Light-Year
Distance light travels in a given time.
Formula
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Example
1 ly ≈ 9.46×10¹² km
How it works
A light-year is not a unit of time; it is a unit of distance. The formula d = c × t works because speed means "distance per time," so multiplying the speed of light by a time gives the distance light covers in that time. In astronomy, this helps describe enormous distances using the fastest known speed in the universe.
When to use it
Use this formula when you want to convert a time interval into the distance light travels, or when interpreting light-year distances in space. It is especially useful for comparing distances between stars, galaxies, and other very large cosmic objects.
Worked example, step by step
Find the distance light travels in 2 hours. Step 1: Write the formula: d = c × t Step 2: Use c = 299,792,458 m/s and t = 2 hours = 7,200 s Step 3: Substitute: d = 299,792,458 × 7,200 Step 4: Calculate: d = 2,158,505,097,600 m Step 5: Convert to kilometres: d = 2,158,505,097.6 km Final answer: light travels about 2.16 × 10^9 km in 2 hours.
Common mistakes to avoid
- Treating a light-year as a measure of time instead of distance
- Using minutes or hours without converting them into seconds first
- Forgetting that the speed of light is constant in vacuum and should be used with consistent units
Did You Know?
A light-year is about 9.46 trillion km — the nearest star beyond the Sun is over 4 of them away.
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