# What is a light-year?

Sep 4, 2014

A light-year is the distance that light travels in a year.

The speed of light is approximately 300,000,000 m/s or $3 \setminus \times {10}^{8}$ m/s. A more accurate number is 299 792 458 m/s.

We can take the speed of light and figure out how much light travels in one year, assuming 365 days in a year:

$3 \setminus \times {10}^{8} m / s \setminus \times 60 s / \min \setminus \times 60 \min / h r \setminus \times 24 h r s / \mathrm{da} y \setminus \times 365 \mathrm{da} y s / y e a r \setminus \approx 9.46 \setminus \times {10}^{15} m$

Some may prefer this formatting to see the cancellations:
$\frac{3 \times {10}^{8} m}{s} \times \frac{60 s}{\min} \times \frac{60 \min}{h r} \times \frac{24 h r s}{\mathrm{da} y} \times \frac{365 \mathrm{da} y s}{y e a r} \setminus \approx 9.46 \setminus \times {10}^{15} m$