Download Lecture 3 Review

Lecture 3 Review
1. Our solar system formed from collapsing gas. Residual angular
momentum causes the Sun, the planets, and the moons to all rotate in the
same prograde motion. Looking down from a point in our northern sky all
objects appear to rotate counterclockwise in the plane of the ecliptic.
If you are attached to the fixed stars, looking down you see the following
sidereal time (with respect to the fixed stars) periods:
Sun
Earth
Moon
A)
25.4 days
365.2564 days
27.3 days
Moon - because the Earth is moving around the Sun, the full-moon
to full-moon month is longer than the sidereal month, i.e.
29.53 days = synodic month = lunar month
B)
Earth - the Earth is tilted 23½o with respect to the ecliptic. This axis
precesses once every 28,000 years. This causes the seasons to
advance and the seasonal or tropical year to change to:
365.2422 days = synodic year = tropical year = equinox year
2. The planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune, Pluto
Mercury, Venus, Earth, and Mars are called the terrestrial planets. They
are rocky planets with similar elemental content. They have iron - nickel
cores
Jupiter, Saturn, Uranus, and Neptune are called gas giants or Jovian
planets. They are large, gaseous, and similar in elemental content.
Pluto is an anomaly.
Planets on the sunward side of the Sun from Earth are said to be in
conjunction.
Planets on the night side of the Sun from the Earth are said to be in
opposition.
Planets revolve more slowly about the Sun the further away from the Sun;
thus, the Earth catches up to an outer (superior) planet as it rotates the Sun.
At opposition an outer planet appears to exhibit retrograde motion. This is
illustrated:
3. Some definitions
our latitude = 42½o = Lo
88 constellations, 12 in the ecliptic called the Zodiac
4. The seasons result from the tilt of 23½o of the Earth’s rotational axis
from Earth’s orbital plane about the Sun = the ecliptic.
Vernal equinox ½ day, ½ night, spring
Autumnal equinox ½ day, ½ night, fall
Summer solstice - longest day of the year
Winter solstice - shortest day of the year
5. The ecliptic is the path of the
Sun through the sky from day to
day. Since the planets, including
the Earth, formed roughly in a
plane, the planets appear to follow
this same path.
The stars rotate about the North
Celestial Pole which today is
Polaris in the Little Dipper. The
pole is at an angle of your latitude
Lo above your horizon looking
towards the North. Stars in a conic
section of angle Lo are always
visible and describe circles about Polaris.
The celestial equator is a great circle about the polar axes. Your zenith is
the point directly you. Stars take an easterly to westerly path through the
sky in a single night. This is called diurnal motion.
The Earth’s 23½o tilt of its rotational axis to the plane of rotation about
the Sun (ecliptic) is the cause of the seasons. Thus, the Sun shines more
directly on the Northern Hemisphere during the summer and more directly
on the Southern Hemisphere during the winter.
The Path of the North Star Celestial Pole - As the Earth precesses, the
north celestial pole slowly traces out a circle among the northern
constellations. At present, the north celestial pole is near the moderately
bright star Polaris, which serves as the “pole star.” Twelve thousand years
from now the bright star Vega will be the pole star.