Seasons Review
• Earth’s rotation axis is tilted 23.5 degrees; tilt
remains the same as it orbits the sun
• Therefore, different parts of the Earth receive
different amounts of daily sunlight throughout the
year, as Earth orbits the Sun.
• If Sun is higher in the sky throughout the day,
incident sunlight is more concentrated, heating
more efficient  summer
• If Sun is lower in the sky throughout the day,
incident sunlight is more dilute, heating less
efficient  winter
1
Seasons Review
• Equinoxes: Sun is on equator (ecliptic and celestial
equator intersect); rises due East, sets due west
– March: moving north
– September: moving south
– Equal daylight / night time
• Solstices: Extreme North and South positions for Sun
– June: farthest North (Tropic of Cancer, latitude +23.5), longest day
– December: farthest South (Tropic of Capricorn, latitude -23.5),
shortest day
2
Seasons Review
• Antarctic circle
(lat. = -(90-23.5)
= -66.5 receives
no sunlight on
June Solstice
• Arctic circle
(lat. = +(90-23.5)
= +66.5 receive
no sunlight on
December
Solstice
3
Phases of the Moon
• The Phases of the Moon are the
variations in the Moon’s
appearance as the Moon orbits
the Earth.
• They are due to the changing
Sun-Earth-Moon angle through
each month.
• The following is the cycle of
lunar phases: new, waxing
crescent, first quarter, gibbous,
full, gibbous, third quarter,
waning crescent, new
4
Moon Phases
• The Moon orbits
the Earth in about
one month (29.5
days)
• Over one orbit the
appearance and
rise and set times
change with the
cycle of Lunar
Phases.
• The figure is not
to scale
5
Light & Dark
How the Earth and Moon would
look high above Earth’s North Pole.
The figure is not to scale.
• We only see the Moon because
sunlight reflects off its surface.
• At any one time, half the Moon
is always lit and half is in
darkness.
• The amount of the illuminated
half we see from Earth depends
on the position of the Moon in
its orbit
• The time of day the Moon is
visible also depends on its
phase.
6
Example: Observing the Moon
• Between September 10-11, at
dusk, the Moon will be
visible in its waxing crescent
phase.
• The first night it appeared
close to the Sun. Subsequent
nights it will appear higher in
the sky (farther from the Sun)
and the lit portion appeared to
be growing larger.
• What do you think you would
see the next night? How
about a week later?
7
The Moon Rises Later Each Day
8
Lunar Phase Terms
• The cycle of Lunar Phases
starts at the New Moon.
• At New Moon the Moon is
not visible to us on Earth.
• From New Moon to Full
Moon the illuminated
fraction of the Moon we
see from Earth grows. It is
said to be waxing.
• After Full Moon the
fraction
of
the
illuminated
We can only see the half of the
Moon above and to the right of the Moon visible from Earth
diagonal line drawn on the Moon shrinks. It is said to be
waning.
in the figure above.
9
The Cycle of Lunar Phases
• The Lunar cycle is
split into quarters
–
–
–
–
New Moon
1st Quarter
Full Moon
3rd (or Last) Quarter
• Since the whole cycle
lasts about a month
each quarter lasts
about a week.
See http://kalender-365.de/lunar-calendar.php for more months. 10
As you view
the moon from
different
directions
(relative to the
Sun) its
appearance
changes.
11
crosses meridian
at sundown
crosses meridian
at about 9:00 pm
crosses meridian
at about 3:00 pm
sunlight
crosses
meridian
at midnight
crosses
meridian
at noon
sunlight
crosses meridian
at about 9:00 am
crosses
meridian
at about 3:00 am
crosses meridian
at dawn
12
Phases of the Moon
What Can You See
http://kids.niehs.nih.gov/lunar/home.htm
13
Phases of the Moon
When Can You See It
14
The Lunar Phases Web Tools
• Lunar Phases -- University of NebraskaLincoln
• Lunar Phases Interactive
15
Lunar Libration
16
Near and Far Sides of the Moon
Robert Gendler, 1999
Apollo 16, 1972
17