The story behind this piece
In 1925 the German engineer Walter Hohmann showed that the cheapest path between two circular orbits is half an ellipse that just touches both. This plate draws that ellipse for Earth and Mars at their true 1 : 1.524 scale, with every figure computed from the Sun's gravitational parameter, μ = 1.32712×10²⁰ m³/s². Leave Earth's orbit with an extra 2.94 km/s, coast 259 days, add 2.65 km/s at Mars — and launch only when Mars leads Earth by 44°, which comes round every 780 days. Gold ticks mark each ten days of flight, placed with Kepler's equation; small marks show where both planets stand at the same moments. The orbits are idealised as circular and coplanar, and the plate says so.
