The speed of light is a fascinating topic that has significance from everyday life to the vast expanse of the cosmos. The speed of light is the speed at which light travels through a medium, and for a vacuum, this speed has been defined as 299,792,458 m/s. This speed is almost incomprehensible to us due to its magnitude. For example, if you moved at the speed of light, you would circumnavigate the equator approximately 7.5 times in one second.
Because of this finite speed, everything we observe is a glimpse into the past. This is particularly applicable to astronomy due to the vast distance of objects observed. Astronomers are able to use this principle and look further back in time by studying more distant objects. Our nearest neighbor, the Andromeda Galaxy, is 2 million light years away, meaning we see what it looked like 2 million years ago. This allows astronomers to garner a better understanding of the universe’s history.
Along with visible light, all forms of electromagnetic radiation travel at the speed of light. Therefore, this speed governs both the light viewed from distant stars on Earth, as well as communications with distant space probes. NASA’s Voyager 1 probe, the most distant satellite in interstellar space which was launched in 1977, takes nearly a day (22.5) hours for a transmission to reach the spacecraft. This delay is due to the spacecraft being more than 15 billion miles away from Earth, and the speed of light, or electromagnetic radiation in this case, sets the ultimate limit of the speed of these communications. This is even applicable to the communication delay during the Apollo missions, where it took 1.255 seconds for communications to reach the moon from Earth, as seen in the graphic below.
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