Define a light-year as a distance
Define a light-year by the distance light travels in vacuum during one year.
Section 6.2.2 · Stars
Core and Extended; Physics 0625, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked.
Before you begin
- Use distance = speed × time.
- Distinguish metres from seconds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does speed multiplied by time give?
What you will learn
- Define a light-year by the distance light travels in vacuum during one year.
- Interpret astronomical light travel without confusing distance with duration.
Read the unit’s definition
A light-year is a unit of distance: the distance light travels in vacuum in one year. Its name includes a time interval because that interval defines a travel distance at a specified speed. It is not a time unit like a year or a second.
A star described as several light-years away is assigned a distance, not an age. Light received now left a distant star earlier. In a simple stationary-distance model, its one-way travel duration in years numerically matches its distance in light-years.
Name the path and model
One-way reception and a request followed by a reply have different total paths. A light-year is not a fixed waiting time for every possible journey: a signal crossing two light-years has twice the one-year path at vacuum light speed.
Travel-time interpretations assume a stated separation and propagation model. They do not automatically include a spacecraft’s slower travel speed, processing waits, moving endpoints or expansion over vast cosmological paths. Quantitative metre conversion is Supplement work in the next lesson.
Pause and explain
Which quantity is measured in light-years?
Worked example
A stationary beacon is four light-years away. What does the distance mean?
Show the worked solution
- One light-year is a one-year vacuum-light path.
- Four light-years is four such path lengths.
- One-way light takes four years in the stated model.
Answer A distance equal to four one-year vacuum-light paths; one-way light time four years.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define one light-year in words without giving a metre conversion.
Give me a hint
Name what travels, the medium condition and the defining interval.
Compare my reasoning
- Identify light.
- Specify propagation in vacuum.
- Use an interval of one year to define its travelled distance.
The distance light travels in vacuum in one year.
Look for these in your work
- State distance, not time.
- Include vacuum and one year.
A static teaching beacon is six light-years away. When did light received now leave it?
Give me a hint
Use the one-way path in the specified stationary model.
Compare my reasoning
- Six light-years is six one-year light paths.
- The travel time is six years.
- Reception occurs after emission.
Six years earlier in the stated model.
Look for these in your work
- Identify earlier emission.
- Retain the model assumption.
A headline says a star at eight light-years must be eight years old. Evaluate it.
Give me a hint
Distance and stellar age are independent descriptions.
Compare my reasoning
- Eight light-years is a distance.
- The observation refers to earlier emitted light.
- It supplies no birth date or evolutionary lifetime.
The distance does not establish the star’s age.
Look for these in your work
- Distinguish distance from age.
- Explain what delayed reception means.
Common mistakes
- A light-year is not a year.
- Received light describes an earlier emission.
- Distance alone does not determine stellar age.
What can you explain now?
Which quantity is measured in light-years?
Compare with the explanation
Distance
The defining year is used to construct a distance travelled by light.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Interpret astronomical light travel without confusing distance with duration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.