Light-Years and Cosmic Distances
A light-year is the distance light travels in one year.
About this topic
A light-year is the distance light travels in a year, about 9.46 trillion km. It measures distance, not time. Looking at distant objects means looking back in time: the light from a star 100 light-years away left 100 years ago.
Other units: the astronomical unit (Earth–Sun distance, 150 million km) and parsec (3.26 ly). Methods of measuring distance include parallax for nearby stars, standard candles (Cepheid variables, supernovae) and redshift. The observable universe is about 93 billion light-years across.
An example: Proxima Centauri is 4.2 light-years away, which is about 40 trillion km. Even the fastest spacecraft ever built, the Parker Solar Probe, at about 190 km/s, would need about 6,600 years to get there, and Voyager 1 would take about 75,000 years. Light from the Sun takes 8 minutes 20 seconds to reach Earth, so we always see the Sun as it was a little while ago. Light from the Andromeda galaxy is 2.5 million years old, which means we see it as it was before humans existed.
Parallax is the apparent shift of a nearby star against distant background stars as Earth moves around its orbit, and the angle in arcseconds is the inverse of the distance in parsecs: a star with a parallax of 0.1 arcseconds is 10 parsecs away. Cepheid variable stars pulse at a rate related to their true brightness, giving a ladder of distances. Students often use the light-year as a measure of time because of its name. Also remember that the edge of the observable universe is larger than 13.8 billion light-years because space has expanded while the light travelled.
Key ideas
- About 9.46 trillion km
- Nearest star Proxima is 4.2 ly away
- Looking far means looking back in time
Quick check
Use cases: where Light-Years and Cosmic Distances helps
- Homework: review the key idea behind Light-Years and Cosmic Distances before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Light-Years and Cosmic Distances without notes.
- Visual learning: play with the interactive lab to see how Light-Years and Cosmic Distances changes when you adjust its inputs.
- Teaching a friend: explain Light-Years and Cosmic Distances in your own words, then compare your version with the key ideas.
Earth science helps with weather forecasts, natural hazards, energy and climate planning.
Frequently asked questions
What is Light-Years and Cosmic Distances?
A light-year is the distance light travels in one year.
What will I learn in Light-Years and Cosmic Distances?
About 9.46 trillion km. Nearest star Proxima is 4.2 ly away. Looking far means looking back in time.
Is Light-Years and Cosmic Distances part of Earth & Space?
Yes. It is one of 60 Earth & Space topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Light-Years and Cosmic Distances lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Light-Years and Cosmic Distances?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.