Every hour, on the hour, a pulse of light leaves Earth for Voyager 1. The width of this stage is one light-day, the distance light covers in 24 hours, so each pulse needs a full day to cross it. Twenty-four of them are always on their way, an hour apart, and the oldest is just reaching the spacecraft. On 18 November 2026 at 10:16 UTC Voyager 1 is one light-day from Earth for the first time. Distances come from JPL's ephemerides, not a sketch, and they show that "one light-day" has three slightly different moments, depending on which way the light is going.
Your pulses are kept in this browser only, so you can come back and see where they are. Sound starts only when you press Sound: a tone for the downlink and a bell each time a pulse reaches Voyager 1.
Real positions. The page loads the positions and velocities of Voyager 1, Voyager 2, Earth, the Sun and the four giant planets from NASA JPL's Horizons system, every 5 days (10 for the Sun and planets) from October 1977 to December 2036, relative to the barycentre of the Solar System. Between samples it interpolates with cubic Hermite curves, which use the velocities too. Clock time (UTC) is converted to the ephemeris time scale (TDB) with the leap-second table.
Light time solved, not looked up. A signal received on Earth at time T left the spacecraft at T − τ, where c·τ = |S(T − τ) − E(T)|. The page solves this by iteration for every frame. A command sent at t reaches the spacecraft at t + τ with c·τ = |S(t + τ) − E(t)|; that is a different number, by a few seconds, because both bodies move while the light travels. The barycentre matters too: the Sun itself moves about a thousand kilometres while the signal is on its way. Checked against JPL's own one-way distances at 4,300 dates: the difference is under 0.1 km from 1990 on.
Three moments. Voyager 1's distance from Earth at one instant equals one light-day on 18 November 2026 at 10:16:07 UTC. A command needs a full day from 17 November at 18:24:45 UTC, about 16 hours earlier, because the spacecraft keeps moving away while the light chases it. A signal arriving at Earth took a full day from 19 November at 02:17:27 UTC, about 16 hours later, because it set out a day before, when the spacecraft was closer. The HUD's light time is the command's (uplink); the readouts give all three. Voyager 2 reaches one light-day on 1 November 2035 at 07:05 UTC.
What you see. Earth is at the left. The horizontal axis is distance from Earth measured in light time, in true proportion, and the stage is a little wider than one light-day. Voyager 1's lines rise to the right and Voyager 2's fall. The lines are drawn as a beam and spread towards the spacecraft; that spread is exaggerated for the eye. A pulse leaves Earth every hour on the hour and moves at the speed of light on this scale. Faint marks moving the other way are the downlink, the signal coming home. Planets are placed at their distance from Earth on the chosen date (they are not on the line to Voyager; the axis is distance only).
The signal itself. Signal strength uses the link budget from the guide, which is calibrated on JPL's design table for Voyager 2 in 1996 (−145.5 dBm received from 48.6 AU) and assumes the 12.3 W X-band transmitter. The Doppler shift is the one-way shift of the 8.4 GHz downlink from the range rate. "Above the horizon" uses the three Deep Space Network complexes and the spacecraft's direction in the J2000 frame (precession since 2000 moves it by about a third of a degree, which is ignored).
A data sculpture in the language of light as material: a black room, fine parallel lines, time made visible as distance. It recreates no specific work.