100 Greatest Discoveries

1. The Planets Move (2000 B.C. – 500 B.C.)
A thousand years of observations reveal that there are stars that move in the sky and follow patterns, showing that the Earth is part of a solar system of planets separate from the fixed stars.

2. The Earth Moves (1543)
Nicolaus Copernicus places the sun, not the Earth, at the center of the solar system.

3. Planetary Orbits Are Elliptical (1605 – 1609)
Johannes Kepler devises mathematical laws that successfully and accurately predict the motions of the planets in elliptical orbits.

4. Jupiter Has Moons (1609 – 1612)
Galileo Galilei discovers that Jupiter has moons like the Earth, proving that Copernicus, not Ptolemy, is right. Copernicus believes that Earth is not unique, but instead resembles the other planets, all of which orbit the sun.

5. Halley's Comet Has a Predictable Orbit (1705 – 1758)
Edmund Halley proves that comets orbit the sun like the planets and successfully predicts the return of Halley's Comet. He determines that comets seen in 1531 and 1607 are the same object following a 76-year orbit. Halley's prediction is proven in 1758 when the comet returns. Unfortunately, Halley had died in 1742, missing the momentous event.

6. The Milky Way Is a Gigantic Disk of Stars (1780 – 1834)
Telescope-maker William Herschel and his sister Carolyn map the entire sky and prove that our solar system resides in a gigantic disk of stars that bulges in the center called the Milky Way. Herschel's technique involves taking a sample count of stars in the field of view of his telescope. His final count shows more than 90,000 stars in 2,400 sample areas. Later studies confirm that our galaxy is disk-shaped, but find that the sun is not near the center and that the system is considerably larger than Herschel's estimation.

7. General Relativity (1915 – 1919)
Albert Einstein unveils his theory of general relativity in which he proposes that mass warps both time and space, therefore large masses can bend light. The theory is proven in 1919 by astronomers using a solar eclipse as a test.

8. The Universe Is Expanding (1924 – 1929)
Edwin Hubble determines the distance to many nearby galaxies and discovers that the farther they are from us, the faster they are flying away from us. His calculations prove that the universe is expanding.

9. The Center of the Milky Way Emits Radio Waves (1932)
Karl Jansky invents radio astronomy and discovers a strange radio-emitting object at the center of the Milky Way. Jansky was conducting experiments on radio wavelength interference for his employer, Bell Telephone Laboratories, when he detected three groups of static; local thunderstorms, distant thunderstorms and a steady hiss-type static. Jansky determines that the static is coming from an unknown source at the center of the Milky Way by its position in the sky.

10. Cosmic Microwave Background Radiation (1964)
Arno Penzias and Robert Wilson discover cosmic microwave background radiation, which they suspect is the afterglow of the big bang. Their measurements, combined with Edwin Hubble's earlier finding that the galaxies are rushing away, make a strong case for the big bang theory of the birth of the universe.

11. Gamma-Ray Bursts (1969 – 1997)
The two-decade-long mystery of gamma-ray bursts is solved by a host of sophisticated ground-based and orbiting telescopes. Gamma-ray bursts are short-lived bursts of gamma-ray photons, which are the most energetic form of light and are associated with nuclear blasts. At least some of the bursts have now been linked with distant supernovae — explosions marking the deaths of especially massive stars.

12. Planets Around Other Stars (1995 – 2004)
Astronomers find a host of extrasolar planets as a result of improved telescope technology and prove that other solar systems exist, although none as yet resembles our own. Astronomers are able to detect extrasolar planets by measuring gravitational influences on stars.

13. The Universe Is Accelerating (1998 – 2000)
Unexpectedly, astronomers find that instead of slowing down due to the pull of gravity, the expansion of the universe at great distances is accelerating. If these observations are correct and the trend continues, it will result in the inability to see other galaxies. A new theory of the end of the universe based on this finding has been called the "big rip."

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