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Motion of the Moon
Review Question
Describe the path the Sun will take across
the sky on the day of the autumnal equinox
as viewed from the Earth’s equator.
Review Question
Describe the path the Sun will take across
the sky on the day of the autumnal equinox
as viewed from the Earth’s north pole.
Review Question
Describe the path the Sun will take across
the sky from a latitude of 40 degrees north
on the day of the autumnal equinox.
Review Question
Describe the path the Sun will take across
the sky from a latitude of 40 degrees north
on the day of the summer solstice.
Review Question
Describe the path the Sun will take across
the sky from a latitude of 40 degrees north
on the day of the winter solstice.
Sky as seen
from 40
degrees north
latitude
Review Question
What is the latitude of the tropic of Cancer?
Review Question
What is the latitude of the artic circle?
Review Question
What is the highest the Sun can get above
the southern horizon at a latitude of 30
degrees north?
Review Question
Why is there a difference between the
tropical year and the sidereal year?
Review Question
What causes the Earth to precess?
The Moon
As we pass through the month, the appearance
of the Moon changes each night. We say the
Moon goes through a series of phases.
Discussion
Many people believe incorrectly that the
phases of the Moon are caused by the
shadow of the Earth on the Moon. What
simple observation of the Moon could you
make to prove that they are wrong?
First quarter Moon
The Earth is round
The “shadow” on a first or last quarter Moon
is a straight line dividing the face of the
Moon in half. The Earth is round and will
always cause a curved shadow on the Moon.
When the shadow of the Earth does fall on
the Moon we call this a lunar eclipse.
The real reason
The Moon’s phases are actually cause by
the changing position of the Moon with
respect to the Sun. The side of the Moon
facing the Sun is always fully illuminated
(except during a lunar eclipse). But because
the angle we view the Moon differs from
that of the Sun, we see varying degrees of
the Moon’s surface facing us illuminated.
New Moon
Occurs when the Moon is in the same part of
the sky as the Sun. Thus the Moon is up
during the day, and the illuminated side of
the Moon faces away from the Earth. The
Moon is not visible when it is new.
Waxing crescent
When less than ½ of the Moon’s face is
illuminated. It is the western side of the
Moon’s face that is illuminated and the Moon
is visible in the sky before Sunset. Waxing
means increasing.
First Quarter
When exactly ½ of the Moon’s face is
illuminated. It is the western side of the
Moon’s face that is illuminated and the Moon
is up before sunset. Although ½ of the Moon’s
face is illuminated it is called a quarter
because the Moon is ¼ of the way through it’s
cycle and occurs about one week after the
new phase. The Moon is 90 degrees away
from the Sun in the sky.
Waxing Gibbous
When more than ½ of the Moon’s western
face is illuminated.
Full Moon
when the Moon’s face is fully illuminated. At
this time the Moon is directly opposite the Sun.
Occurs about 2 weeks after the new phase.
Waning Gibbous
when more than ½ of the Moon’s face is
illuminated. But now it is the eastern face that
is illuminated and waning means decreasing.
Last Quarter
when ½ of the Moon’s face is illuminated. Again
it is the eastern half that is illuminated and the
Moon is visible after sunrise. Occurs about 3
weeks after the new phase.
Waning Crescent
When less than ½ of the eastern half of the
Moon’s face is illuminated.
Discussion
If the Moon is in its full phase on March 21st
and the Sun sets at 6:00 P.M. what time does
the Moon rise? What time does the Moon
reach the meridian? What time does the
Moon set?
The full Moon is exactly opposite the Sun or
180 degrees away in the sky. Astronomers
would measure this angle in hours and say it is
12 hours from the Sun. If sunset is at 6:00 P.M.
the full Moon rises at 6:00 P.M., crosses the
meridian at midnight and sets at sunrise at
6:00 A.M., 12 hours different from the Sun.
Discussion
If the Moon is in its last quarter phase on
March 21st and the Sun sets at 6:00 P.M.
what time does the Moon rise? What time
does the Moon reach the meridian? What
time does the Moon set?
The last quarter Moon is 90 degrees to the
west of the Sun in the sky, or 6 hours before
the Sun. If sunset is at 6:00 P.M. and sunrise is
at 6:00 A.M. (we are at the equinox) the last
quarter Moon rises at midnight, crosses the
meridian at 6:00 A.M and sets at noon.
Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the
Moon if the Moon is located at the position
of the vernal equinox?
Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the
Moon if the Moon is located at the position
of the vernal equinox?
If the Moon is at the position of the vernal
equinox it must be 180 degrees from the Sun
and therefore full.
Discussion
Suppose it is the first
day of autumn in the
northern hemisphere.
What phase is the
Moon if the Moon is
located at the position
of the winter solstice?
Explain.
Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the
Moon if the Moon is located at the position
of the summer solstice? Explain.
Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the
Moon if the Moon is located at the position
of the summer solstice? Explain.
The Moon is 90 degrees behind the Sun.
Because the Sun is moving eastward relative
to the stars, the Moon is 90 degrees to the
west of the Sun and therefore in 3rd or last
quarter phase.
What time is it in this drawing?
What time is it in this drawing?
Discussion
The Moon goes through its phases in 29.53
days. This is called the synodic month, i.e.
the month relative to the Sun. The actual
orbital period of the Moon is 27.32 days, the
sidereal month. Why is there a difference in
the synodic and sidereal months?
Synodic and Sidereal Month’s
Discussion
The Moon always keeps the same side facing
the Earth. Does this mean the Moon does not
spin on its axis? Explain?
Synchronous rotation
Orbital period is the same as the rotational
period
Discussion
What is the mean solar day on the Moon?
What is the sidereal day on the Moon?
Mean solar day on the Moon
29.5 Earth days, the Sun rises in the east sets in
the west 14.75 days later, followed by 14.75
days of darkness.
The Moon’s sidereal day is its actual rotation
(and revolution) period of 27.32 days.
Eclipses
If the Sun, Earth and the Moon line up so that
the Moon’s shadow falls on the Earth or the
Earth’s shadow falls on the Moon, we see an
eclipse.
Solar eclipse
Earth passes through the Moon’s shadow.
Lunar eclipse
Moon passes through Earth’s shadow.
Discussion
What phase must the Moon be in for a solar
eclipse to occur?
What phase must the Moon be in for a lunar
eclipse to occur?
The Moon’s orbit
The Moon’s orbit is tilted 5 degrees relative to
the ecliptic. Thus the Moon spends ½ of its
orbit above the ecliptic and ½ below.
Because the ecliptic is the path of the Sun
with respect to the stars, eclipses can only
occur when the Moon lies in the ecliptic.
Hence, the name ecliptic.
Discussion
From a latitude of 40 degrees, how high above
the south horizon can the Moon get in the
sky?
Discussion
What time of the year can the full Moon be
its highest above the horizon? Explain.
Discussion
The Moon crosses the ecliptic twice each
orbit or twice a month. Why don’t we get an
eclipse twice a month?
Types of Solar eclipses
Total – the Moon completely covers the
surface of the Sun.
Partial – the Moon covers only part of the Sun,
i.e. the Moon and Sun are not perfectly
aligned.
The parts of a shadow
Penumbra – the outer, lighter part of the
shadow, corresponds to the Sun’s surface
being only partially covered
Umbra – the central, darkest part of the
shadow, corresponds to the entire disk of the
Sun being covered
Types of Solar eclipses
Annular – the Moon and Sun are perfectly
aligned but the Moon is furthest away from
Earth in its orbit and appears too small to
completely cover the Sun, leaving the rim of the
Sun exposed.
Lunar Eclipse
Discussion
During a total lunar eclipse the Moon does not
get completely dark. Instead it glows an
orangish-red color. Why?
Total Lunar eclipse July 1982
Why do you think the bottom of
the Moon is lighter than the top?
Total Lunar eclipse of July 16, 2000
The motions of the Planets
Each planet follows the same diurnal
motion as the Sun, Moon and stars, rising
in the east and setting in the west each
day.
Like the Sun and Moon, each planet moves
west to east with respect to the stars. This
is called direct motion. In addition, all the
planets stay close to the ecliptic.
Path of Mars
Discussion
Suppose Mars is moving in retrograde
motion and will rise at midnight.
Since Mars is moving with retrograde
motion, that means that during the night it
will be moving west-to-east rather than
east-to-west. So at midnight it will rise in
the west and move across the sky and then
set in the east
Do you agree or disagree?
Summary
Motion of stars, Sun, Moon, and planets
Definition of day, year month
Causes of seasons, moon’s phases
Solar and lunar eclipses