This is over 360°, so you subtract 360° from theThe result is 46°59’29”.Meridian AngleThe meridian angle, like the LHA, is measuredbetween the observer’s celestial meridian and the hourcircle of the observed body. The meridian angle,however, is measured east or west from the celestialmeridian to the hour circle, through a maximum of 180°,instead of being measured always to the west, as donefor the LHA, through 360°.Polar DistanceThe polar distance of a heavenly body at a giveninstant is simply the complement of its declination atthat instant; that is, polar distance amounts to 90° minusthe body’s declination. The conventional symbol usedto indicate polar distance is the letter p.Altitude and Altitude CorrectionsThe angle measured at the observer’s position fromthe horizon to a celestial object along the vertical circlethrough the object is the altitude of the object. Altitudesare measured from 0° on the horizon to 90° at the zenith.The complement of the altitude is the zenith distance,which is often more convenient to measure and to usein calculations. Your horizontal plane at the instant ofobservation is, of course, tangent to the earth’s surfaceat the point of observation; however, the altitude valueused in computations is related to a plane parallel to thisone but passing through the center of the earth. Thedifference between the surface-plane altitude value andthe center-of-the-earth-plane altitude value is theparallax correction.Because of the vast distance between the earth andthe fixed stars, the difference between the surface-planealtitude and the center-of-the-earth-plane altitude issmall enough to be ignored. For the sun and for planets,however, a correction for parallax must be applied to theobserved altitude (symbol ho) to get the true altitude (h,).A second altitude correction is the correction forrefraction– a phenomenon that causes a slight curve inlight rays traveling to the observer from a body observedat low altitude.A third altitude correction, applying to only the sunand moon, is semidiameter correction. The stars andthe planets Venus, Mars, Jupiter, and Saturn, arepinpoint in observable size. The sun and moon,however, show sizable disks. The true altitude of eitherof these is the altitude of the center of the disk; but youcannot line the horizontal cross hair accurately on thecenter. To get an accurate setting, you must line the crosshair on either the lower edge (called the lower limb) orthe upper edge (called the upper limb). In either caseyou must apply a correction to get the altitude of thecenter.A combined parallax and refraction correction forthe sun and planets and a refraction correction for starskeyed to observed altitudes are given in the two insidecover pages in the Nautical Almanac. Semidiametercorrections for the sun and moon are given in the dailypages of the almanac. If you observe the lower limb, youadd the semidiameter correction to the observedaltitude; if you observe the upper limb, you subtract it.The correction appears at the foot of the Sun or Mooncolumn, beside the letters S.D.Zenith DistanceThe zenith distance of an observed body amounts,simply, to 90° minus the true (or corrected) altitude ofthe body. The letter z is the conventional symbol used torepresent zenith distance.DETERMINING LATITUDETo determine the true azimuth of a line on theground from a celestial observation, you must know thelatitude of the point from which the celestial observationis made. If you can locate the point of observationprecisely on an accurate map, such as a U.S. GeologicalSurvey (USGS) quadrangle map, you can determine thelatitude from the marginal latitude scale. If no such mapis available, you can determine the latitude through ameridian observation of a heavenly body.Latitude by Meridian Altitude ObservationIn a meridian observation you determine the altitudeof the body at the instant it crosses your celestialmeridian. At this instant the body will be at themaximum altitude observable from your position.When you are applying a meridian altitude to get thelatitude, there are three possible situations, eachillustrated in figure 15-12 and explained in the followingparagraphs.CASE I. When the body observed is toward theequator from the zenith, you can use the followingformula to get the latitude:Q=8+Z= a + (90° - h),15-14

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