Diurnal Motion E to W at about 360°/day
Length of day using Meridian Crossing
Annual Motion of Sun w.r.t. stars is
about 1°/day W to E
The Moon moves about 13°/day w.r.t. the stars, on a path
near, but not on, the ecliptic. The inclination of the moons
orbit to the ecliptic varies somewhat from year to year, but is
approximately 5°. The intersection of the Moon's orbital
plane with the ecliptic defines the Line of Nodes. The Ascending
Node is that which the moon passes through when going through the
ecliptic from south to north, while at the descending node it
goes through the ecliptic from north to south.
As the Moon travels around the earth, we see various degrees
of its surface illuminated, what we call phases.
Due to the fact the Earth moves around the Sun as the Moon
orbits the Earth, the length of time it takes for the moon to
pass through all of its phases is slightly longer that its true
orbital period in space. We therefore distinguish between the
Sidereal (true orbit) Period - 27.3 days, and the Synodic Period
- 29.5 days. One way of thinking of this is that it takes 27.3
days for the moon to line up with the same star in the sky, but
in the meantime, the sun has moved a little bit, so, for example
at New Moon, it takes longer for the Moon to catch up to the
Looking at how this is seen deom the vantage point
of the earth:
Note: The Vernal Equinox is chosen as the zero point for RA.
Precession changes the RA, DEC of an object with time.
Seasons are NOT due to variations in the earth-Sun distance,
but due to the obliquity of the ecliptic.
When the Moon passes between the Earth and the Sun, we may see
a Solar Eclipse. When the Moon passes into the shadow cast by the
Earth, we get a Lunar Eclipse. If an observer is located inside
the Umbra, where the light from the Sun is totally blocked out,
they would see a Total Solar Eclipse. If some sunlight is
visible, the observer is in the Penumbra, and it is a Partial
Solar Eclipse. Due to the varying distance of the Moon from the
Earth, sometimes its shadow does not reach the surface of the
earth, and and a narrow ring or annulus of the Sun is visible -
an Annular Eclipse. Similarly, the Moon can be totally within the
Earth's Umbra (total lunar eclipse) or not (partial lunar
eclipse). Notice that when a total lunar eclipse occurs, it is
visible from half the Earth, while only a small part of the earth
can witness a total solar eclipse.
Furthermore, because of the tilt of the Moon's orbit
w.r.t. the ecliptic, an eclipse does not occur at every new or
full phase. It can only happen when the Moon is near the ecliptic
(near one of the nodes).
1. Planets usually move W to E with respect to stars -
2. Sometimes move E to W w.r.t. stars - Retrograde
3. Always near the ecliptic (but not always on it).
4. All reach Conjunction with the Sun
5. Not all reach Opposition Mercury - 23° maximum elongation from Sun
Venus - 46° maximum elongation from Sun
6. Mars, Jupiter, Saturn are brightest at Opposition