Space · Distance
Light travel time calculator.
How long does light take to get from the Sun to Earth, or across any distance you type in? Pick a preset or enter your own.
Inputs
Light travel time
Enter a distance.
How light travel time is calculated
Time equals distance divided by the speed of light. The speed of light in vacuum is exactly 299,792,458 metres per second (about 299,792 km/s), and one astronomical unit (AU) is exactly 149,597,870,700 m, so light crosses 1 AU in about 499 seconds.
Worked example. Sun to Earth is 1 AU = 149,597,870.7 km. 149,597,870.7 km / 299,792.458 km/s = 499.0 s, which is 8 min 19 s. Sun to Mars at NASA's mean distance of 228.0 million km takes 12.7 min, and Earth to Moon at 384,400 km takes 1.28 s.
Planet distances in the presets are the mean distances from the Sun in NASA's fact sheet. The distance between two planets changes as both orbit, so for a signal between Earth and Mars enter the current distance yourself.
Frequently asked questions
How long does sunlight take to reach Earth?
About 8.3 minutes, or 8 minutes 19 seconds, over an average distance of 1 AU.
How long does light take to travel from the Moon?
About 1.28 seconds over the Moon's mean distance of 384,400 km. The Moon's distance varies, so the real figure ranges a little either side.
How far is a light-year?
A light-year is the distance light travels in one Julian year of 365.25 days: about 9.46 trillion kilometres (9,460,730,472,580.8 km).
Why do signals to Mars take a variable time?
Earth and Mars orbit the Sun at different speeds, so their separation changes continuously. Enter the actual distance for the date you care about.
Sources
Checked
- NASA JPL Solar System Dynamics: Astrodynamic Parameters
- NIST CODATA: speed of light in vacuum
- NASA NSSDCA: Planetary Fact Sheet (metric)
- NASA NSSDCA: Moon Fact Sheet
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Scope
Straight-line vacuum travel time. It ignores delays added by equipment, relays or the atmosphere.