47

Example : To' find a Virginis (Spica) at 11 o’clock mean time on June 30th, 1875, or at XVII. 35m. 25s. as shown by the sidereal clock.

A. —The declination of Spica on June 30th was io° 30' 43", and the

vernier was set to this position.

B. —The hour-circle which read from W. to E. was revolved till it

read to the upper vernier, the right ascension of Spica XIII. r 8m. 38s.

C. —The telescope was turned, so that the lower vernier read XVII.

35m. 25s. and the star was in the field of view.

If the circle had read from E. to W. the right ascension setting could have been effected thus :—

B. —The hour-circle is revolved till it reads XIII. 18m. 38s. to the

lower vernier.

C. —The telescope is turned till the movable vernier reads XVII.

35m. 25s., and the star is in the field.

Twenty-four hours of sidereal time may be roughly estimated as four minutes shorter than twenty-four hours of mean time, or a gain of ten seconds every hour; a watch thus regulated will keep fairly correct sidereal time. If time is not accurately known a slight movement in right ascension will often bring the object into the field. The correctness of the local time of any place may be easily ascertained by setting one vernier to the right ascension of a known star in the field, when the other vernier will show correct sidereal time. If this operation is quickly performed, a clock or watch may be accurately set at the commencement of the evening’s work. Time however can be most accurately ascertained by a meridian passage.

Example : Spica being brought in the centre of the field, and the right ascension circle set to XIII. i8m. 38s., the time shown by the other vernier was XVIII. 3m.

To convert Mean into Sidereal Time.

Mean time can be easily converted into sidereal; but data must be obtained from an almanac. Example :—To convert VII 50m. Greenwich mean time to sidereal time on July 10th, 1875 :—

Sidereal time at mean noon, July 1 oth ...VII. 12m. is.

Add sidereal interval for 7I1. 50m. ... VII. 51m. 17s.

Sidereal time

... XV. 3m. 18s.

---

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 97,70 %.

La langue de reconnaissance de l'OCR est l'Anglais.

---