Astronomy Principles and Practice Fourth Edition.pdf

416 Practical projects
Summer Time was in operation, where
British Summer Time (BST) = Universal Time (UT) + 1 hour.
(iv) Procedure. Data concerning the Sun’s position are tabulated in The Astronomical Almanac
(AA)—(see pages C4–C19 therein)—and are referred to TDT with
UT = TDT − T.
To a good approximation for the summer of 2000, T = 66 s. Values of the numerical correction
for other years can also be found in the AA—pages K8–K9.
The daily solar data (C4–C19) refer to the solar crossing of the meridian as the ‘ephemeris
transit’, this corresponding to the TDT at which the Sun crosses an ephemeris meridian at longitude
1·002 738 × T east of the Greenwich meridian. Its occurrence is, thus, T earlier on the TDT time
scale than the transit of the Greenwich meridian. To all intents and purposes, to the accuracy that can be
achieved with this experiment, the time of the Greenwich transit in UT can be taken as the ephemeris
transit in TDT.
An approximate position of Glasgow University Observatory to be used as the dead reckoning
position (DRP) can be taken as
Longitude (λ D ) = 4 ◦ 22 ′ W
Latitude (φ D ) = 55 ◦ 53 ′ N.
Two sets of observations are required—A and B(i) or B(ii).
One set, A, is to be taken at the local meridian transit, when the Sun’s altitude is changing
slowly with time. Determination of the Sun’s maximum altitude at the transit allows a position line
to be determined. (The time at which the transit occurs cannot be measured accurately and only an
approximate position of the observatory could be obtained if this information were to be used.) The
advantage of a meridian set of observations is that the reduction of the observations is particularly
simple.
Observations should be made at approximately three minute intervals, and cover a period from at
least twenty minutes before to at least twenty minutes after meridian transit.
The second set, B, is to be taken over a period when the Sun’s altitude is changing quite quickly
with time. Using the curve which depicts this variation, a second position line can be obtained. Make
a set of observations EITHER according to the scheme B(i) OR to scheme B(ii).
For B(i), obtain a series of measurements of the altitude of the Sun’s centre during a period in
the morning. Observations should be made at least 2 hr before meridian transit, at approximately two
minute intervals, for a run of at least 10 minutes.
For B(ii), obtain a series of measurements of the altitude of the Sun’s centre during a period in
the afternoon. Observations should be made at least 2 hr after meridian transit, at approximately two
minute intervals, for a run of at least 10 minutes.
IMPORTANT REMINDER
NEVER OBSERVE THE SUN WITHOUT THE SHADES
COVERING BOTH MIRRORS.
(a) A sextant, an artificial horizon and a chronometer are required. The artificial horizon, a trough
of water, is required for sextant observations made on land. (In some old books, a trough of mercury
is recommended for the artificial horizon. This is no longer accepted practice because of mercury’s
toxicity.) At sea a horizon is available. Any chronometer error must be checked before and after the
series of observations.