The large yellow shell depicts a light-year; the smaller yellow covering depicts a light-month. Read more about this image at Wikimedia Commons.

You are watching: Difference between astronomical unit and light year

Objects in our cosmos are extremely far away. They’re so far away the kilometers or miles aren’t a advantageous measure of your distance. So we speak of space objects in regards to light-years, the distance light travel in a year. Light is the fastest-moving ingredient in our universe. It travels at 186,000 miles per 2nd (300,000 km/sec). And thus a light-year is 5.88 sunshine miles (9.46 trillion kilometers).

But stars and also nebulae – no to mention distant galaxies – room vastly farther 보다 one light-year away. And, if we try to refer a star’s distance in miles or kilometers, we soon finish up through impossibly large numbers. However miles and kilometers are what many of us use to comprehend the distance from one location on planet to another. In the late 20th century astronomer Robert Burnham, Jr. – author of Burnham’s Celestial Handbook – devised one ingenious method to portray the street of light-years in regards to miles and kilometers.

Keep reading, because that a method to recognize the vastness the the universe, using systems of street we know and also use every day.

Burnham began by relating the light-year come the astronomical unit – the Earth-sun distance.

One huge unit, or AU, equals around 93 million mile (150 million km).

Another method of looking in ~ it: the astronomical unit is a bit an ext than 8 light-minutes in distance.

A irradiate beam bring away 8 minutes to travel the 93 million miles (150 million km) native the sunlight to the Earth. Photo via Brews Ohare/ Wikimedia Commons.

A light-year, pictured together a mile

Robert Burnham i found it that, fairly by coincidence, the variety of astronomical systems in one light-year and the number of inches in one mile space virtually the same.

For basic reference, there space 63,000 huge units in one light-year, and also 63,360 customs (160,000 cm) in one mile (1.6 km).

This exorbitant coincidence permits us to lug the light-year under to Earth. If we scale the astronomical unit – the Earth-sun street – at one inch, climate the light-year top top this range represents one mile (1.6 km).

The closest star come Earth, various other than the sun, is Alpha Centauri at part 4.4 light-years away. Scaling the Earth-sun distance at one inch locations this star in ~ 4.4 miles (7 km) distant.


The red star in the facility of this picture is Proxima Centauri, our sun’s nearest neighbor among the stars. A beam of irradiate from this star takes around 4 years to take trip to Earth. Image via Max Planck Institute for Extraterrestrial Physics.

Familiar space objects, conceptualized

Scaling the expensive unit in ~ one inch (2.5 cm), below are distances to various bright stars, star clusters and also galaxies:

Alpha Centauri: 4.4 mile (6.4 km)

Sirius: 8.6 mile (14 km)

Vega: 25 miles (40 km)

Pleiades open up star cluster: 444 mile (715 km)

Antares: 555 mile (893 km)

Hercules globular star cluster (aka M13): 22,200 mile (35,700 km)

Center of ours Milky way galaxy: 26,100 mile (42,000 km)

Great Andromeda galaxy (M31): 2,540,000 miles (4,100,000 km)

Sombrero galaxy (M104): 28,000,000 miles (45,000,000 km)

Whirlpool galaxy (M51): 31,000,000 mile (50,000,000 km)

And so on, ago to around 13 billion+ light-years to the the furthest galaxies: 13,000,000,000 miles (21,000,000,000 km)

Okay, the numbers room still quite big! yet hopefully castle can help you check out that our world is really vast.

There space 33 stars within 12.5 light years of ours sun. Click into Atlas the the world to discover an interactive web page that’ll allow you save zooming farther and farther out. Image via Atlasoftheuniverse.com.

The fastest-moving stuff in the universe

As mentioned above, irradiate travels at an remarkable 186,000 mile per second (300,000 km/sec). That’s really fast. If you can travel in ~ the rate of light, girlfriend would have the ability to circle the Earth’s equator about 7.5 time in simply one second!

In other words, a light-second is the distance light travel in one second, or 7.5 times the distance approximately Earth’s equator. A light-year is the street light travels in one year.

How much is that? main point the variety of seconds in one year by the variety of miles or kilometers that light travel in one second, and also there you have actually it: one light-year. It’s around 5.9 trillion miles (9.5 sunshine km).

This scale starts close to home however takes us all the means out come the Andromeda Galaxy, the most far-off object most world can see through the unaided eye. Image via Bob King/ Skyandtelescope.com.

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Bottom line: Here’s a means to recognize the range of light-years in miles and kilometers.