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Background Information ~ The Speed of Light

Before we can get started with the study of light in astronomy, let's talk briefly about the speed of light.

300,000 kilometers per second

186,000 miles per second




Light cannot go faster than this, so the speed of light is often called the speed limit of the universe.





Since the speed of light is finite, when we look out into the sky, we are seeing light that produced billions of years ago. It has been traveling the vast distances of the universe ever since and just now is reaching an object (your eye or a telescope).

This idea is called "Look Back Time". It is the time it took from the time was produced until it is detected on Earth.

For example, light from the sun takes 8 minutes to get to Earth, so the sunlight that is filling your room during the day left the surface of the sun 8 minutes ago. The sun is 93 million miles away (on average) and it takes a whole 8 minutes for the light to travel to Earth.


What can slow down the speed of light?


Can you measure the speed of light...with chocolate?!

This experiment came recommended by a former student. Check it out!

Watch this video for a brief introduction to the speed of light. Answer these questions in your notebook before moving on:
  1. What was first used to measure the speed of light?
  2. Einstein said that matter and energy cannot travel faster than the speed of light. What was his reason?
  3. Define a light year.
  4. How large is the Milky Way galaxy in light years?


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