2. Speed of Light

The speed of light may not be an actual "thing" like most of the other astronomical discoveries on our list, but it's no less important. The speed of light is a constant (c), and used in many areas of study, including physics, astronomy and computing. It is part of Einstein's famous mass-energy equivalence (E=mc²). No matter what type of light or other forms of electromagnetic radiation, in empty space light will always travel at 299,792,458 meters per second (186,282 miles per hour).

Galileo tried unsuccessfully to measure the speed of light several times. Danish astronomer Ole Rømer succeeded in 1676 after observing the lunar eclipses of Jupiter, especially those of its moon, Io. When the Earth was closer to Jupiter, the times between the eclipses were shorter than when the two planets were farther apart. Rømer believed that this must mean that the speed of light was finite, as it was taking longer to reach Earth when Jupiter was farther away. His measurements were very close to the actual speed of light as we know it today. Other the centuries, increasingly precise measurements were obtained. In 1975, an instrument called a laser inferometer was used to obtain the current measurement, and subsequently the definition of a meter was changed to "the length of the path traveled by light in vacuum during a time interval" of one-299,792,458th of a second [source: BIPM].