The elements of astronomy; a textbook . Fig. 3. — Dip of the Horizon. 1 The Dip (in minutes) = y/h (in feet) nearly. 12 VERTICAL CIRCLES. [§16 sextant the height of the sun or other heavenly body, in theoperations by which he determines the place of his ship. 17. Vertical Circles; the Meridian and the Prime Vertical.— Vertical Circles are great circles drawn from the zenith at rightangles to the horizon. Their number is indefinite : each starhas at any moment its own vertical circle. That particularvertical circle which passes north and south is known as theCelestial Meridian, and is evidently
The elements of astronomy; a textbook . Fig. 3. — Dip of the Horizon. 1 The Dip (in minutes) = y/h (in feet) nearly. 12 VERTICAL CIRCLES. [§16 sextant the height of the sun or other heavenly body, in theoperations by which he determines the place of his ship. 17. Vertical Circles; the Meridian and the Prime Vertical.— Vertical Circles are great circles drawn from the zenith at rightangles to the horizon. Their number is indefinite : each starhas at any moment its own vertical circle. That particularvertical circle which passes north and south is known as theCelestial Meridian, and is evidently the circle which would beobtained by continuing to the sky the plane of the terrestrialmeridian upon which the observer is located. The verticalcircle at right angles to the meridian is the Prime Fig. 4. — The Horizon and Vertical Circles. O, the place of the , the Observers , the Zenith; P, the , the , the , the Prime Vertical. M, some Star. ZMH, arc of the Stars Vertical Circle. TMR, the Stars Almucantar. Angle TZM, or arc SH, Stars Azimuth. Arc HM, Stars Altitude. Arc ZM, Stars Zenith Distance. 18. Parallels of Altitude, or Almucantars. — These are smallcircles of the celestial sphere drawn parallel to the horizon justas the parallels of latitude on the earths surface are drawnparallel to the equator. The term Almucantar (Arabic) isseldom used. § 19] ALTITUDE AND ZENITH DISTANCE. 13 19. We are now prepared to designate the place of a bodyin the sky by telling how many degrees it is above the horizon,and how it bears from the observer. Altitude and Zenith Distance. — The Altitude of a celestialbody is its angular elevation above the horizon ; , the numberof degrees between it and the horizon, measured on a vert
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