lesson
A light-year measures distance
1 · Learn
A light-year is the distance light travels in vacuum in one year, not a unit of time. Distant light takes time to reach us, so observing a distant object shows light emitted earlier.
Avoid confusing travel time with a current snapshot at the source. A simplified example can ignore expansion and relative motion, but those assumptions should be clear.
2 · Worked example
In a simple stationary model, light from an object 10 light-years away takes 10 years to reach us.
3 · Your turn
In the same simple model, how old is light arriving from an object 4 light-years away?
Check your answer
It was emitted about 4 years earlier.
4 · Apply your learning
Compare metres, seconds and light-years, explaining which quantities each unit measures.
Adult guidance and safety
Use supplied evidence and approved models. Practical science needs a qualified adult, appropriate equipment and a risk assessment. Do not attempt chemical reactions, heating, mains electricity or solar observation independently. Never request personal health or genetic disclosures.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
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