What Is A Light Year?

Vast distances in interstellar space are measured in light years

What is a light year? The short answer to that question is that a light year is a measure of distance – the distance that light can travel in a year, 365.25 days. Light travels at the speed of 186282 miles per second in the vacuum of interstellar space. So in a year’s time light will travel about 5.8786 trillion miles; this is roughly 63240 times the mean distance of the Earth from the Sun(93000000 miles), 2.461 million times the mean distance of the moon from the Earth(238855 miles), and 23.608 million times the distance around the Earth at the equator. These are gigantic numbers which give us some indication of how vast the distance between stars is in interstellar space – a light year is easily the most practical way to measure these incredible distances.

Light Year Distance

The vast distances of interstellar and intergalactic space

The gigantic and vast distance of a light year, 5.8786 trillion miles, has already been established with a few examples given. Let’s go ahead and give some more examples of different orders of magnitude.

Light traveling from the Earth to the Moon will take an average of about 1.28 seconds, assuming the Moon is at its mean distance of 238855 miles.

It takes about 8.32 minutes for light to travel the mean distance of the Earth from the Sun, 93000000 miles.

Assuming a mean distance of Saturn from Earth of 793.8 million miles(the distance will vary greatly according to which side of the Sun Saturn is on) it takes light about 1.1834 hours – 1 hour and 11 minutes – to travel this distance.

Radio waves from the New Horizons space probe took about 4.417 hours, 4 hours and 25 minutes, to travel 2.92 billion miles from Pluto to the Earth. Note that radio waves are part of the electromagnetic spectrum as is light waves and so travel at exactly the same speed.

The space probe Voyager 1 has traveled farther from Earth than any other spacecraft, about 13.3 billion miles as of early 2019 – it entered the interstellar medium of space in late August 2012 becoming the first manmade object to leave the Solar System. At this great distance, it takes light about 19.63 light hours( 19 hours and 38 minutes) to travel this great distance.

Interstellar Space

The depths of interstellar space

The Oort Cloud

The Oort Cloud is a cloud of icy planetesimal bodies, including comets, which lies far beyond the heliosphere of the Sun and the Kuiper belt in interstellar space. Its inner boundary is estimated at about 4.65 trillion miles(.79 light years) and its outer boundary is estimated at about 9.3 trillion miles(1.58 light years). In fact, the Oort Cloud is about one thousand times further away from the Sun than the Kuiper belt is. The Voyager 1 probe won’t reach the Oort Cloud for about another 300 years. Although not known precisely, it is estimated that the Oort Cloud is about two light years in diameter. An interesting fact here is that the Sun’s gravitational reach is about 11.63 trillion miles – about 1.98 light years. It is at this point that the gravitational effect of the interstellar medium, the matter and electromagnetic radiation between star systems, and of the stars themselves starts to become dominant.

Proxima Centauri

Proxima Centauri, a low mass red dwarf star, is the nearest known star to our Solar System and is about 4.22 light years away.


Sirius is the brightest star in Earth’s night sky

Sirius is the brightest star in the sky with an apparent magnitude of -1.46, almost twice as bright as Canopus, the next brightest star. It has twice the mass and is about 25 times as luminous as our Sun. Sirius is approximately 8.7 light years from Earth.

Tau Ceti

One of the planets of Tau Ceti could have some form of alien life

Tau Ceti is a star about 11.9 light years from Earth and is currently believed to have 5 planets, one of which is 6.6 times the mass of Earth and is in a habitable zone around the star meaning it could have some form of life.


Canopus, the second brightest star in the night sky and the brightest star in the southern hemisphere(apparent magnitude -.74), is a yellow giant star which is 310 light years from Earth. It is over 13000 times as luminous as our Sun and over 65 times the size.

Black Hole

A0620-00 is the closest known black hole to Earth and is in a binary star system in the constellation Monoceros – it is about 3000 light years from Earth.

Milky Way

Stars in our Milky Way galaxy fill the night sky

The center of our Milky Way galaxy is about 26000 light years from Earth, and the diameter of the Milky Way galaxy is about 100000 light years.

Large Magellanic Cloud

The Large Magellanic Cloud is actually a satellite galaxy of our Milky Way and is about 163000 light years away.

Andromeda Galaxy

The Andromeda galaxy is the closest major galaxy to Earth

The Andromeda galaxy, about 2.544 million light years from Earth, is the closest major galaxy. Its name comes from the fact that it can be seen in the constellation of Andromeda.

Triangulum Galaxy

The Triangulum Galaxy is the furthest known object in the night sky that is visible to the naked eye and is about 3 million light years from Earth.

Virgo Cluster

The Virgo Cluster is a group of about 1300 galaxies grouped together and is the closest major galaxy cluster to Earth, about 59 million light years away.

Sloan Great Wall

The Sloan Great Wall is a strange cosmic structure that is formed by a gigantic wall of galaxies – it is about one billion light years from Earth.


3C-273 is the brightest quasar optically visible from Earth with an apparent magnitude of 12.9 – it is about 2.4 billion light years away.

The Universe

The comoving distance from the Earth to the edge of the observable Universe is currently believed to be about 45.7 billion years. Note that this is much greater than the current estimate for the age of the Universe, about 13.8 billion years, possibly because of the nontrivial topology of the Universe which may cause light to circumnavigate different circles of a smaller sphere of the Universe causing a different light year value. Thus the furthest part of the Universe which can currently be observed is roughly 13.8 billion light-years away.

Light Seconds – Light Years – Light – Centuries – How To Measure Extreme Distances Video

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