How fast is light, exactly?
Light in a vacuum travels at 299,792.458 kilometres per second. That number is not a measurement that gets refined over the years: since 1983 the metre itself is defined from the speed of light, so 299,792,458 m/s is exact by definition. For back-of-the-envelope work, 300,000 km/s is close enough. At that speed a beam could circle the Earth’s equator (about 40,075 km) roughly 7.48 times in one second. This calculator uses a single relationship, time = distance ÷ speed of light, to give a one-way time and a round-trip time, and it converts the distance into kilometres, astronomical units and light-years. It is meant for homework, writing, curiosity about how old starlight is, and for estimating the minimum delay of a radio signal.
Light-years, astronomical units and their relatives
A light-year is a unit of distance, not time: the distance light covers in vacuum in one year. Astronomers use the Julian year of exactly 365.25 days (31,557,600 seconds), so one light-year is 299,792.458 × 31,557,600 ≈ 9.4607e+12 km, which is about 9.46 trillion kilometres or 5.8786e+12 miles. The astronomical unit (AU) is another exact, defined length: 149,597,870.7 km, close to the average Earth–Sun distance. One light-year is therefore about 63,241 AU. Inside the Solar System kilometres and AU are convenient; between stars the light-year takes over. Professional astronomers also use the parsec, which is about 3.26 light-years. The table below applies the same idea to shorter spans of light travel.
| Unit | Distance in km | Distance in AU |
|---|---|---|
| Light-second | 299,792.5 | 0.002 |
| Light-minute | 17,987,547.5 | 0.1202 |
| Light-hour | 1.0793e+9 | 7.2144 |
| Light-day | 2.5902e+10 | 173.1446 |
| Light-year (Julian year) | 9.4607e+12 | 63,241.0771 |
Light travel time across the Solar System
The table gives one-way and round-trip times for a handful of distances. Distances to the planets change constantly because everything is in motion, so the planet rows use each planet’s mean distance from the Sun in AU. They show how long sunlight needs to reach that orbit. For the actual Earth–planet delay on a given day you would need that day’s distance from an ephemeris, which you can type into the calculator above.
| Object | Distance (km, approx.) | Light, one way | Round trip | In light-years |
|---|---|---|---|---|
| Low Earth orbit altitude (about 420 km) | 420 | 1 ms | 3 ms | 4.44e-11 |
| Geostationary orbit altitude | 35,786 | 119 ms | 239 ms | 3.78e-9 |
| Moon (mean distance) | 384,400 | 1.28 s | 2.56 s | 4.06e-8 |
| Sun (1 AU) | 149,597,871 | 8 min 19 s | 16 min 38 s | 1.58e-5 |
| Sun to Mars (mean orbit, 1.52 AU) | 227,388,763 | 12 min 38 s | 25 min 17 s | 2.40e-5 |
| Sun to Jupiter (mean orbit, 5.20 AU) | 777,908,928 | 43 min 15 s | 1 h 26 min | 8.22e-5 |
| Sun to Saturn (mean orbit, 9.58 AU) | 1,433,147,601 | 1 h 20 min | 2 h 39 min | 1.51e-4 |
| Sun to Neptune (mean orbit, 30.1 AU) | 4,502,895,908 | 4 h 10 min | 8 h 21 min | 4.76e-4 |
The values for the Moon and the planets are rounded averages, since orbits are ellipses and the real distance swings above and below them. The Moon, for example, is closer than the mean at perigee and farther at apogee.
Distances to stars and galaxies
| Object | Distance (light-years, approx.) | In km | In AU |
|---|---|---|---|
| Proxima Centauri | 4.2465 | 4.017e+13 | 268,553 |
| Sirius | 8.6 | 8.136e+13 | 543,873 |
| Milky Way (diameter) | 100,000 | 9.461e+17 | 6,324,107,708 |
| Andromeda Galaxy | 2,500,000 | 2.365e+19 | 158,102,692,711 |
Stellar distances are refined as measurements improve, so decimals vary between catalogues, and the figures for the Milky Way and Andromeda are round, order-of-magnitude values. Treat the table as a way to get a feel for the scale; for anything you plan to cite, check a current astronomical catalogue.
