Motion of the
Sun
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Sidereal Motion
The word sidereal derives from the Latin word for
“star”. This is because sidereal motion is motion
with respect to the stars. One sidereal day is
the time it takes for a star in the sky to come
back to the same place in the sky. Because, for
all intents and purposes, the sky is “fixed”, a
sidereal day is when Earth rotates 360°. A
sidereal day is 23 hours 56 minutes and 4.09
seconds long. 2
A Sidereal year
the time it takes for the sun to return to the same position
with respect to the stars. Due to the precession of the
equinoxes the sidereal year is about 20 minutes longer
than the tropical year. The tropical year is the interval at
which seasons repeat and is the basis for the calendar
year.
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Synodic Motion
The word synodic derives from the
Greek word for meeting or assembly.
It is motion relative to a conjunction
or alignment of sorts. A synodic or
solar day is the time it takes the sun
to successively pass the meridian
(astronomical noon). A mean solar
day is 24 hours (the “mean” is there
to average over the effect of the
analemma). Earth has to rotate more
A synodic year is the time it takes for a planet-
sun alignment to reoccur. For the case of the sun,
it is the time it takes the sun to come to the same
place on the ecliptic (equinox to equinox) and is
called a Tropical Year. A tropical year is 365.242
mean solar days (366.242 sidereal days). It is just
over 20 minutes shorter than a sidereal year
(again, the effect of precession).
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Ecliptic
The ecliptic is the plane in which Earth orbits the
sun. The Zodiac is the twelve constellations that
contain the ecliptic. The planets also orbit very
close to the ecliptic. Thus the zodiacal
constellations also are where one observes the
planets in the sky. The twelve constellations of the
Zodiac are:
Sun's Motion Along the
Zodiac
Because Earth takes about 4 minutes longer to
repoint towards the sun (the solar day is longer
than the sidereal day), the location of the sun with
respect to the Zodiac changes. The sky rotates
from east to west. Because the sun lags behind
the stars/Zodiac, the sun lags easterly every day
by 4 minutes which is just under 1° per day. In 365
days, the sun will have completed one full circuit
around the Zodiac.
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Circumpolar & never-Rise
Stars
Stars near the celestial poles make small
circles and may not pass the horizon plane. If
they are always above the horizon they are
called circumpolar stars. If they are always
below the horizon they are never rise stars.
Circumpolar stars for the northern hemisphere
are never-rise stars for the southern
hemisphere and vice versa.
Bands in the Sky
Object location in the sky
The ecliptic is the plane of Earth's orbit about
the sun. Because Earth is tilted (its obliquity)
about 23.5° to this orbital plane, the sun's
declination is always between +23.5° on the
summer solstice and -23.5° on the winter
solstice. Thus one can picture a band 47° wide
centered on the celestial equator. The sun is
Meridian
The meridian is the north-south line of the
observer in the horizon coordinate system. It is
the “highest” point an object will be in the sky. As
such, it is a convenient reference point for such
things as the passage of time.
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Meridional Altitude
1. Draw and label the poles.
Label the altitude arc of the
pole above the horizon.
The north celestial pole points directly
north and the south celestial pole
points directly south. The altitude of
the poles is equal to the latitude of the
observer. For northern hemisphere
observers, the north celestial pole will
be above the horizon and the south
celestial below the horizon and vice
versa for southern hemisphere 13
Meridional Altitude
2. Draw and label the celestial
equator. Label its altitude arc
above the horizon.
The celestial equator is 90° from
either pole. The altitude of the
celestial equator will simply by 90° −
the altitude of the pole above the
horizon.
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Meridional Altitude
3. Draw the object whose
meridional altitude is being
measured. Label its
declination arc.
Declination is measured from the
celestial equator. Positive declination
is always towards the north celestial
pole and negative declination is
always towards the south celestial
pole irrespective of the observer's
latitude.
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Meridional Altitude
4. Calculate meridional
altitude: altitude of celestial
equator + declination of star.
If the meridional altitude is above 90°
one must subtract the number from
180° as altitude is only defined up to
90°. That is an altitude of 100° is
really 80°.
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