Celestial Night during the Year
A: the Earth takes 1 year to complete 1 solar revolution, and,
B: by definition, the night sky is only viewable on the spinnning
side of the earth progressively opposite from the sun,...so,
Is it then accurate to declare, progressively from east to west,
that each point along the ecliptic of the night sky's celestial
sphere, if viewed at the same hour each night, will achieve zenith
visible night sky orientation, at some point along the earth's
annual solar transit?
If the Earth's orbit were perfectly circular (or if the orbit were a
non-rotating ellipse), and if the Earth axis were not wobbling so that its
tilt direction was constant (or if the wobbling was in perfect coincidence
to the Earth's revolution around the sun), then your conclusions would be
However, the anomalistic year (the time it takes to revolve around the sun,
perihelion to perihelion say) does not coincide with the tropical year (the
time for the Earth to complete a revolution such that its axis is appearing
to point in the same zodiac, solstice to solstice say). The lag is small,
but it does mean that the constellations are not in the same exact location
in the night sky at a particular time of night in a particular time of year.
On average the anomalistic year is 25min longer than a tropical year, so
over 58 years there is a regression of 1 day.
Even if we were to focus entirely on the anomalistic year as our sole
time-measure, the elliptical orbit of the Earth, and the fact that the orbit
itself revolves around the sun (with the sun at one of the focus and the
center of the orbital revolution) should cause variation in the night sky at
each point in the orbit. I don't have data on the orbital period of the
Earth's ellipse, so I can't tell you how strong this effect is.
Greg (Roberto Gregorius)
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Update: June 2012