The speed of light in a vacuum is a constant of our universe; no matter where you are or how you’re moving, light will always travel at the same speed.
But what is that speed? It’s 299,792,458 meters per second, or 299,792.458 kilometers per second. Really fast.
Using the speed of light can be quite convenient for astronomical purposes, because we can begin to use it as an extremely long ruler. Since the speed at which light travels is always the same in the vacuum of space, the distance it will cover in a fixed amount of time is also always the same. If you’re driving 60 miles an hour exactly, then it will always take you exactly one hour to travel 60 miles. It’s the same principle with light, except on a much grander scale.
The distance between the Earth and the Sun is about 93 million miles, or 149.6 million kilometers. Light travels about three hundred thousand kilometers a second, but even at that insane speed, the distance between the Earth and the Sun is large enough that there will be lag. A little over 8 minutes of lag, in fact. 8 minutes isn’t very long, but it’s certainly noticeable. To get to Jupiter from the Sun, light takes 43 minutes - to get to Saturn, it’s about 80 minutes - almost an hour and a half. Mars gets light from the sun about 12.5 minutes later, and Neptune, the most distant major planet in our solar system, has to wait about 250 minutes - a little over four hours.
This kind of time delay is something all scientists working with spacecraft orbiting other worlds (or roving on their surfaces) must deal with. The delay will be at minimum 8 minutes less than the numbers above, but depending on where the planets are in their orbits, it can be significantly longer. It’s the root behind the “Seven Minutes of Terror” video NASA released before the Curiosity rover landed; they knew the rover should have landed, unfolded, and radioed home, but they had to wait seven minutes for light to make the trip between Earth and Mars.
Most distances in astronomy are too small to be using light minutes, seconds, or hours. The nearest star to our own is four light years away. From the Sun to the center of our galaxy is about 30,000 light years. Our galaxy is so large that it would take about 100,000 years for light to travel from edge to edge. And the distances between galaxies are even greater.
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