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

The 365 Days of Astronomy · Apr 15, 2026 · 14m

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…