# Light year facts for kids

Kids Encyclopedia Facts

A light year (symbol: ly) is the distance that light travels in empty space in one year. Since the speed of light is about 300,000 km per second (about 186,000 miles per second), then a light year is about 10 trillion kilometers (about 6 trillion miles). A light year is not a length of time.

The light year is used in astronomy because the universe is huge. Space objects such as stars and galaxies may be hundreds, thousands or millions of light years away.

Think of a star at a distance of 100 light years from us on Earth. Light leaves the star and takes 100 years to get to us. When we see the star's light, we are seeing that star as it was 100 years ago.

## History

The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a 160-millimetre (6.2 in) heliometre designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9×1013 km; 6.1×1013 mi). Bessel added that light takes 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

The speed of light was not yet precisely known in 1838; the estimate of its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.

The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.

Although modern astronomers often prefer to use the parsec, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.

## Usage of term

Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.

List of orders of magnitude for length
Scale (ly) Value Item
10−9 4.04×10−8 ly Reflected sunlight from the Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly 350000 to 400000 kilometres).
10−6 1.58×10−5 ly One astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.
1.27×10−4 ly The Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.
5.04×10−4 ly New Horizons encounters Pluto at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−3 2.04×10−3 ly The most distant space probe, Voyager 1, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014. It will take about 17500 years to reach one light-year at its current speed of about 17 km/s (38000 mph, 61 200 km/h) relative to the Sun. On September 12, 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on August 25, 2012, becoming the first manmade object to leave the Solar System.
2.28×10−3 ly Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.
100 1.6×100 ly The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 au, with its outer edge at 100000 au.
2.0×100 ly Maximum extent of the Sun's gravitational dominance (Hill sphere/Roche sphere, 125000 au). Beyond this is the deep ex-solar gravitational interstellar medium.
4.24×100 ly The nearest known star (other than the Sun), Proxima Centauri, is about 4.24 light-years away.
8.6×100 ly Sirius, the brightest star of the night sky. Twice as massive and 25 times more luminous than the Sun, it outshines more luminous stars due to its relative proximity.
1.19×101 ly HD 10700 e, an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of star Tau Ceti.
2.05×101 ly Gliese 581, a red-dwarf star with several detectable exoplanets.
3.1×102 ly Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9 bright giant 10700 times more luminous than the Sun.
103 3×103 ly A0620-00, the second-nearest known black hole, is about 3000 light-years away.
2.6×104 ly The centre of the Milky Way is about 26000 light-years away.
1×105 ly The Milky Way is about 100000 light-years across.
1.65×105 ly R136a1, in the Large Magellanic Cloud, the most luminous star known at 8.7 million times the luminosity of the Sun, has an apparent magnitude 12.77, just brighter than 3C 273.
106 2.5×106 ly The Andromeda Galaxy is approximately 2.5 million light-years away.
3×106 ly The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.
5.9×107 ly The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.
1.5×1082.5×108 ly The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).
109 1.2×109 ly The Sloan Great Wall (not to be confused with Great Wall and Her–CrB GW) has been measured to be approximately one billion light-years distant.
2.4×109 ly 3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1. 3C 273 is about 2.4 billion light-years away.
4.57×1010 ly The comoving distance from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving radius of the observable universe. This is larger than the age of the universe dictated by the cosmic background radiation; see here for why this is possible.

## Calculation

The way to calculate the light-year distance:

$299 792 458 \times 60 \times 60 \times 24 \times 365.25$

where:

• 299 792 458 is the number of meters light travels in one second
• 60 is the number of seconds in a minute
• The other 60 is the number of minutes in an hour
• 24 is the number of hours in a day
• 365.25 is the number of days in a Julian year

## Related units

• Light-second - used in astronomy, telecommunications and relativistic physics, is exactly 299792458 metres or 131557600 of a light-year.
• Light minute - The distance that light travels in one minute (about 18,000,000 km per minute, or 11,160,000 miles per minute).
For example, our Sun is about 8 light minutes from Earth
• Light second - The distance that light travels in one second (about 300,000 km per second, or 186,000 miles per second)
For example, our Moon is about 1¼ light seconds from Earth
• Light-month - used occasionally for approximate measures. It's roughly one-twelfth of a light-year. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.