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50 free Light-Year trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This light year trivia quiz covers the most misunderstood unit in astronomy: a measure of distance that sounds like a measure of time. The easy questions are the ones that clear up the confusion: what a light-year actually measures, roughly how many kilometres it is, how long sunlight takes to reach Earth, and how far away the nearest star is. From there it moves into the history: the German writer who first used the term in 1851, the astronomer who measured the first stellar distance but refused to use the unit, and the Danish astronomer who first showed light has a finite speed at all. The harder end is for space enthusiasts: the exact IAU definition of the light-year, why professional astronomers prefer the parsec and who coined that word, the 1983 redefinition of the metre, Fizeau's cogwheel, the communication delay to Apollo 8 and to Mars, the size of the Milky Way and Andromeda in light-years, and the 93-billion-light-year diameter of the observable universe. Because you searched for it, there are also a few questions on Buzz Lightyear, the space ranger named after a real astronaut. Every answer was checked against encyclopaedia entries on the light-year, the parsec, the speed of light and the objects mentioned before publishing, and the supporting sentence is stored with each question. If you enjoy this one, try our astronomy, solar system and space exploration quizzes next.
30 of 50 questions with answers and explanations. Play the quiz
Q 01A light-year is a unit of what?
Length
Despite the word year, it measures how far light travels in one Julian year of 365.25 days.
Q 02Approximately how many kilometres is one light-year?
9.46 trillion
That is about 5.88 trillion miles.
Q 03Which body's definition of the light-year is based on the Julian year of 365.25 days?
The International Astronomical Union
The Julian year is used rather than the 365.2425-day Gregorian year or the tropical year.
Q 04Neighbouring galaxies are typically separated by distances of what order?
Millions of light-years
Galaxies themselves span a few thousand to a few hundred thousand light-years.
Q 05What is the IAU abbreviation for light-year?
ly
French, Spanish and Italian use al, and German uses Lj for Lichtjahr.
Q 06How many light-years is one Gly, or gigalight-year?
A billion
A megalight-year (Mly) is a million light-years and a kilolight-year (kly) is a thousand.
Q 07Who made the first successful measurement of the distance to a star other than the Sun, in 1838?
Friedrich Bessel
He noted light would take 10.3 years to cross the gap he measured, but declined to use the light-year as a unit.
Q 08Which star's distance did Bessel measure using a heliometer designed by Fraunhofer?
61 Cygni
Its parallax was 0.314 arcseconds.
Q 09The light-year first appeared as a unit in an 1851 popular astronomy article by which German writer?
Otto Ule
He compared the odd distance-unit-ending-in-year to a walking hour, or Wegstunde.
Q 10Which famous astrophysicist called the light-year 'an inconvenient and irrelevant unit'?
Arthur Eddington
He complained it had crept from popular use into technical work.
Q 11A light-second is what fraction of a light-year?
1/31,557,600
That denominator is the number of seconds in a Julian year of 365.25 days.
Q 12Roughly how far does light travel in one nanosecond?
One foot
The light-foot is sometimes used informally as a measure of time.
Q 13How does the Hayden Planetarium define a light-month?
30 days of travel time
It is roughly, but not exactly, a twelfth of a light-year.
Q 14What is the exact speed of light in a vacuum?
Q 21What spinning device did Hippolyte Fizeau use to measure the speed of light on Earth?
A cogwheel
He aimed a beam at a mirror 8 km away and got 315,000 km/s; Foucault later used a rotating mirror.
Q 22How long does light take to travel from the Sun to the Earth?
8 minutes 19 seconds
Newton gave 'seven or eight minutes' in his 1704 Opticks.
Q 23Ground control had to wait at least how long for a reply from Apollo 8 as it orbited the Moon?
Three seconds
The delay was the round-trip light time to the Moon and back.
299,792,458 m/s
It is exact because the metre is now defined from it.
Q 15Roughly what is the speed of light in kilometres per hour?
1 billion
That is about 700 million miles per hour.
Q 16Since which year has the metre been defined by the distance light travels in a fraction of a second?
1983
Before that the metre was defined separately and the speed of light was measured experimentally.
Q 17Who made the first quantitative estimate of the speed of light, in 1676?
Ole Rømer
He noticed the eclipses of Jupiter's moon Io ran early or late depending on Earth's distance from Jupiter.
Q 18Which moon of Jupiter did Rømer study to show light has a finite speed?
Io
He estimated light took 22 minutes to cross the diameter of Earth's orbit.
Q 19Which ancient Greek thinker first proposed that light has a finite speed?
Empedocles
Aristotle disagreed, arguing light was a presence rather than a movement.
Q 20Which scientist proposed in 1638 an experiment using lanterns to time light between two hilltops?
Galileo
Isaac Beeckman had proposed a cannon-flash-and-mirror version in 1629.
Q 24How long can a one-way radio signal between Earth and Mars take?
Five to twenty minutes
A rover in trouble would not be heard from for 4 to 24 minutes, and commands take as long again.
Q 25Which physicist proposed in 1865 that light was an electromagnetic wave travelling at c?
James Clerk Maxwell
Einstein later postulated the speed was the same in every inertial frame.
Q 26How far away is Proxima Centauri, the nearest star after the Sun?
4.25 light-years
That is about 1.3 parsecs; it is too faint to see with the naked eye.
Q 27Who discovered Proxima Centauri in 1915?
Robert Innes
It has about 12.5% of the Sun's mass.
Q 28What kind of star is Proxima Centauri?
Red dwarf
It will remain on the main sequence for another four trillion years.
Q 29How many light-years is one parsec, approximately?
3.26
It is also 206,265 astronomical units.
Q 30The parsec is defined as the distance at which one astronomical unit subtends what angle?
One arcsecond
The name is a contraction of parallax of one arcsecond.