This is over 360°, so you subtract 360° from the The result is 46°59’29”. Meridian Angle The meridian angle, like the LHA, is measured between the observer’s celestial meridian and the hour circle  of  the  observed  body.  The  meridian  angle, however, is measured east or west from the celestial meridian to the hour circle, through a maximum of 180°, instead of being measured always to the west, as done for the LHA, through 360°. Polar Distance The polar distance of a heavenly body at a given instant  is  simply  the  complement  of  its  declination  at that instant; that is, polar distance amounts to 90° minus the body’s declination. The conventional symbol used to indicate polar distance is the letter  p. Altitude and Altitude Corrections The  angle  measured  at  the  observer’s  position  from the horizon to a celestial object along the vertical circle through the object is the altitude of the object. Altitudes are 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 use in calculations. Your horizontal plane at the instant of observation is, of course, tangent to the earth’s surface at the point of observation; however, the altitude value used in computations is related to a plane parallel to this one but passing through the center of the earth. The difference  between  the  surface-plane  altitude  value  and the  center-of-the-earth-plane  altitude  value  is  the parallax correction. Because of the vast distance between the earth and the fixed stars, the difference between the surface-plane altitude  and  the  center-of-the-earth-plane  altitude  is small enough to be ignored. For the sun and for planets, however, a correction for parallax must be applied to the observed altitude (symbol  ho) to get the true altitude (h,). A second altitude correction is the correction for refraction– a phenomenon that causes a slight curve in light rays traveling to the observer from a body observed at low altitude. A third altitude correction, applying to only the sun and moon, is semidiameter correction. The stars and the  planets  Venus,  Mars,  Jupiter,  and  Saturn,  are pinpoint  in  observable  size.  The  sun  and  moon, however,  show  sizable  disks.  The  true  altitude  of  either of these is the altitude of the center of the disk; but you cannot line the horizontal cross hair accurately on the center. To get an accurate setting, you must line the cross hair on either the lower edge (called the lower limb) or the  upper  edge  (called  the  upper  limb).  In  either  case you must apply a correction to get the altitude of the center. A  combined  parallax  and  refraction  correction  for the sun and planets and a refraction correction for stars keyed to observed altitudes are given in the two inside cover pages in the Nautical Almanac. Semidiameter corrections for the sun and moon are given in the daily pages of the almanac. If you observe the lower limb, you add  the  semidiameter  correction  to  the  observed altitude; if you observe the upper limb, you subtract it. The correction appears at the foot of the Sun or Moon column, beside the letters  S.D. Zenith Distance The zenith distance of an observed body amounts, simply, to 90° minus the true (or corrected) altitude of the body. The letter z is the conventional symbol used to represent zenith distance. DETERMINING  LATITUDE To determine the true azimuth of a line on the ground from a celestial observation, you must know the latitude of the point from which the celestial observation is  made.  If  you  can  locate  the  point  of  observation precisely on an accurate map, such as a U.S. Geological Survey (USGS) quadrangle map, you can determine the latitude from the marginal latitude scale. If no such map is available, you can determine the latitude through a meridian observation of a heavenly body. Latitude by Meridian Altitude Observation In  a  meridian  observation  you  determine  the  altitude of  the  body  at  the  instant  it  crosses  your  celestial meridian.  At  this  instant  the  body  will  be  at  the maximum altitude observable from your position. When you are applying a meridian altitude to get the latitude,  there  are  three  possible  situations,  each illustrated in figure 15-12 and explained in the following paragraphs. CASE I. When the body observed is toward the equator from the zenith, you can use the following formula to get the latitude: Q=8+Z =   a  +  (90°  -  h), 15-14