Presets
Playback
Distance and medium
The distance chooses the trip; the refractive index slows light inside glass.
Mars geometry
Where Earth and Mars are on their circular orbits.
Layers
Model checks
The science
Light travel time
Light travels in vacuum at the exact defined speed c = 299,792.458 km/s, so a signal over distance d takes t = d / c. In a transparent medium of refractive index n it is slower, v = c / n; glass fibre has n of about 1.49, so v is about 0.67 c (the race shows the difference over 7,500 km). Round-trip delay for a question-and-answer exchange is 2 d / v.
Earth-Mars geometry
Planets are placed on circular, coplanar orbits (Earth 1 AU, period 365.256 d; Mars 1.524 AU, period 686.98 d). The separation follows the law of cosines, d = sqrt(r1^2 + r2^2 - 2 r1 r2 cos(delta phi)) with delta phi = 2 pi t (1/T1 - 1/T2). The synodic period between oppositions is 1 / (1/T1 - 1/T2), about 780 days. 1 AU is 149,597,870.7 km, 499.0 light-seconds.
Honest limits
Real orbits are ellipses, so the real Earth-Mars distance ranges from about 54.6 to 401 million km (3 to 22 minutes one way); circular orbits give 78 to 378 million km. The distance slider and the preset buttons use the real values (as in the 3D page), the map uses circles. Relativistic effects, light bending and signal processing delays are ignored.
What to try
- Press Mars closest and then Mars farthest: the round-trip time changes from about 6 to 45 minutes, which is why rovers are not driven live.
- Drag the Mars date slider through a whole synodic period and watch the delay graph oscillate.
- Pick Fibre NYC-Kyiv and press Race: the vacuum pulse arrives first by about 12 ms.
- Pick Proxima or Milky Way to see why the numbers become years.