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Naut. Almanac |
Celestial Navigation
Hour Angles
Since the
In contrast to stars, the GHA’s of sun, moon, and
planets increase at slightly different (and variable) rates.
This is attributable to the revolution of the planets
(including the earth) around the sun and to the revolution of the moon around
the earth, resulting in additional apparent motions of these bodies in the sky.
It is sometimes useful to measure the angular distance
between the hour circle of a celestial body and the hour circle of a reference
point in the sky instead of the
The angular distance of a body (X) westward from the
hour circle (upper branch) of the first point of Aries, measured from 0°
through 360° is called sidereal hour angle, SHA.
The first point of Aries is the fictitious point in
the sky where the sun passes through the plane of the earth’s equator in spring
(vernal point). The GHA of a body is the sum of the SHA of the body and the GHA
of the first point of Aries, GHA Aries:
(If the resulting GHA is greater than 360°, subtract
360°.)
GHA Aries, measured in time units (0-24h) instead of
degrees, is called Greenwich Sidereal Time, GST:
The angular distance of a body measured in time units
(0-24h) eastward from the hour circle of the first point of Aries is called
right ascension, RA
Declinations are not affected by the rotation of the
earth.
The concept
of the earth’s axial rotation causing change in the hour angle of bodies
Since the
In contrast to stars, the GHA’s of sun, moon, and
planets increase at slightly different (and variable) rates. This is due to the
revolution of the planets (including the earth) around the sun and to the
revolution of the moon around the earth, resulting in additional apparent
motions of these bodies in the sky.
It is sometimes useful to measure the angular distance
between the hour circle of a celestial body and the hour circle of a reference
point in the sky instead of the
The angular distance of a body westward from the hour
circle (upper branch) of the first point of Aries, measured from 0° through
360° is called siderial hour angle, SHA.
The first point of Aries is the fictitious point in
the sky where the sun passes through the plane of the earth’s equator in spring
(vernal point).
The GHA of a body is the sum of the SHA of the body
and the GHA of the first point of Aries
‘Greenwich Hour Angle (GHA)’, ‘Local Hour Angle
(LHA)’and longitude, and their relationship
The Greenwich hour angle, GHA, is the angular distance
of a Geographical Position westward from the upper branch of the Greenwich
meridian (0°), measured from 0° through 360°.
The declination, Dec, is the angular distance of
Geographical Position from the plane of the equator, measured northward through
+90° or southward through -90°.
GHA and Dec are geocentric coordinates (measured at
the center of the earth).
The great circle going through the poles and
Geographical Position is called hour circle.
The rate of
change of GHA of the sun and Aries
Since the Greenwich meridian rotates with the earth
from west to east, whereas each hour circle remains linked with the almost stationary
position of the respective body in the sky, the GHA’s of all celestial bodies
and the imaginary point of aries increase as time progresses (approx. 15° per
hour).
In contrast to stars, the GHA’s of sun, moon, and
planets increase at slightly different (and variable) rates. This is
attributable to the revolution of the planets (including the earth) around the
sun and to the revolution of the moon around the earth, resulting in additional
apparent motions of these bodies in the sky.
It is sometimes useful to measure the angular distance
between the hour circle of a celestial body and the hour circle of a reference
point in the sky instead of the
The
geographical position of a body for any given GMT
Star
From
the Almanac take down the GHA (Aries) for the GMT in question
Take
down the SHA of the star
Add
the two and GHA (Star) is obtained,
Take
down the Declination of the Star.
Use
the GHA (Star) as the Longitude and the Declination as the Latitude and plot on
the chart; this would be the GP of the Star.
Sun
Take down the GHA (Sun) from the almanac and the
Declination. Use the Declination as the Latitude and the GHA (Sun) as the
longitude to plot on the chart.