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)

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

Kepler’s 2nd 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 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.

But if you threw it fast enough, it would keep falling until it got all the way back to where you started. On all these trajectories, the projectile is in free fall. On all these trajectories, the projectile is in free fall.

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

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.

Why do astronauts in the Space Shuttle in Earth orbit feel weightless? 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. That is why they experience weightlessness: just as a platform diver feels while diving down towards a pool, or a sky diver feels while in free fall. On all these trajectories, the projectile is in free fall.