The Earth-Moon-Sun The Earth-Moon-Sun SystemSystem
Chapter 22, Section 2Chapter 22, Section 2
Motions of the EarthMotions 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
RotationRotationThe main results of the earth’s rotation are
night and dayThere 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
Solar Day vs. Sidereal DaySolar Day vs. Sidereal Day
RevolutionRevolution 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 MotionEarth’s Orbital Motion
Earth’s Axis and SeasonsEarth’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
Earth’s Axis and SeasonsEarth’s Axis and Seasons
Concept CheckConcept Check
What phenomena result from Earth’s rotation and revolution?
Rotation: night and dayRevolution: seasons
PrecessionPrecessionThe 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)
PrecessionPrecession
Concept CheckConcept Check
In the year 14,000, what star will be the new “North Star”?
Vega
Earth-Sun MotionEarth-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
Earth-Sun MotionEarth-Sun Motion
Concept CheckConcept 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.
Motions of the Earth-Moon SystemMotions of the Earth-Moon System
Earth has one natural satellite, the moonOur moon orbits Earth within a period of
about one monthBecause 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
Earth-Moon SystemEarth-Moon System
Phases of the MoonPhases of the Moon 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 MoonPhases of the Moon
Lunar MotionsLunar Motions
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 MotionsLunar Motions
Concept CheckConcept 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.
EclipsesEclipses 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 EclipseSolar Eclipse
Lunar EclipseLunar Eclipse
Concept CheckConcept 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.
AssignmentAssignment
Read Chapter 22, Section 2 (pg. 622-629)Read Chapter 22, Section 2 (pg. 622-629)Do Chapter 22 Assessment #1-34 (pg. 639-Do Chapter 22 Assessment #1-34 (pg. 639-
640)640)
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