PLANETRY MOTION

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Planetary Motions

Sun's Motion

Diurnal Motion E to W at about 360°/day

Length of day using Meridian Crossing

Solar Day

Sidereal day

Annual Motion of Sun w.r.t. stars is

about 1°/day W to E

Ecliptic

Equinoxes

Solstices

 

MOON'S MOTION

 

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.

LUNAR PHASES

As the Moon travels around the earth, we see various degrees of its

surface illuminated, what we call phases.

LUNAR MONTHS

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 Sun.

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

Seasons are NOT due to variations in the earth-Sun distance, but due

to the obliquity of the ecliptic.

 

ECLIPSES

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).

 

PLANETARY MOTION

Observations:

 

1. Planets usually move W to E with respect to stars - Direct Motion

2. Sometimes move E to W w.r.t. stars - Retrograde Motion

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