Cheap Astronomy - Dear CA # 133: Why It's Always C

Cheap Astronomy - Dear CA # 133: Why It's Always C

Author: 365DaysOfAstronomy.org April 15, 2026 Duration: 14:40

Cheap Astronomy investigates why light moves at light speed in a vacuum.

Hosted by Steve Nerlich.

Shining a light!

Dear Cheap Astronomy – Why does a photon travel at speed c, part1

It may sound a bit strange to ask why light travels at the speed of light, but that's because c isn't really the speed of light – it's really a measure of the relationship between distance (or space) and time. It's become customary to call c the speed of light, but in reality light moves at different and slower speeds through air, water or glass, it only moves at c in a vacuum.

 

Dear Cheap Astronomy – Why does a photon travel at speed c, part1

Last week we grappled with the issue of why light is able to move at the speed c, which is around 300,000 km/sec in a vacuum. Some key points we landed were that c is actually the fastest speed that anything can move and that light isn't the only thing that moves at c, gravity does too. The constant c is perhaps better thought of as defining the fundamental relationship between space (or distance) and time, where it must take at least one second to cross 300,000 kilometers – meaning it must take at least 0.000000033 second to cross one meter, and you can add even more zeros to cross centimeters and then nanometers.

 

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Imagine a daily conversation about the cosmos, where each sunrise brings a fresh perspective from a different corner of the globe. That’s the essence of The 365 Days of Astronomy. Born from the spirit of the 2009 International Year of Astronomy, this enduring project has evolved into a genuine community effort, releasing a new episode every single day. You won’t hear the same host each time; instead, the microphone passes to astronomers, educators, enthusiasts, and scientists, each sharing their unique passion and expertise. This structure means the tone and topic can shift wonderfully from one day to the next-you might delve into the mechanics of a black hole on Tuesday and then explore ancient celestial myths on Wednesday. The consistent thread is a shared wonder for the universe, presented in accessible, bite-sized pieces. Managed by Avivah Yamani and edited by Richard Drumm, this daily podcast is a production of the Planetary Science Institute, sustained by listener support. It’s a calendar of the cosmos, offering a daily moment of reflection and discovery that proves you don’t need a telescope to have your perspective shifted. Tuning in regularly builds a habit of cosmic curiosity, connecting the vastness of space to the rhythm of our own days.
Author: Language: English Episodes: 100

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