Worked examples
Example 1: sunlight. 149,597,870.7 km ÷ 299,792.458 km/s ≈ 499.0 s, which the calculator shows as 8 min 19 s. The sunlight reaching you right now left the Sun a little over eight minutes ago. Because Earth’s orbit is slightly elliptical, the delay ranges from about8 min 11 s when Earth is closest to about 8 min 27 s when it is farthest, using distances of roughly 147.1 and 152.1 million km.
Example 2: a conversation with the Moon. At the mean distance of 384,400 km, light needs1.282 s one way and 2.56 s there and back. That is the minimum gap between a question sent to a lunar base and its answer, before any equipment delay is added.
Example 3: a trip to the nearest star. Proxima Centauri is about 4.2465 light-years away, which is 4.2465 × 9.461e+12 ≈ 4.017e+13 km. A vehicle moving at a constant 10,000 km/h (a deliberately simple assumption) would need about 460,000 years. The gap between “4 years for light” and “hundreds of thousands of years for a fast vehicle” is the real message of interstellar distances.
Example 4: a satellite hop. A geostationary satellite sits about 35,786 km above the equator, so a signal needs at least 0.119 s to go up. A ground-to-satellite-to-ground path of two legs takes at least0.24 s, and the slant path to a place away from the equator is longer than the straight-up altitude, so the true delay is a bit more.
Looking into the past
Because light takes time, you always see objects as they were. The Sun appears as it was about eight minutes ago. Sirius, roughly 8.6 light-years away, shows you light that left it about 8.6 years ago. Light from the Andromeda Galaxy is about two and a half million years old. As a rule of thumb for homework and writing, an object N light-years away is seen as it was about N years ago.
Common mistakes and limits
- A light-year is not a time. “It takes ten light-years to get there” is wrong; “it is ten light-years away” is right.
- Vacuum only. The result assumes a vacuum. In glass fibre, whose refractive index is typically around 1.47, light travels roughly one third slower: 33.4 ms for 10,000 km in vacuum versus about 49 ms in such fibre.
- Not an internet speed test. Real network delay adds routing, equipment and processing time on top of the physical limit, so measured latency is always higher than the number here.
- Relativity is ignored. The calculator does simple division. It does not model time dilation for fast travellers, nor how cosmic expansion changes the meaning of distance for the farthest galaxies.
- Distances are not fixed. The Moon, planets and spacecraft all move, so the same object has a different distance next week. For accurate planning use a distance for the specific date.
Putting the result to use
For astronomy coursework the answer to “how old is the light I am seeing?” is just the distance in light-years read as years. For communication delay, double the one-way time to get the best possible question-and-answer gap, then add whatever processing you expect. For fiction and games, enter a delay you want (for instance a message that takes several minutes to arrive) and use the light-year or AU conversion to find a distance that makes the setting feel consistent.
Frequently Asked Questions
Is anything faster than light?
According to special relativity, an object with mass cannot reach or exceed the speed of light in vacuum, and no signal can carry information faster than it. That is why the speed of light is treated as the cosmic speed limit.
Why is a light-year sometimes quoted as 9.46 trillion km and sometimes as 9,460,730,472,581 km?
They are the same number rounded differently. Using the Julian year of 365.25 days gives 9,460,730,472,580.8 km, and shorter forms simply round that value.
Does sunlight really take 8 minutes and 20 seconds?
With the mean Earth–Sun distance the calculation gives about 499 seconds, or 8 minutes 19 seconds; “8 minutes 20 seconds” is the usual rounded figure. Over the year it varies from roughly 8 minutes 11 seconds to 8 minutes 27 seconds.
Does the expansion of the universe change these numbers?
For very distant galaxies, “how far the light travelled” and “how far away the galaxy is now” are different distances. This calculator does the plain conversion of the distance you enter and does not apply cosmological corrections.
Where can I convert ordinary units?
For general length and speed conversions use the Unit Converter.