50 Fun Facts About Light-Year

A light-year is a unit of what?

Despite the word year, it measures how far light travels in one Julian year of 365.25 days.

Approximately how many kilometres is one light-year?

That is about 5.88 trillion miles.

Which body's definition of the light-year is based on the Julian year of 365.25 days?

The Julian year is used rather than the 365.2425-day Gregorian year or the tropical year.

Neighbouring galaxies are typically separated by distances of what order?

Galaxies themselves span a few thousand to a few hundred thousand light-years.

What is the IAU abbreviation for light-year?

French, Spanish and Italian use al, and German uses Lj for Lichtjahr.

How many light-years is one Gly, or gigalight-year?

A megalight-year (Mly) is a million light-years and a kilolight-year (kly) is a thousand.

Who made the first successful measurement of the distance to a star other than the Sun, in 1838?

He noted light would take 10.3 years to cross the gap he measured, but declined to use the light-year as a unit.

Which star's distance did Bessel measure using a heliometer designed by Fraunhofer?

Its parallax was 0.314 arcseconds.

The light-year first appeared as a unit in an 1851 popular astronomy article by which German writer?

He compared the odd distance-unit-ending-in-year to a walking hour, or Wegstunde.

Which famous astrophysicist called the light-year 'an inconvenient and irrelevant unit'?

He complained it had crept from popular use into technical work.

A light-second is what fraction of a light-year?

That denominator is the number of seconds in a Julian year of 365.25 days.

Roughly how far does light travel in one nanosecond?

The light-foot is sometimes used informally as a measure of time.

How does the Hayden Planetarium define a light-month?

It is roughly, but not exactly, a twelfth of a light-year.

What is the exact speed of light in a vacuum?

It is exact because the metre is now defined from it.

Roughly what is the speed of light in kilometres per hour?

That is about 700 million miles per hour.

Since which year has the metre been defined by the distance light travels in a fraction of a second?

Before that the metre was defined separately and the speed of light was measured experimentally.

Who made the first quantitative estimate of the speed of light, in 1676?

He noticed the eclipses of Jupiter's moon Io ran early or late depending on Earth's distance from Jupiter.

Which moon of Jupiter did Rømer study to show light has a finite speed?

He estimated light took 22 minutes to cross the diameter of Earth's orbit.

Which ancient Greek thinker first proposed that light has a finite speed?

Aristotle disagreed, arguing light was a presence rather than a movement.

Which scientist proposed in 1638 an experiment using lanterns to time light between two hilltops?

Isaac Beeckman had proposed a cannon-flash-and-mirror version in 1629.

What spinning device did Hippolyte Fizeau use to measure the speed of light on Earth?

He aimed a beam at a mirror 8 km away and got 315,000 km/s; Foucault later used a rotating mirror.

How long does light take to travel from the Sun to the Earth?

Newton gave 'seven or eight minutes' in his 1704 Opticks.

Ground control had to wait at least how long for a reply from Apollo 8 as it orbited the Moon?

The delay was the round-trip light time to the Moon and back.

How long can a one-way radio signal between Earth and Mars take?

A rover in trouble would not be heard from for 4 to 24 minutes, and commands take as long again.

Which physicist proposed in 1865 that light was an electromagnetic wave travelling at c?

Einstein later postulated the speed was the same in every inertial frame.

How far away is Proxima Centauri, the nearest star after the Sun?

That is about 1.3 parsecs; it is too faint to see with the naked eye.

Who discovered Proxima Centauri in 1915?

It has about 12.5% of the Sun's mass.

What kind of star is Proxima Centauri?

It will remain on the main sequence for another four trillion years.

How many light-years is one parsec, approximately?

It is also 206,265 astronomical units.

The parsec is defined as the distance at which one astronomical unit subtends what angle?

The name is a contraction of parallax of one arcsecond.

Who coined the word parsec in 1913?

Dyson had proposed astron and Charlier suggested siriometer, but Turner's name stuck.

Which Star Wars character famously misused the parsec as a unit of time?

He claimed the Millennium Falcon made the Kessel Run in less than 12 parsecs.

One astronomical unit is roughly how many light-seconds?

It was redefined in 2012 as exactly 149,597,870,700 metres.

The astronomical unit was historically conceived as the average distance between which two bodies?

It is now defined as an exact number of metres.

Approximately how far away is the Andromeda Galaxy?

It is the nearest major galaxy to the Milky Way and about 152,000 light-years across.

Which Persian astronomer described the Andromeda Galaxy as a 'small cloud' around 964 CE?

He recorded it in his Book of Fixed Stars.

What is the estimated diameter of the observable universe?

Its radius is about 46.5 billion light-years, larger than the age of the universe times c because space has expanded.

How far is the Solar System from the centre of the Milky Way?

We sit on the inner edge of the Orion Arm.

What is the estimated isophotal diameter of the Milky Way?

The disc is only about 1,000 light-years thick at the spiral arms.

The Toy Story space ranger is named after which real astronaut?

The producers thought Aldrin had the coolest astronaut name; he received no fee.

What was Buzz Lightyear originally going to be before he became a space ranger?

Director John Lasseter thought a one-man band was outdated and wanted something like G.I. Joe.

Which actor turned down the role of Buzz Lightyear and later called it the biggest regret of his career?

Lasseter had used audio from When Harry Met Sally for the screen test.

Who voiced Buzz in the 2022 spin-off film Lightyear?

The film is presented as the in-universe movie that inspired the toy.

On which hostile planet is Buzz marooned in the film Lightyear?

James Brolin voices Zurg, an elderly nihilistic Buzz from an alternate future.

Bessel found 61 Cygni lay about 660,000 astronomical units away. How long did he say light took to cross that gap?

He offered the figure as a mental picture but refrained from using the light-year as a unit.

Otto Ule explained the odd 'year' in light-year by comparing it to which everyday German unit?

His 1851 popular astronomy article is the first known appearance of the unit.

Before 1984, the IAU defined this distance unit using which calendar period, not the Julian one?

The IAU (1964) constants also used a measured, not defined, speed of light.

In German, the light-year is abbreviated how?

It comes from Lichtjahr; French, Spanish and Italian use 'al'.

In 1729, which astronomer used stellar aberration to show light travels about 10,210 times faster than Earth orbits?

That implied light takes 8 minutes 12 seconds to reach Earth from the Sun; the modern figure is 10,066 times.

Rømer's study of Io led him to conclude light takes how long to cross the diameter of Earth's orbit?

Huygens combined this with an orbit-size estimate to get about 220,000 km/s, roughly 27% too low.