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Celestial
Motions:
Two major kinds motion
in the sky are evident:
-
Diurnal
(Daily) Motion: The Sun, Moon, and Stars move overhead from east
to west. This motion is due to the rotation
of the Earth on its axis.
-
Orbital
(Annual) Motion: The Sky changes while looking at the same
location every night. It repeats itself (more or less) once each year.
This motion is due to the orbit of the Earth
around the Sun.
There are also some other
types of motion you might notice:
-
Lunar
(Orbital) Motion: The Moon Cycles through our piece of the
sky once every 27 days. This is due to the
orbit of the Moon around the Earth.
Planetary
(Ecliptic) Motion: This motion is more complex since the planets
don't follow an obvious consistent period (which is because Each planet
orbits the Sun at a different distance!). They do all appear to move
fairly
close to the plane of the Ecliptic (as do both the Sun and
Moon).
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