Light-Year
Distance light travels in one year.
Formula
Variables
Example
≈ 9.46×10¹² km
How it works
A light-year is a unit of distance, not time: it tells you how far light travels in one year at the constant speed c. Since distance equals speed multiplied by time, multiplying the speed of light by one year gives a standard astronomical distance. This is useful because space is so large that kilometers or miles would be unwieldy for measuring gaps between stars and galaxies.
When to use it
Use this when comparing distances between stars, galaxies, or other large-scale cosmic objects. It is also helpful when converting between the time light takes to travel and the distance across space.
Worked example, step by step
Take the speed of light as c = 299,792,458 m/s One year ≈ 365.25 days × 24 hours × 3600 s = 31,557,600 s Distance = c × time Distance = 299,792,458 m/s × 31,557,600 s Distance ≈ 9.4607 × 10^15 m Convert to km: 9.4607 × 10^15 m ÷ 1000 = 9.4607 × 10^12 km So, 1 light-year ≈ 9.46 × 10^12 km
Common mistakes to avoid
- Thinking a light-year is a measure of time rather than distance
- Using 365 days instead of accounting for the approximate length of a year
- Forgetting to convert meters to kilometers when comparing with large astronomical distances
Did You Know?
A light-year is about 9.5 trillion kilometres, yet the nearest star beyond the Sun is over four of them away.
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