One Light
How far has light travelled since a given day?
Pick a date and work out how far light could have travelled since then, using the exact speed of light. Comes with a table of distance scales from the Moon to Sirius and a small calculator that runs in your browser.
The calculation
Light in a vacuum travels at exactly 299,792.458 km per second. That value is exact by definition in the SI, not a measurement with error bars. So the distance light covers in any stretch of time is just the elapsed seconds multiplied by that speed.
A light-year is the distance light covers in one Julian year of 365.25 days, which is 31,557,600 seconds. That works out to about 9.46 trillion km, or 63,241 times the Sun–Earth distance.
distance (light-years) = seconds since the date ÷ 31,557,600
distance (AU) = seconds since the date ÷ 499.005
The second line uses the fact that light takes 499.005 seconds (8 min 19 s) to cross one astronomical unit, the mean distance from the Sun to Earth, which the IAU fixed in 2012 at exactly 149,597,870.7 km.
Try it
If you enter only a day, the calculator does what the One Light app does: it treats the moment as falling somewhere in that whole day, shows the midpoint, and only counts a distance scale as passed once even the latest possible moment has passed it.
Distance scales, and how long light needs to reach them
| Distance scale | How far | Light needs |
|---|---|---|
| Earth–Moon mean distance | ≈ 384,000 km (1.28 light-seconds) | 1.28 seconds |
| 1 astronomical unit (mean Sun–Earth distance) | 149,597,870.7 km | 8.3 minutes |
| Neptune's average orbital distance | 30.06 AU | 4 hours 10 minutes |
| Voyager 1's heliopause crossing | 121.7 AU | 16.9 hours |
| Estimated inner Oort Cloud begins | ≈ 2,000 AU | 11.6 days |
| Estimated outer Oort Cloud begins | ≈ 10,000 AU | 57.8 days |
| One light-year | 63,241 AU | 1 year |
| Proxima Centauri, the nearest star | 4.25 light-years | 4 years 91 days |
| Sirius, the Dog Star | ≈ 8.60 light-years (± 0.036) | about 8 years 7 months (± 13 days) |
Two things to notice. First, how quickly the solar system ends: light covers Neptune’s distance in about four hours and Voyager 1’s in under a day. Second, how slowly the stars arrive. After a whole year, light has covered about 11.6% of the way to Sirius.
A distance scale, not a destination
None of these numbers says that light from any particular place actually passed the Moon or Neptune. Light leaving Earth goes in every direction, and in most of them there is nothing. What the table gives is a scale: in this much time, light can cover this much space. That is the honest form of the statement, and it is the one used throughout the app.
Why Sirius is given as a window, not a date
Sirius’s distance comes from its parallax measured by the Hipparcos satellite: 379.21 milliarcseconds, ± 1.58. Turning that into a distance gives 8.60 light-years with an uncertainty of about ± 0.036 light-years, which is roughly ± 13 days of light travel. So any “arrival” at the Sirius distance is honestly a window of about 26 days, and anyone who gives you a single exact date is claiming more precision than the measurement has.
Questions
How far does light travel in one day?
About 173 astronomical units, or roughly 25.9 billion km. That is nearly six times Neptune's distance from the Sun.
How many kilometres is a light-year?
About 9.46 trillion km: 299,792.458 km/s multiplied by the 31,557,600 seconds of a Julian year.
How long does light take to reach Sirius?
About 8.60 years, with a measurement uncertainty of roughly ± 13 days.
- Speed of light 299,792.458 km/s (SI exact); astronomical unit 149,597,870.7 km (IAU 2012 Resolution B2); light-year = light travelling one Julian year of 365.25 days (IAU). The same constants the One Light app computes with.
- Moon, AU and Neptune distances: NASA, Basics of Space Flight and NASA’s Webb explainer (1.28 light-seconds).
- Voyager 1 heliopause crossing at 121.7 AU: NASA Voyager 1.
- Oort Cloud inner/outer edges (estimates): NASA, Oort Cloud facts.
- Proxima Centauri 4.25 light-years: NASA, What is a light-year?
- Sirius: Hipparcos parallax 379.21 ± 1.58 mas → 2.637 pc ≈ 8.60 ± 0.036 light-years (ESA Hipparcos).
Also in this series
One Light is a quiet pet memorial built on this calculation: from the day you said goodbye, it shows how far light has travelled since, using Sirius as the measuring rod. A calculation, not a claim about where they are.