You might be surprised to know that a light-year is just one normal Earth year long (a Julian year of 365.25 days)! Confused? Think of it this way, a light-year isn’t a measure of time, but a measure distance within a certain amount of time (a year).
If you’ve kept up with all my other posts on space, you’ll know that distances get pretty big in space. Even within our own solar system, common units of measurement start to break down and become impractical. We’ve discussed the use of the Astronomical Unit as a compensation for the vast distances in our own solar system, but when you move beyond our solar system and start working with the distances between stars, even the AU stops being practical. Enter the light-year which quite simply uses the distance light travels in one year as a means of measuring things that are really far apart.
Our previous post gave the speed of light as 186,282 miles per second (and some change). To determine how far light travels in one year, multiply 186,282 by 60 to get a Light-minute of 11,176,943.82 miles per minute. Now multiply that figure by 60 to get a light-hour of 670,616,629.2 miles per hour. Multiply that figure by 24 to get a light-day of 16,094,799,100.8 miles per day. Finally, multiply that figure by 365.25 and you wind up with a light year which is a measure of distance covering roughly 5,878,625,371,567.2 miles per year.
So how does that stack up in our solar system? To reach each of the planets and Kuiper Belt objects like Pluto it takes light leaving the Sun:
- Mercury - About 3 minutes
- Venus - About 6 minutes
- Earth - About 8 minutes
- Mars - About 12 ½ minutes
- Jupiter - About 43 minutes
- Saturn - About 1 1/3 hours
- Uranus - Just shy of 3 hours
- Neptune - A bit over 4 hours
At 4.583 billion miles from the Sun (at maximum distance) it takes light from the surface of the Sun a little under 7 hours to reach Pluto.