Long-Term Ephemerides for the Sun
Scope and Purpose
Due to the relative smooth and regular motion of the Earth around the Sun, compact long-term ephemeris data for the Sun can be compiled. This section describes how such tables can be constructed and also how they are used. These compact Sun Ephemeris may be used as a backup alternative if a regular Nautical Almanac is not available. The accuracy of the obtained GHA and Declination data is better than 2'. The effort to obtain the ephemeris data is higher compared to using a regular Almanac in which the data is recorded at integral hour intervals with an accuracy of 0.1' for both GHA and Declination.
Time is experienced in the first place by the rotation of the Earth
resulting in the day-night succession and in the second place by the
orbiting of the Earth around the Sun manifesting itself primary in
the succession of the different seasons over the year but also in the
changing star constellations at night.
Unfortunately there is no direct relationship between the two time
periods of one day and one year.
For the Earth, it takes about 365.25 rotations to arrive at the same
point on it's orbit around the Sun.
Construction of the Tables
The main table,
from which the primary ephemeris information is obtained,
is based on the values of the Equation-of-Time and Declination of the Sun
recorded for the beginning of each day (00:00 hour) over one year.
This main table is constructed for a reference year, which may be chosen
at about half of the required time range.
The data for the other years is obtained by entering the main table
with a slightly adjusted time called Orbit Time.
This time adjustment or hour offset (in integral hours) is obtained
from table (a) at the entry for the year of interest.
The other tables (b), (c) and (d) are used to interpolate the data obtained from the main table.
Explanation of the Tables
The time correction of table (a) is the time difference compared to the GMT of the reference year, needed to obtain the same E and Dec data as recorded for the reference year. With this correction the Orbit Time is obtained, which is the adjusted GMT time required to enter the fundamental table.
The axis of the Earth is undergoing a precessional
motion similar to that of a top spinning with its axis tilted.
In about 25800 years the axis completes a cycle and returns
to the position from which it started. Since the celestial
equator is 90° from the celestial poles, it too is moving. The
result is a slow westward movement of the equinoxes and
solstices, which has already carried them about 30°, or one
constellation, along the ecliptic from the positions they
occupied when named more than 2000 years ago. Since
sidereal hour angle is measured from the vernal equinox,
and declination from the celestial equator, the coordinates
of celestial bodies would be changing even if the bodies
themselves were stationary. This westward motion of the
equinoxes along the ecliptic is called precession of the
equinoxes. The total amount, called general precession, is
about 50 seconds of arc per year. It may be considered
divided into two components: precession in right ascension
(about 46.10 seconds per year) measured along the celestial
equator, and precession in declination (about 20.04" per
year) measured perpendicular to the celestial equator. The
annual change in the coordinates of any given star, due to
precession alone, depends upon its position on the celestial
sphere, since these coordinates are measured relative to the
polar axis while the precessional motion is relative to the
|Cover << Sail Away << Celestial Navigation << .||last updated: 27-Apr-2017|