How fast is the speed of light?
Light travels through a vacuum at about 299,792 km per second (roughly 300,000 km/s). That's fast enough to circle the Earth about seven and a half times in a single second.
What is a light-year?
A light-year isn't a unit of time β it's a unit of distance meaning "the distance light travels in one year," equal to about 9.46 trillion km. Because the universe is almost unimaginably vast, using light-years instead of kilometers makes distances far easier to express.
We're actually looking at the past
Sunlight takes about 8 minutes 20 seconds to reach Earth, so the Sun you see right now is actually how it looked 8 minutes and 20 seconds ago. Starlight in the night sky may have left those stars decades, centuries, or even millions of years ago.
Frequently Asked Questions
Is anything faster than light?
According to current physics (special relativity), nothing with mass can reach or exceed the speed of light. The speed of light is the theoretical maximum speed at which information and matter can travel in the universe.
How far is a light-year in kilometers or miles?
One light-year is about 9.46 trillion km (roughly 5.88 trillion miles). Even the fastest spacecraft ever built would need tens of thousands of years to travel that far.