Light Travel Time Calculator
How long light takes to cross any distance.
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Light is fast, but space is vast. Enter any distance — in kilometres, astronomical units, light-years, or parsecs — and this calculator tells you how long a beam of light would take to cross it, revealing just how far away things really are.
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Light Travel Time — The Quick Answer
Divide the distance by the speed of light (299,792 km/s). Sunlight takes about 8 minutes 20 seconds to reach Earth; moonlight about 1.3 seconds.
Shortcut
Time = distance ÷ speed of light (299,792.458 km/s)
Light travel times across space
| Journey | Light travel time |
|---|---|
| Earth to Moon | 1.28 seconds |
| Sun to Earth | 8 min 20 sec |
| Sun to Mars | ~12.7 minutes |
| Sun to Neptune | ~4.1 hours |
| Sun to Proxima Centauri | 4.24 years |
| Across the Milky Way | ~100,000 years |
Because light takes time to travel, we always see distant objects as they were in the past. The Sun you see is 8 minutes old; the nearest star, over 4 years old.
How It Works
The Formula
Distance over speed
Formula
Time = distance ÷ c c = 299,792.458 km/s
Variables
The gap to cross
Converted to kilometres. 1 AU = 149.6 million km; 1 light-year ≈ 9.46 trillion km.
Speed of light
299,792.458 km/s in a vacuum — the universe's ultimate speed limit.
Light travel time
How long light needs to make the trip. Also how far back in time you're seeing when you look at the object.
Note: A light-year is a distance, not a time — the distance light travels in one year. This calculator works the other way, turning a distance into the time light needs to cross it.
Worked Example: Why Sunlight Is 8 Minutes Old
Dividing one astronomical unit by the speed of light
Take the distance
The Sun is about 1 AU away = 149,600,000 km.
Divide by the speed of light
149,600,000 km ÷ 299,792 km/s ≈ 499 seconds.
Read it in minutes
499 s ≈ 8 min 19 s — so the sunlight you see left the Sun over 8 minutes ago.
Reference Guide
| Journey | Light travel time |
|---|---|
| Earth to Moon | 1.28 seconds |
| Sun to Earth | 8 min 20 sec |
| Sun to Neptune | ~4.1 hours |
| Sun to Proxima Centauri | 4.24 years |
| Across the Milky Way | ~100,000 years |
Looking Back in Time
Light-time is also how far into the past you're seeing
Light takes time to arrive, so every object is seen as it was when its light left it — 'now' is only ever local.
Best for: Grasping why distant 'now' doesn't exist for us
Within the Solar System, light-time ranges from ~1.3 s (the Moon) to hours (the outer planets) — and sets the lag for commanding spacecraft.
Best for: Understanding communication delays
Stars and galaxies are seen years to billions of years in the past — a direct window into cosmic history.
Best for: Appreciating astronomical distances
Common Mistakes
Thinking a light-year is a time
A light-year is a distance — how far light travels in a year (~9.46 trillion km). This tool turns a distance into the time light needs to cross it.
Forgetting we see the past
Distant objects are shown as they were when their light left. A star 100 light-years away might no longer look the same — or even exist.
Using light-speed for spacecraft travel
Light-time is the fastest-possible signal delay. Real probes travel far slower than light, so their journey times dwarf the light-travel figure.
Ignoring that c is a vacuum speed
Light slows in glass, water, or air. These figures assume a vacuum — fine for space, but not for light moving through a dense medium.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [bipm]
BIPM (SI Brochure, 9th edition) (2019). Definition of the metre fixing the speed of light c = 299,792,458 m/s exactly. Bureau International des Poids et Mesures.
The speed of light used here is fixed exactly by the SI definition of the metre.
- [iau]
International Astronomical Union (2012). Resolution B2 defining the astronomical unit as 149,597,870,700 m exactly. IAU.
Basis for converting AU and light-years to kilometres.
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 8, 2026
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Frequently Asked Questions
How long does light take to reach Earth from the Sun?
About 8 minutes and 20 seconds. The Sun is roughly 150 million km away (1 AU), and dividing by the speed of light gives around 500 seconds.
What is a light-year?
A light-year is the distance light travels in one year — about 9.46 trillion kilometres. It's a unit of distance, not time, used because space distances are too vast for kilometres to be practical.
Why do we see stars as they were in the past?
Because their light takes years, centuries, or millennia to reach us. When you look at a star 100 light-years away, you see light that left it 100 years ago — so you're literally looking into the past.
How long would light take to cross the galaxy?
The Milky Way is about 100,000 light-years across, so light takes roughly 100,000 years to cross it. Even at the universe's top speed, our galaxy is almost unimaginably large.