Field notes

The Light Delay Today

Live · drawn in your browser from the orbits, a year back and a year ahead · updated every minute

Light takes about 1.3 seconds to reach the Moon. It takes between 3 and 22 minutes to reach Mars, and which end of that range you are at depends on where the two planets are in their orbits. This page works it out for the moment you opened it.

Why a bandwidth protocol cares

Every number in the Bandwidth Swap pilot was chosen for a bench where the answer comes back in microseconds: an hourly window, a five-second clock tolerance, a Mint that is always reachable. Version 0.3 of the paper now treats those as parameters of a link rather than constants of the protocol (Appendix A), and this chart is the reason. A relay serving Mars has a clock tolerance that moves with the orange line, a window that is a contact pass rather than an hour, and a Mint it reconciles with when the link allows. The previous note, Receipts Across the Light Delay, walks through what survives that and what has to change.

How it is computed

No server and no ephemeris download. The page places Earth and Mars on their orbits from their mean orbital elements at the J2000 epoch, solves Kepler's equation for each day, and divides the distance by the speed of light. The Moon's distance is its mean distance with the perigee-to-apogee swing on its 27.55-day anomalistic month. Against the almanac this is accurate to about a minute for Mars and a few hundredths of a second for the Moon: good for the shape and the day's figure, not for navigation. The code is in this page.

Sources: mean orbital elements from JPL Solar System Dynamics, approximate positions of the planets; lunar constants from the NASA Moon fact sheet; the Bandwidth Swap whitepaper, v0.3.