Understanding the Universe: An Introduction to Astronomy, 2nd Edition

You embark on a fantastic voyage throughout the Universe, starting in this lecture with a whirlwind tour of the course, which extends from phenomena in Earth's atmosphere to events at the farthest reaches of space and time.

The daytime sky contains intriguing natural sights that offer a colorful introduction to astronomy. One such phenomenon is the rainbow. This lecture examines how a rainbow is created and how it appears under different circumstances.

Continuing the study of the daytime sky, this lecture examines phenomena associated with sunrise and sunset, including the elusive green flash, Buddha's rays, and Earth's own shadow at sunset.

Many objects in the night sky can be enjoyed with the naked eye, even from the city. These include several famous constellations, bright stars, planets, and satellites such as the International Space Station.

Far from city lights, the night sky becomes spectacular and includes such sights as the Milky Way, the Magellanic Clouds, zodiacal light, and comets. Though not technically "celestial," auroras are also wondrous spectacles.

A relatively inexpensive telescope and even a simple pair of binoculars greatly increase the number of celestial objects you can study, such as the craters on the Moon, the Orion Nebula, and the Andromeda Galaxy.

As Earth rotates on its axis and orbits the Sun, the night sky changes over a 24-hour period, as well as throughout the year. This lecture explains how to locate celestial bodies and why the sky appears different from place to place.

Contrary to popular belief, the seasons are not caused by changes in the distance between Earth and the Sun over the course of a year. Instead, the tilt of Earth's axis of spin relative to the axis of its orbital plane produces seasons.

Lunar phases occur as the Moon orbits Earth, changing the viewing perspective of the Moon's illuminated hemisphere. Lunar eclipses take place when Earth, the Sun, and Moon are almost exactly aligned.

Total solar eclipses are stunning celestial sights, which occur when the Moon comes between Earth and the Sun, totally blocking the Sun for a small portion of the Earth. These rare events reveal beautiful and thrilling phenomena.

Solar eclipses also come in annular and hybrid varieties, reflecting the varying distance of the Moon from Earth. A famous total solar eclipse in 1919 provided observational evidence for Einstein's general theory of relativity.

Astronomy has its roots in the ancient civilizations of Mesopotamia, Babylon, India, Egypt, and China. The Greeks in particular developed sophisticated and sometimes surprisingly accurate theories of the nature of the heavens.

The Greek philosopher Aristotle concluded that Earth is stationary at the center of the Universe, surrounded by 55 nested spheres. Ptolemy elaborated this geocentric model into a system that held sway for nearly 1,500 years.

In 1543, Copernicus proposed a heliocentric system, in which Earth and other planets orbit the Sun, which is at the center of the Universe. In the early 1600s Galileo provided strong evidence for this model with the newly invented telescope.

This lecture explores the refinements in the Copernican model made possible by Tycho's extremely accurate measurements of planetary positions, which were analyzed by Kepler to produce his laws of planetary motion.

According to legend, Newton saw a falling apple and realized that the force that pulled it toward Earth also pulled on the Moon, keeping it in its orbit. Building on the work of Kepler and Galileo, he revolutionized astronomy with his law of universal gravitation and laws of motion.

Observations of the transit of Venus across the face of the Sun in the 18th century helped determine the scale of the Solar System. In the wider Universe, distances are so vast that the finite speed of light means we are literally looking back in time.

The best way to understand the size of the Universe is to investigate it in a series of steps, each 10 times larger or smaller than the one preceding. In this way, you explore the cosmos from the smallest to the largest scale.

Astronomers obtain most of their information through the analysis of light. This lecture introduces the electromagnetic spectrum and the technique of spectroscopy, in which light is dispersed into its component colors, as in a rainbow.

Delving deeper into the nature of light, you explore the strange duality between electromagnetic waves (light waves) and particles (photons), which is a fundamental feature of quantum theory.

The surface temperature of a star determines its apparent color. The hottest stars are bluish in color, and the coldest stars are reddish; stars at intermediate temperatures appear white. The Sun is a white star.

Electrons jumping between different energy levels in atoms emit and absorb photons in a characteristic way for each element. Thus, astronomers can use the light from distant objects to deduce their chemical compositions.

Today's telescopes are designed to provide huge light-gathering mirrors at relatively low cost. The mirrors focus light from distant objects onto sensitive electronic detectors that are far more efficient than traditional photographic film.

This lecture looks at radio telescopes, adaptive optics for ground-based infrared telescopes, and NASA's Great Observatories, which include the Hubble Space Telescope, Chandra X-Ray Observatory, and Spitzer Space Telescope.