45

another star twelve hours distant, and observe the declination error Half this will be the vernier error. The vernier can be altered this amount, and the star refound by again shifting the foot. All that now will be requisite will be to re-adjust the polar axis on a meridian star. Example 3.

Example i.

Meridian East West ... ... a Tauri ... 160 15' read as 160 15' 1 ... a Ursa Majoris 62° 25' ,, 540 20' ... a Andromeda 28° 24' ,, 26° 9' 1 1 Error 0 8° S' 2° 15'

2 | — IO° 20'

Vernier error — 5° io'

Example 2.

Meridian East West ... ... i Cancri ... 290 14' read as 290 14' ... k Bootis ... 52° 24' ,, 5o°4o' ... y Andromeda 410 42' ,, 420 56' Error + 0 1° 44' i° 14'

2 — 3°'

Vernier error — 15'

Example 3.

Meridian East ... West ... ... a Leonis ... 120 36'read as 12° 36' ... i\ Draconis ... 6i° 48' ,, 6i° 48' ... £ Pegasi ... 390 38' ,, 410 24' Error + 0 0 i° 46'

Vernier error 53' 2 + i° 46'

Set vernier to 40° 31' + 53'

The foot was shifted until t Pegasi was in the centre of the field, the vernier altered so that it read 390 38', and finally the latitude screw re-corrected.

To set the Right Ascension Vernier.

The right ascension and hour-circle of an equatorial should always be movable, for, if not, a distinct calculation must be made before each observation. A movable hour-circle is fitted with two verniers, one reading time, and the other right ascension, and one of these can be shifted for preliminary adjustment. Two data are requisite before this can be done. Greenwich time can generally be obtained at the nearest Postal Telegraph Office, but as this will be only correct for places of the same longitude, the longitude of the place of observation can be easily determined from an ordnance map, and the distance converted to time (15' longitude == im time). If the place is east of Greenwich, this amount must be added; if west, the contrary. Accurate mean local time having been thus ascertained, look in an almanac opposite the date of observation for the mean time of transit of a convenient star or planet, which is given for every day of the year.

---

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 95,14 %.

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

---