What is the speed of light in a vacuum, to the nearest round figure?

Light travels at almost exactly 300,000 kilometres per second in a vacuum, or about 186,000 miles per second. The precise value is 299,792,458 metres per second, and that number is exact by definition, because the metre is now defined as the distance light covers in a set fraction of a second rather than the other way round. The speed matters far beyond optics. In relativity it is the maximum speed at which any information or influence can travel, which is why nothing with mass can be accelerated up to it: the energy required grows without limit. It also sets the scale of astronomy, since a light year is simply how far light goes in a year, and looking at distant objects means seeing them as they were when the light left. Light slows down in glass, water or any transparent material, and that change of speed is what bends rays and makes lenses work.

Sunlight takes a little over eight minutes to reach Earth, so you always see the Sun slightly in the past.