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(66.) July 4, 1853, at 3h 42m A.M. apparent solar time, in long. 84° 42′ W., required sidereal time.
Ans., 22h 35m 115.53.
(67.) Oct. 21, 1853, at 8h 48m P.M. apparent solar time, in long. 88° 8' E., required sidereal time.
Given, mean time, or apparent time at the ship, to find what heavenly body will pass the meridian the next after that
1. Get a Greenwich date (p. 70).
2. Find the right ascension of the mean sun (and, if the Greenwich date is in apparent time, find also the equation of time, p. 77) for that date, so as to get mean time (p. 88). 3. Add together ship mean time and the right ascension of mean sun.
4. The sum (rejecting 24 hours if greater than 24 hours) will be sidereal time, or the right ascension of the meridian.
5. Then that star found in some catalogue of fixed stars, whose right ascension is the next greater, will be the star required.
Feb. 24, 1853, at 4h 42m P.M. mean time nearly, in long. 100° E., find what bright star will pass the meridian the next after that date.
Looking into the "Catalogue of the mean places of 100 principal fixed stars" (Nautical Almanac, p. 432), we find the star whose right ascension is next greater than 2h 58m is a Persei; therefore a Persei is the bright star that will come to the meridian the next after 4h 42m P.M. on Feb. 24.
Sometimes it is required to find what principal stars will pass the meridian between certain convenient hours for observing their transits: as, for instance, between 8h and 11h P.M. To do this, we must find the right ascension of the meridian for these two times by the above rule; then the stars whose right ascensions lie between will be the stars required.
Oct. 3, 1853, in long. 90° W., find what bright stars put down in the Nautical Almanac will pass the meridian between the hours of 9 and 12 P.M.
In Catalogue p. 432, Nautical Almanac, the stars whose right ascensions lie between 21h 50m 51s and Oh 51m 21s are from a Aquarii to 6 Ceti inclusive.*
* In the "Handbook for the Stars," published by the author, there is a table of the times of the transits of the principal fixed stars. This
(68.) What bright stars put down in the Nautical Almanac will pass the meridian of a ship in long. 40° E., between 8h and 10h P.M. mean time on Nov. 20, 1853 ?
Ans., From a Andromeda to a Arietis. (69.) What bright star will pass the meridian of a ship in long. 30° W. the first after 10h 30m P.M. on Oct. 10, 1853 ? Ans., a Andromedæ.
(70.) What bright stars will pass the meridian of a ship in long. 56° W. between the hours of 6 and 10 P.M. on March 10, 1853 ? Ans., From 6 Tauri to y Argûs. (71.) What bright stars put down in the Nautical Almanac will pass the meridian of Greenwich between the hours of 7 and 9 P.M. mean time, on August 20, 1853 ? Ans., From Ursa Minoris to ẞ Lyræ.
(72.) What stars named in the Nautical Almanac will pass the meridian of a ship in long. 86° E., on Oct. 20, 1853, between the hours of 10 P.M. and midnight?
Ans., From a Andromeda to a Eridani. (73.) What bright star will pass the meridian of Greenwich the first after 9h P.M. on Sept. 12, 1853 ?
table enables the observer to find the name of the bright star that is on the meridian at any given time, and at any place, without calculation.
Given, sidereal time, to find mean time.
1. Take out of the Nautical Almanac the right ascension of the mean sun (called in the Nautical Almanac sidereal time), for noon of the given day.
2. From sidereal time (increased if necessary by 24 hours) subtract the quantity just taken out, the remainder is mean time nearly.
3. Find, in the table of the acceleration of sidereal on mean solar time, the correction for this time, and subtract it from the mean time nearly.
4. The remainder is the mean time required.
NOTE. In strictness we ought to have entered the table with the correct mean time, instead of that used; but it is evident we may obtain a still closer approximation to the truth by repeating the work, using the last approximate value instead of the preceding one. For all practical purposes this repetition is unnecessary.
1. April 27, 1846, when a sidereal clock showed 3h 40m 45s, required mean time.
2. March 2, 1848, when a sidereal clock showed 3h 40m 453,
required mean time.
3h 40m 458.0
Right asc. m. sun at m. noon 22 41 35.94
The clock of an observatory used for taking the transits of a heavenly body is generally adjusted to sidereal time: the above rule will show how to determine the error of a solar clock, or a chronometer regulated to mean time; for we have only to note the time shown by the two instruments at the same instant by comparing the chronometer with the sidereal clock at some coincident beat, the error of the latter being supposed to be known.
Greenwich, March 3, 1853, when a sidereal clock showed 6h 10m 203 a chronometer showed 7h 32m 10s, required the error of the chronometer on Greenwich mean time; the error of sidereal clock being 2m 428.5 slow.
Right asc. mean sun at mean noon 22 44 41.48