Astronomy from ancient Times Ancient Astronomy Stonehenge on

Ancient Astronomy Stonehenge on the summer solstice. As seen from the center of the

Astronomy in Early Americas • Maya Indians developed written language and number system. •

Constellations • People drew pictures of constellations; created stories to account for the figures

Aristotle’s Universe: A Geocentric Model • Aristotle proposed that the earth was at the

Planetary Motion • From Earth, planets appear to move different from stars. • Most

Heliocentric Model - Copernicus • In 1543, Copernicus proposed that: the Sun, not the

Galileo • Galileo used a telescope to observe 4 points of light that changed

Johannes Kepler: Laws of Planetary Motion

Kepler: Elliptical orbits The amount of "flattening" of the ellipse is the eccentricity. In

Kepler’s 1 st Law: The orbits of the planets are ellipses, with the Sun

Kepler’s 2 nd Law • The line joining the planet to the Sun sweeps

Newton had an interesting ‘thought experiment’: Is it possible to throw an object into

If a ball is thrown, it usually falls back to earth. The harder you

you threw it fast enough, it would On. But all ifthese trajectories, the projectile

… that as it falls, the Earth curves away underneath On all these trajectories,

Gravity and Orbits The Sun’s inward pull of gravity on the planet competes with

Apparent Weightlessness in Orbit This astronaut on a space walk is also in free

Let’s take a little time to answer the following question: • Why do astronauts

• Some common misconceptions which become apparent in answers to this question are:

In spacecraft (like the Shuttle) in Earth orbit, astronauts are in free fall, at

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Astronomy from ancient Times

Ancient Astronomy Stonehenge on the summer solstice. As seen from the center of the stone circle, the Sun rises directly over the "heel stone" on the longest day of the year. The Big Horn Medicine Wheel in Wyoming, built by the Plains Indians. Its spokes and rock piles are aligned with the rising and setting of the Sun and other stars.

Astronomy in Early Americas • Maya Indians developed written language and number system. • Recorded motions of Sun, Moon, and planets -especially Venus. • Fragments of astronomical observations recorded in picture books made of tree bark show that Mayans had learned to predict solar and lunar eclipses and the path of Venus. • One Mayan calendar more accurate than those of Spanish.

Constellations • People drew pictures of constellations; created stories to account for the figures being in the sky. • Used stars and constellations for navigation.

Aristotle’s Universe: A Geocentric Model • Aristotle proposed that the earth was at the center of the universe with everything orbiting around it. (Geocentric) • Everything moved in perfect circles.

Planetary Motion • From Earth, planets appear to move different from stars. • Most of the time, planets move west to east (compared to stars) • Occasionally, they appear to change direction and temporarily go backwards. (Retrograde motion) (retrograde-move) retrograde-move

Heliocentric Model - Copernicus • In 1543, Copernicus proposed that: the Sun, not the Earth, is the center of the solar system. • Such a model is called a heliocentric system. • Ordering of planets known to Copernicus in this new system is illustrated in the figure. • Represents modern ordering of planets. • (copernican-move)

Galileo • Galileo used a telescope to observe 4 points of light that changed their positions with time around the planet Jupiter. • He concluded that these were objects in orbit around Jupiter.

Johannes Kepler: Laws of Planetary Motion

Kepler: Elliptical orbits The amount of "flattening" of the ellipse is the eccentricity. In the following figure the ellipses become more eccentric from left to right. A circle may be viewed as a special case of an ellipse with zero eccentricity, while as the ellipse becomes more flattened the eccentricity approaches one. (eccentricity-anim)

Kepler’s 1 st Law: The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

Kepler’s 2 nd Law • The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. Orbit-anim

Newton had an interesting ‘thought experiment’: Is it possible to throw an object into orbit around the Earth?

If a ball is thrown, it usually falls back to earth. The harder you throw, the farther it goes. But it eventually falls to Earth.

you threw it fast enough, it would On. But all ifthese trajectories, the projectile is inkeep free falling until it got all the way back to where you started.

… that as it falls, the Earth curves away underneath On all these trajectories, the projectile is in free fall. it, and the projectile completes entire orbits without ever hitting the Earth.

Gravity and Orbits The Sun’s inward pull of gravity on the planet competes with the planet’s tendency to continue moving in a straight line.

Apparent Weightlessness in Orbit This astronaut on a space walk is also in free fall. The astronaut’s “sideways” velocity is sufficient to keep him or her in orbit around the Earth.

Let’s take a little time to answer the following question: • Why do astronauts in the Space Shuttle in Earth orbit feel weightless?

• Some common misconceptions which become apparent in answers to this question are: (a) there is no gravity in space, (b) there is no gravity outside the Earth’s atmosphere, or (c) at the Shuttle’s altitude, the force of gravity is very small.

In spacecraft (like the Shuttle) in Earth orbit, astronauts are in free fall, at the same rate as their spaceships. On allisthese That why they trajectories, experience the weightlessness: projectile is in free just fall. as a platform diver feels while diving down towards a pool, or a sky diver feels while in free fall.