Transcript Scales of the Universe - University of Massachusetts Amherst

Our Moon
Goals of this Lecture
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Understand the Lunar Phases
Introduce tides and tidal forces
Understand Lunar Eclipses
Understand Solar Eclipses
Assigned Reading
and Announcements
Chapter 3, all of it
Homework #1 will be posted today
Due in class on Thursday 18
Lunar Phases
Earth & Moon
Like planets, the moon does not
emit its own right.
The Moon is ~1/4 of the Earth in
diameter and
~1 percent in mass.
The rotation of the Moon and its
revolution around the Earth are
synchronized and take 27
days,7 hours, and 43 minutes.
The length of the Lunar orbit
around Earth (Sidereal Period)
is 27.32 days.
The length of the Lunar Phase
(Synodic Period) is 29.53 days.
We only see one side of the moon.
The Phases of the Moon (1)
• The Moon orbits Earth in a sidereal period of
27.32 days.
Earth
Moon
27.32
days
Fixed direction in space
The Phases of the Moon (2)
Fixed direction in
space
Moon
29.53
days Earth
Earth orbits around Sun =>
Direction toward Sun changes!
• The Moon’s
synodic period (to
reach the same
position relative to
the sun, i.e., the
same lunar phase)
is 29.53 days (~ 1
One rotation
in 24 hours
One orbit in 27.32 days
Earth Rising on the Moon
Apollo 11
Survey Question
In this picture of the Earth rising, what is the
predominant direction of the sun in relation to
the observer?
1) above the observer
2) below the observer
3) behind the observer
4) behind the Earth
How long does it take for the
moon to rotate once?
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One day
About 27 days
Half a year
One year
How long does it take for the
moon to rotate once?
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One day
About 27 days
Half a year
One year
Lunar and Solar Eclipses
Lunar, from “Luna”
Solar, from “Sol”
Lunar Eclipse
Whose shadow is this?
Why is the shadow curved?
Reminder: angular (apparent)
diameter
• Angular diam / 206,265” = linear diam./distance
• NOTE: linear diam. and distance MUST be in the
same units
• It so happens that both the Moon and the Sun have
angular diameter of about 0.5 degree, the exact
value at a given time depending on the exact
distance.
Shadows
A Total Lunar Eclipse (1)
Motion of the moon against
the background of stars
Motion of the moon
against the horizon
A Total Lunar Eclipse (2)
A total lunar
eclipse can
last up to 1
hour and 40
min.
During a total
eclipse, the
moon has a
faint, red
glow,
reflecting sun
light scattered
in Earth’s
atmosphere.
Partial lunar eclipse
During a lunar eclipse, what
would you see from the moon?
Geometry of Solar Eclipses
Umbra (shadow): sun is completely blocked out.
Penumbra (almost shadow): sun is partially blocked out.
Earth and Moon’s Orbits
Are Slightly Elliptical
Apogee =
position furthest
away from Earth
Perihelion = position
closest to the sun
Earth
Perigee = position
closest to Earth
Moon
Sun
(Eccentricities greatly exaggerated!)
Aphelion
= position
furthest
away
from the
sun
Annular Solar Eclipses
The angular sizes of the moon and
the sun vary, depending on their
distance from Earth.
Perig Apog
ee
ee
Perihe Aphel
lion
ion
When Earth is
near perihelion
and the moon is
near apogee,
we see an
annular solar
eclipse.
Annular eclipse
Looking Back on an Eclipsed Earth; 1999 August 11
Credit: Mir Space Station Crew
Total Eclipse of June 21st 2001
The umbra is never
more than 270 km in
diameter and sweeps
across Earth's surface
at 1600 km/hr (1000
mph)!
Survey Question
The phase of the moon at a solar eclipse is?
a) new
b) 1st quarter
c) full
d) it depends on the year
How come there isn't an eclipse
every full moon and new moon?
• The main reason is that the moon’s orbit is
tilted from the earth’s orbit.
• A total eclipse from a given point on the
surface of the Earth is not a common
occurrence. For example, it will be two
decades before the next total solar eclipse
visible in North America occurs.
Effects of Moon on Earth:
Tides
To sun
Spring Tides
Occur at every new and full moon
Neap tides
Occur at every first- and third-quarter moon
Friction drags the tidal bulges eastward
out of the direct earth-moon line
Earth's rotation slows down by 0.0023 s/100 years.
Only 900 million years ago, Earth' day was 18 hrs long.
The moon's orbit is growing larger by about 4 cm/yr.
We see only one side of the moon, because the tidal friction
has slowed the rotation of the moon so that its rotation and
revolution are synchronized
The Tidally-Locked Orbit of the Moon
The Earth exerts tidal
forces on the moon’s
rocky interior.
 It is rotating with
the same period
around its axis as it is
orbiting Earth (tidally
locked).
 We always see the
same side of the moon
facing Earth.
Solar day and Sidereal day
• The apparent solar day = the time from
high noon to high noon.
• The average length of the apparent solar
day = 24 hours
• Sidereal day = the time from a star to the
same position.
• Because the sun moves in respect to stars,
the sidereal day is different from the solar
day.
The Length of the Solar Day
Earth revolves about 360o/365 ~ 1o around Sun
Earth rotates at a rate of 360o/(24x60)=0.25o/minute
So need an extra 1o/0.25o =4 minutes to complete a solar
day