Light-time and signal delay calculator
Find how long light or a radio signal takes to reach the Moon, the Sun, a planet or any distance you type, one way and there and back.
Runs entirely in your browser
One-way delay
One-way light time to each body
| Body | Distance | One-way light time |
|---|
Planet distances use circular, coplanar orbits and the semi-major axes of the planets, so closest and farthest values are approximate; the real distance on a given day changes continuously. Not for navigation or mission planning.
How to calculate light travel time
- Choose a destination from the list, or choose Custom and type a distance with its unit.
- For the Moon, the Sun and the planets, pick whether to use the closest, typical (midpoint) or farthest distance from Earth.
- Read the one-way delay and the round-trip time. The table shows the same for every body.
The formula
The delay is t = d / c, where d is the distance and c = 299,792.458 km/s is the speed of light in vacuum, exact by the 1983 definition of the metre (CGPM). A round trip is 2d / c. An astronomical unit is exactly 149,597,870.7 km (IAU 2012 Resolution B2).
The Moon's distances are the typical perigee, mean and apogee values of 363,300 km, 384,400 km and 405,500 km from the NASA Moon Fact Sheet, and the Sun's are the perihelion, mean and aphelion distances of about 147.1, 149.6 and 152.1 million km. For the planets, the semi-major axes in astronomical units come from the NASA planetary fact sheets (Mercury 0.387, Venus 0.723, Mars 1.524, Jupiter 5.203, Saturn 9.537, Uranus 19.19, Neptune 30.07, Pluto 39.48). The closest and farthest distances from Earth are estimated as |a − 1| and a + 1 astronomical units, assuming circular orbits in one plane, and the typical value is their midpoint.
These are averages for a quick idea. Real distances depend on the date, the eccentricity and the inclination of the orbits, and radio links also add processing and relay delays. Not for navigation or mission planning.
Questions
How fast is the speed of light?
Exactly 299,792.458 kilometres per second, which is the definition of the metre. Radio waves, which carry spacecraft signals, travel at the same speed in space.
How long does light take to reach the Moon?
About 1.28 seconds one way on average (384,400 km), and 2.56 seconds for a round trip. It ranges from about 1.21 to 1.35 seconds because the Moon's orbit is elliptical.
How long does sunlight take to reach Earth?
About 8 minutes and 19 seconds, since the Sun is on average one astronomical unit (149,597,870.7 km) away. Light from the Sun takes between 8 minutes 10 seconds and 8 minutes 27 seconds over the year.
Why is the delay to Mars not one number?
The distance between Earth and Mars changes from about 0.52 AU at closest to about 2.52 AU at the farthest, so the one-way delay varies from roughly 3 to 22 minutes. Real values depend on the date and are slightly different from these circular-orbit approximations.
Does a signal travel faster than light?
No. Nothing carries information faster than light, so a round trip to a planet always takes at least twice the one-way light time, plus time for processing.
What is a light-minute?
It is the distance light travels in one minute, about 17,987,547 km. A light-second is 299,792.458 km, and a light-year is about 9.46 trillion km.