Transcript Rotation - Cloudfront.net

The Earth-Moon-Sun
System
Chapter 22, Section 2
Motions of the Earth
The two main motions of Earth are rotation
and revolution
 Rotation – the turning, or spinning, of a
body on its axis
 Revolution – the motion of a body, such
as a planet or moon, along a path around
some point in space
 Precession – the slight movement, over a
period of 26,000 years, of Earth’s axis
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Rotation
The main results of the earth’s rotation are
night and day
 There are two kinds of days!
 Mean Solar day – time interval from one
noon to the next (~ 24 hours)
 Sidereal day – the time it takes for the
Earth to make one complete rotation with
respect to a star other than our sun (23
hours, 26 minutes, and 4 seconds)
 We use the mean solar day, and most
astronomers use the sidereal day
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Solar Day vs. Sidereal Day
Revolution
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Earth revolves around the sun in an elliptical
orbit at an average speed of 107,000 km per
hour
At perihelion, Earth is closest to the sun (~147
million km away), this occurs ~ January 3rd each
year
At aphelion, Earth is farthest from the sun (~152
million km away), this occurs ~ July 4th each
year
The apparent annual path of the sun against the
celestial sphere (stars) is called the ecliptic
The paths of the planets travel near the ecliptic
Earth’s Orbital Motion
Earth’s Axis and Seasons
From the reference angle of the ecliptic,
the Earth’s axis of rotation is tilted 23.5
degrees
 Because of this tilt, we have our yearly
seasons
 The equinoxes mark when the sun has
reached the intersection of the celestial
equator, and the solstices are when the
sun has reached 23.5 degrees with
respect to the celestial equator
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Earth’s Axis and Seasons
Concept Check
What phenomena result from Earth’s
rotation and revolution?
 Rotation: night and day
 Revolution: seasons
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Precession
The Earth’s axis maintains around the
same angle, but the direction it points is
always changing
 The result is our axis “drawing” a circle in
the sky
 Think of a spinning top!
 It has a period of 26,000 years, meaning in
the year 14,000 the axis won’t be pointing
at the North Star (Polaris), but instead at
Vega (which then becomes our North Star)
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Precession
Concept Check
In the year 14,000, what star will be the
new “North Star”?
 Vega
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Earth-Sun Motion
In addition to its own movements, the
Earth accompanies the sun as it speeds
toward the star Vega, at ~ 20km/s
 The sun, like other stars around it, revolve
around the galaxy, this trip takes 230
million years at speeds approaching 250
kilometers per second
 Earth is presently approaching the Great
Galaxy in Andromeda
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Earth-Sun Motion
Concept Check
In what ways does Earth move?
 Earth revolves around the sun, rotates on
its axis, and moves slightly on its axis. It
also revolves around the Milky Way.
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Motions of the Earth-Moon System
Earth has one natural satellite, the moon
 Our moon orbits Earth within a period of
about one month
 Because the moon’s orbit is elliptical, its
distance to Earth varies averaging at
384,401 kilometers
 Perigee – the moon’s closest approach to
Earth
 Apogee – the moon’s farthest approach
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Earth-Moon System
Phases of the Moon
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Phases of the Moon – a change in the amount
of the moon that appears lit, happens on a
monthly basis
Lunar phases are a result of the motion of the
moon and the sunlight that is reflected from its
surface
The percentage that is visible depends on the
location of the moon with respect to Earth and
the sun
When the moon lies between the sun and Earth,
none of its bright side faces Earth (New Moon)
When Earth is between the moon and the sun,
all its lighted side face Earth (Full Moon)
Phases of the Moon
Lunar Motions
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Synodic Month – the cycle of the moon through
its phases (~ 29 ½ days)
Sidereal Month – the true period of the moon
(amount of time it takes to revolve around Earth)
is ~ 27 1/3 days
The moon’s period of rotation about its axis is
the same as its revolution around Earth,
resulting in us only ever seeing one side of the
moon
Only satellites and astronauts have ever seen
the “far” side of the moon
Lunar Motions
Concept Check
Why does the same side of the moon
always face Earth?
 The moon’s period of rotation about its
axis and its revolution around Earth are
the same.
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Eclipses
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Solar Eclipse – when the moon moves in a line
directly between Earth and the sun it casts a dark
shadow on Earth (occurs during new moon phases)
Lunar Eclipse – the moon is eclipsed when it
moves into Earth’s shadow (occurs during full moon
phases)
The moon’s orbit is inclined about 5 degrees to the
plane that contains Earth and the sun, so its shadow
will often miss Earth
During a new-moon or full-moon phase, the moon’s
orbit must cross the plane of the ecliptic for an
eclipse to take place
This only occurs four times a year (in pairs, a solar
and a lunar) at the equinoxes
Solar Eclipse
Lunar Eclipse
Concept Check
Why don’t eclipses happen during every
full-moon or new-moon phase?
 The moon’s orbit must cross the plane of
the ecliptic for an eclipse to happen.
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Assignment
Read Chapter 22, Section 2 (pg. 622-629)
 Do Chapter 22 Assessment #1-34 (pg. 639640)
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