Chapter

2929Our Solar System

Ancient Greeks

Early Astronomy

Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and phenomena.

Early astronomers used philosophical arguments to explain natural phenomena and some observational data.

Solar System Models Geocentric Model

• In the ancient Greeks’ geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth.

Heliocentric Model• In the heliocentric model, Earth and the other

planets orbit the sun.

What Is Retrograde motion?

It was noticed that the planets move eastward among the stars.

However, each planet then seems to stop, reverse direction for a time and then resume an eastward motion.

This apparent westward motion of the planets with respect to the stars is called Retrograde motion.

Retrograde Motion Animation

Kepler’s 3 Laws of Planetary Motion

1st LawOrbits of the planets are elliptical.An ellipse is an oval-shaped path.

2nd LawPlanets revolve faster when they are closer to the sun.

Planet Revolution 1st & 2nd Law Animations

Kepler’s 3 Laws of Planetary Motion

3rd LawAn orbital period is the time (in Earth years) it takes a planet to make one revolution around the sun.The further the planet is from the sun, the longer the orbital period is. p2 = a3

3rd Law Animation

What Is an Astronomical Unit?

It is the average distance between Earth and the sun

It is about 150 million kilometers (93 million miles)

An astronomical unit (AU)

•

Newton’s Accomplishment

Universal Gravitation

Universal GravitationGravity decreases with distance. The greater the mass of an object, the greater is its gravitational force.

Gravity’s Influence on Orbits

Motions of Earth

Rotation…is the turning, or spinning, of a body on its axis.Rotation is responsible for our days & nights. Mean solar day is the time interval from one noon to the next, about 24 hours.

Motions of Earth

Revolution is the motion of a body, such as a planet or moon, along a path around some point in space.

Perihelion is the time in January when Earth is closest to the sun.Aphelion is the time in July when Earth is farthest from the sun.

Motions of Earth

Precession• the change in direction in which the axis of

the Earth points.• This change occurs about every 26,000

years.• When this happens, Polaris will no longer

be our “North Star.” Instead it will be the star Vega.

Precession Animation

Motions of Earth

Earth’s TILT on its axis causes the seasons! As Earth moves around the sun, the tilt changes the angle of light received from the sun.Light hitting at direct angle = summerLight hitting at indirect angle = winter

Earth’sSeasons

Phases of the Moon

The phases of the moon are due to the amount of the moon that is lit by the sun.

These changes occur monthly.

Phases of the Moon

Phases of the Moon

animation

YouTube:Song

Moon Phases Animation

Lunar Motion

Lunar Rotation

• The moon’s period of rotation on its axis and its revolution around Earth are the same (271/3 days)

• SO… we always see the same side of the moon

• A volunteer?

Why We SeeThe Same

Side ofThe Moon

Eclipses

The Earth–Moon–Sun System

Solar eclipse: when the moon is directly between Earth and the sun, casting a shadow on part of Earth.

Lunar eclipse: when Earth is directly between the moon and the sun, casting a shadow on the moon.

Only happens during a new-moon or full-moon phase, when Earth/sun/moon are all on the same plane.

Solar Eclipse

Solar Eclipse

Eclipse’s For 2012

Lunar Eclipse

Lunar Eclipse

Most widely accepted history…

Origin of the Moon

When the solar system was forming, a body the size of Mars hit Earth. The resulting debris was ejected into space, began orbiting around Earth, and eventually united to form the moon.

Origin of the Moon

Why Is the Moon So Important?

• The moon calms Earth’s tilt; controls our tides, and so much more!

• While watching the movie, describe at least 10 ways our Earth would be different if we had no moon (page ___ in your notebook)

The Day the Earth Was Gone

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