Transcript: Light and all other forms of electromagnetic radiation travel at 300 thousand kilometers per second or 186 thousand miles per second. This is the speed of light denoted by the small letter “c”. The speed of light is so fast that it was not possible to measure it in ancient times. Four hundred years ago Galileo tried to measure it by positioning a friend on a hill top five miles away with a lantern. Galileo also had a lantern with a shutter. Galileo opened the shutter of his lantern and the light traveled to the distant hill top. His friend was instructed as soon as he saw the light to open his shutter and Galileo saw a light pulse back. Galileo hoped to measure the time it took light to travel back and forth between the two hill tops. In fact the time was so short that he could not have possibly measured it using his pulse as a timing device. The first estimate of the speed of light came from the astronomer Romer in 1675 who used a timing argument on eclipses of the moons of Jupiter by the main planet. The eclipses occur at a slightly different time when the moons are on the far side of Jupiter compared to the near side, the difference being caused by the light travel time across the orbit of Jupiter, about five hours. This was the first estimate of the speed of light, fairly crude. In the modern day with electronics we have measured the speed of light with a very high degree of precision to tiny fractions of a percent. In metric numbers it is almost exactly 300 thousand kilometers per second.