A diagram depicting 12ly. Credit: Atlas of the Universe.
A diagram depicting 12ly. Credit: Atlas of the Universe.

What is a Lite Year?

A light-year, also low-cal twelvemonth or lightyear (symbol: ly), is an astronomical unit of length equal to merely nether x trillion kilometres (or nigh 6 trillion miles). As defined past the International Astronomical Marriage (IAU), a light-year is the distance that light travels in a vacuum in one Julian yr. The calorie-free-year is by and large used to express distances to stars and other distances on a galactic scale, especially in non-specialist and popular scientific discipline publications. The preferred unit of measurement in astrometry is the parsec (approximately 3.26 light-years), because it tin can be more than hands derived from, and compared with, observational data.

Annotation that the light-twelvemonth is a measure of distance. It is not a measure of fourth dimension, for which information technology is sometimes mistaken.

Putting Light Years into Perspective:

The closest star to earth  (other than our Sun) is Alpha Centauri, which is four.3 low-cal years away (or approximately 25 trillion miles way). Considering the speed of lite is but 186,000 miles per 2nd, it tin only travel six trillion miles in ane year . Therefore, it takes 4.three years for Alpha Centauri's light to achieve earth. For human being technology, however, the story is quite unlike. The Voyager Spacecraft, for instance, is currently traveling at approximately 38,000 miles per hour. At this speed, it would have Voyager about 77,000 years to reach Alpha Centauri!

So how big is the Observable Universe?

I of the farthest known objects in the observable universe is the milky way MACS0647-JD, which is about thirteen.3 billion lite-years away (or 76,306,276,800,000,002,359,296 sextillion miles away). It would take Voyager more than than the historic period of the universe to get in that location.

MACS0647-JD is a candidate for the farthest known galaxy from Earth. Image from NASA.
MACS0647-JD is a candidate for the uttermost known galaxy from World. Image from NASA.