Light—that is, any kind of electromagnetic radiation—travels through space at more than 186,000 miles per second. That means that if we see an object 186,000 miles away, it took one second for the light from that object to reach us. This, in turn, means that we are actually seeing the object as it existed one second ago. This effect is called look-back time.

For astronomical distances, look-back time can be a significant effect. The look-back time for the Sun is eight minutes; the look-back time for the planet Jupiter is almost an hour; and for the Alpha Centauri star system, the look-back time is nearly four and a half years.