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)