Estimate one-way network latency from distance, medium and processing delay.
One-way network latency is the sum of propagation delay (the time for a signal to physically travel the distance at a speed determined by the transmission medium) and processing delay (time spent in router/switch queues, serialization, and packet handling). Propagation speed is the speed of light divided by the medium's refractive index — light travels slower in fiber (n≈1.47) or copper (n≈1.5) than in free space (n=1.0). Round-trip time (RTT) is approximately twice the one-way latency when the path is symmetric.
One-way latency
latency = distance / (speed_of_light / refractive_index) + processing_delay
Light travels slower in a medium than in a vacuum, by a factor equal to the medium's refractive index. For typical fiber, that index is about 1.47, so light travels roughly 68% as fast as it does in a vacuum.
Only approximately, and only for symmetric paths. In practice, upload and download paths often differ (asymmetric routing, different congestion in each direction), so measured RTT can deviate from a clean 2x multiple.
Router and switch forwarding lookups, packet serialization onto the physical link, queuing behind other traffic, and any middlebox processing (firewalls, NAT, deep packet inspection) all add processing delay on top of pure propagation delay.
Geostationary satellites orbit about 35,786 km above the equator, so a signal travels roughly 71,572 km round-trip to the satellite and back just one way, adding over 200ms of pure propagation delay even though the signal moves at nearly the speed of light.