How to use it
- Choose the Moon, the Sun or a planet. Planets are measured from the Sun along their mean orbit radius.
- Or choose Custom distance and enter a distance in kilometres, miles, astronomical units, light years or parsecs.
- Read the travel time; the headline switches between seconds, minutes, hours, days and years so the number stays readable.
Formula
t = d ÷ c, with c = 299,792.458 km/s, the exact speed of light in vacuum.
Distances used: the Moon 384,400 km (mean), the Sun 1 AU = 149,597,870.7 km, planets at their semi-major axis (Mercury 57.909, Venus 108.21, Mars 227.956, Jupiter 778.479, Saturn 1432.041, Uranus 2867.043, Neptune 4514.953 and Pluto 5869.656 million km). For the range from Earth the calculator takes |a − a⊕| and a + a⊕, with Earth at 149.598 million km.
Worked example
Sunlight: 149,597,870.7 km ÷ 299,792.458 km/s = 499.005 s, which is 8.32 min.
Mars: 227,956,000 km from the Sun is 12.67 min of light time. From Earth the distance swings between 78.358 and 377.554 million km, so a signal to a Mars rover takes from 4.4 to 21 minutes one way.
Neptune is 4.18 h of light time from the Sun.
Tips
- Round trips double the time: a command to a spacecraft and its reply take twice the one-way time.
- We see the past. The Sun we see is 8 minutes old; a star 100 light years away, as it was a century ago.
- Real distances vary along elliptical orbits, so treat planetary figures as averages.