This is over 360°, so you subtract 360° from the
The result is 46°5929.
The meridian angle, like the LHA, is measured
between the observers 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°.
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 bodys declination. The conventional symbol used
to indicate polar distance is the letter p.
Altitude and Altitude Corrections
The angle measured at the observers 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 earths 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
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
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.
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.
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
CASE I. When the body observed is toward the
equator from the zenith, you can use the following
formula to get the latitude:
= a + (90° - h),