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

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. We've added a new way to donate to 365 Days of Astronomy to support editing, hosting, and production costs. Just visit: and donate as much as you can! Share the podcast with your friends and send the Patreon link to them too! Every bit helps! Thank you! ------------------------------------ Do go visit for cool Astronomy Cast and CosmoQuest t-shirts, coffee mugs and other awesomeness! This show is made possible through your donations. Thank you! (Haven't donated? It's not too late! Just click !) ------------------------------------ The 365 Days of Astronomy Podcast is produced by the Planetary Science Institute. Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org .

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Cheap Astronomy - Dear CA # 133: Why It's Always C is an episode from The 365 Days of Astronomy by 365DaysOfAstronomy.org.

This episode is 14:40 long.

This episode was published on Apr 15, 2026.

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