring gave anything but a stable footing to the instrument and
caused great difficulty with the levels. The noise in the small court of people
moving about during the operation was, it may well be imagined, highly detri-
mental to such delicate work as observations for longitude, particularly where a
pocket chronometer had to be used.

At Kashghar, on the other hand, the court-yard was much larger and
quieter and the ground more stable, and altogether the surroundings were very
much more favorable.

These circumstances, combined with the results obtained from the computa-
tions, have induced me to employ the longitude of Kashghar as the origin for
all my positions in Turkestan.

I have merely employed the other observed longitudes as checks upon the
general accuracy of the positions of those points as determined by other methods,
for which vide the details on the construction of the map which are given in
the body of the report.

(Sd.) Henry Trotter, Capt., R.E.

National Institute of Informatics - Digital Silk Road Project
Digital Archive of Toyo Bunko Rare Books

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National Institute of Informatics - Digital Silk Road Project Digital Archive of Toyo Bunko Rare Books

Citation Information

OCR Text

National Institute of Informatics - Digital Silk Road Project
Digital Archive of Toyo Bunko Rare Books