Motion and Gravity in Space • Each planet spins on its axis. The spinning of a body, such a planet, on its axis is called rotation.

• The orbit is the path that a body follows as it travels around another body in space.

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• A revolution is one complete trip along an orbit. Kepler’s Three Laws of Motion  Johannes Kepler • discovered three laws of planetary motion:

1. Orbits of the planets are elliptical. • An ellipse is an oval-shaped path. Kepler’s 2nd Law 2. planet moves fastest when it is near perihelion and slowest when it is near aphelion

Perihelion – moment closest to sun Aphelion – moment furthest from sun Kepler’s Third Law

3. planets more distant from the sun take longer to orbit the sun.. For example, Mercury, the closest planet to the Sun, takes just 88 Earth days to complete an orbit. But Neptune, the furthest out, takes 164 Earth years to

The graph above shows the distances of the planets from the Sun and the speed at which they travel. The shape of the curve shows that the Kepler Video further out a planet is the longer it takes to http://youtu.be/dRT3m2Wzyh4 orbit the Sun. • Kepler did not understand what causes the planets farther from the sun to move slower than the closer planets.

• Sir Isaac Newton’s description of gravity provides an answer.  Sir Isaac Newton • Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation.

 Universal Gravitation 1. Gravitational force decreases with distance.

2. The greater the mass of an object, the greater is its gravitational force. The direction of the pull of gravity is always towards the center of the mass If gravity is pulling the Earth into the center of the Sun, why does the Earth move in a circle? Why doesn't the Earth crash into the centre of the Sun?

If the forces acting on the Earth were balanced, then the Earth would move through space in a straight line at a constant velocity. The Earth is trying to move past the Sun, the Sun's gravity is pulling the Earth sideways.

Bill Nye - gravity http://youtu.be/6f6maa9xPDM • Orbits Falling Down and Around - Inertia is an object’s resistance to change in speed or direction until an outside force acts on the object.

1. Gravitational attraction keeps the planets in their orbits. Inertia keeps the planets moving along their orbits. 2. There are no other forces acting on the Earth, no friction because the Earth is moving through empty space. The Earth moves in a direction between the two arrows. 3. The attraction of gravity between the two masses (Sun and Earth) stays the same as long as the distance between them stays the same. 4. The Earth must move in a circle to remain the same distance from the Sun. 5. The force which keeps an object moving in a circle is called the centripetal force.

Weightlessness Video http://youtu.be/d57C2drB_wc Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Mass 0·05 0·8 1·0 0·1 318 95 14·5 17 0·002

Diameter 4·9 12·0 12·8 6·8 143 120 51 49·5 2.3

Orbit Time 0·24 0·62 1·0 1·9 12 29·5 84 165 248

Distance 58 108 150 228 780 1430 2870 4500 5900

Orbital Speed 48 35 30 24 13 10 7 5·5 4·8 Mass is given relative to the Earth. Diameter is given in thousands of kilometres. Orbit time is given in years. Distance from the planet to the Sun is given in millions of kilometres. The distance is an average value for the slightly elliptical orbit. Orbital speed is given (approximately) in kilometres per second. Notice that the orbital speed decreases as the distance from the Sun increases Retrograde Motion 6. Retrograde motion is the apparent westward motion of the planets with respect to the stars.

retrograde motion usually means motion which is contrary to the rotation of the primary, that is, the object which forms the system's hub. The Solar & Heliospheric Observatory, is a project of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind.

SOHO was launched in December 1995 by an Atlas Centaur rocket and became operational in March 1996. SOHO weighs about two tons and with its solar panels extended stands about 25 feet across. It was launched in December, 1995. SOHO will continue operating well past the next solar maximum in 2001. SOHO was designed to answer the following three fundamental scientific questions about the Sun: • What is the structure and dynamics of the solar interior? • Why does the solar corona exist and how is it heated to the extremely high temperature of about 1 000 000°C? • Where is the solar wind produced and how is it accelerated? SOHO IMAGES and VIDEOS

CLICK TO VIEW Hubble Space Telescope