Speculum nauticum a looking glasse for sea-men : wherein they may behold a small instrument called the plain scale whereby all questions nautical and propositions astronomical are very easily and demonstratively wrought ... / by Iohn Aspley.

CHAP. XVIII.

The right ascention of the Sun or Starre being given, together with the difference of their ascentions, to finde the oblique ascention.

THe right ascention of any point of the Heavens being knowne, the dif∣ference of the ascention is either to be added thereunto or else to be sub∣stracted from it, according as the Starre is situate in the Northerne or Southerne Signes: As for example, if the Sun be in a∣ny of these six Signes, Aries, Taurus, Gemini, Can∣cer, Leo, or Virgo, then the difference of the ascenti∣ons is to be substracted from the right ascention, and the remainder is the oblique ascention. Suppose there∣fore the Sunne to be in the fourth degree of Gemini, where the right ascention is found to be foure houres, and 8 minutes, or 62 degrees, and the difference of ascention where the Pole is elevated 51 degrees, is found to be one houre 53 minutes otherwise 28 de∣grees 53 minutes, which being taken from the right ascention, leaves two houres, and 16 minutes, or 33 degrees, and 42 minutes, which is the oblique ascen∣tion of the Sun in the fourth degree of Gemini. But if the Sun be upon the South side of the Equinoctiall, either in Libra, Scorpio, Sagitarius, Capricornus, A∣quarius, or Pisces, then the difference of the ascenti∣ons is to be added unto the right ascention, and the Product will be the oblique ascention. Suppose the fourth degree of Sagitarius, is given, for which signe and degree the oblique ascention of the Sun is desi∣red,

his right ascention being then found to be 242 degrees, or 16 houres, 8, minutes, the difference of the ascention is one houre, 53 minutes, or 28 degrees, 18 minutes, which being added unto the right ascen∣tion, makes 18 houres, and one minute; or in degrees 270 degrees, and 18 minutes: which is the oblique ascention of the Sun, when he is in the fourth degree of Sagitarius. And if you would finde the oblique de∣scention, you must adde the difference of the ascenti∣ons unto the right ascention, when the Sun is in these six Signes, Aries, Taurus, Gemini, Cancer, Leo, Virgo: and contrariwise, when the Sunne is in the other six Signes, you must substract the difference from the right ascention, and you shal have the oblique descen∣tion of the Sun, or any Starre, whose right ascention and difference of ascentions is knowne. But it is to be understood, that this manner of operations doth serve no longer then you are upon the North side of the Equinoctiall. For if the South Pole be elevated, the worke is contrary: for so long as the Sun is in any of the Northerne Signes, the difference of the ascenti∣ons is to be added unto the right ascention, to finde the oblique ascention. And contrariwise, substracted to finde the oblique descention. Likewise if the Sun or Starre be in the Southerne Signes, then is the diffe∣rence of ascentions, substracted from the right ascen∣tion, to finde the oblique ascention, and added, to finde the oblique descention.