There is a speed limit in the universe. While many great science fiction stories require this limit to be broken, right now, we are stuck at not going any faster than the speed of light.
It is exactly (actually defined to be) 299,792,458 meters/second. (This is about 671 million miles per hour if you don’t drive with the metric system.) The value for the speed of light is so commonly used in science, it is usually just denoted by the letter c.
The earliest successful estimate of the speed of light was made by Ole Roemer in 1676. He used a rather elegant technique that relied on observations of Jupiter’s moons and came up with an answer that is within 70 percent of the value we now use.
In the late 19th century, the most accurate measurements were made using a rotating mirror. Light from the sun could bounce off the rotating mirror toward a distant mountain, where a fixed mirror would send the light back to the rotating mirror. In the time the light traveled to the distant mountain and back, the rotating mirror would have slightly changed positions, making the spot from the second reflection shift locations slightly. The faster the rotation, the more the spot would move.
In the 20th century, the speed of light was measured to greater and greater precision until 1983. It was then agreed that while all standard lengths (particularly the meter) were human-made, the speed of light in a vacuum was one of the fundamental properties of the universe. After 1983, rather than using the meter to measure the speed of light, the length of a meter was now defined by how far light in a vacuum would travel in 1/299,792,458th of a second.
This defined speed of light is how fast visible light, radio waves, infrared, ultraviolet, X-rays and all other types of electromagnetic radiation travel through a vacuum. No matter the wavelength, they all travel the same speed.
Astronomers care about light because the only way we know anything at all about distant objects is by the light we receive from them.
By studying the light, we can determine the temperature, the composition and whether or not the object is moving toward us or away from us.
This monthJupiter has finally set in the early evening, but Venus will become a more and more prominent evening star as the month progresses.
This week, Mars, Saturn and Antares are making a nice equilateral triangle in the southern sky. Mars is the brighter one on the left, but I think Saturn, up and to the right, is more fun to look at with a small telescope. Even with some binoculars you can tell Saturn isn’t round.
Along with the approaching equinox, we can look forward to cooler evenings and more darkness for your astronomical pursuits. And time to ponder just how long that light from those distant galaxies has traveled just to end its journey inside your eye.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.