# How do scientists measure a light year?

#### MathematicsMeasurementAstronomyAstronomical Distance

Distances between stars and galaxies in the universe are quite literally "astronomical". For example, the distance from the Earth to our Sun is 149,597,871 km, which is actually quite small on the scale of the universe. Our solar system is just about 40 trillion kilometers to the nearest star, Proxima Centauri and the distance between our galaxy, the Milky Way, and the nearest galaxy, the Canis Major Dwarf galaxy, is about 400 quadrillion kilometers!

Did You Know?
the distance from the Earth to the Sun is also a unit used by astronomers and is known as an astronomical unit.

As you can see, these numbers are really large and the use of kilometers is no longer a sensible unit for measuring distances. To overcome this, astronomers came up with the light-year. One light-year is the distance light travels through space in one year. This value can be calculated by multiplying the speed that light travels in a vacuum (remember space is nearly a perfect vacuum), which is about 299,792 kilometers per second (km/s) by the number of seconds in a year: 31,557,600 s. Doing so gives you about 9,461,000,000,000 km which is more than nine trillion kilometers!

Did You Know?
the first successful attempt to determine the speed of light was done in the late 17th century and was found to be about 220,000 km/s, not too far off from the currently accepted value.

In order to convert distances measured in kilometers to light-years, you divide the distance in kilometers by 9,461,000,000,000 km per light-year. Considering the numbers above, the distance from our solar system to Proxima Centauri is 4.2 light-years, and the distance from the Milky Way to the Canis Major Dwarf galaxy is 42,000 light-years.

Did You Know?
the Milky Way galaxy is about 100,000 light-years across.

Since it takes light 4.2 years to reach the Earth from Proxima Centauri, the light that we are currently looking at from the star itself actually left the star 4.2 years ago. What does this mean? We are looking back in time! Today, scientists are able to see light from the universe from when it was just a newborn, which has taken nearly 14 billion years to reach us. Imagine what we can find out by looking 14 billion years back into the history of our universe!

http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/SpeedOfLight/measure_c.html

http://en.wikipedia.org/wiki/Speed_of_light#cite_note-NIST_heterodyne-100

http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html