Tools / Interstellar message calculator
Interactive tool
Interstellar Message Calculator
Choose a distance and see how slowly an interstellar conversation would unfold—even at the speed of light.
Message arrives after
Earliest reply returns after
How it works
A light-year measures distance: how far light travels in one year. A light-speed signal therefore takes one year to travel one light-year.
One-way time = distance in light-years.
Earliest round-trip time = twice that distance.
This model assumes a fixed distance, travel at light speed and an immediate reply. It ignores relative motion and processing delays. It does not imply anyone lives at the destination or that a signal would be detectable. Arrival years are approximate decimal years, not calendar dates.
Distance references
Star distances are rounded educational presets.