# Question #da174

Jan 26, 2018

Light minutes (light years, also) is simply used as a measurement unit for distance, mainly those on such a massive scale that our traditional units are no longer useful.

#### Explanation:

Light has a finite velocity of roughly $3.0 \cdot {10}^{8}$ meters per second. This is a MASSIVE velocity (enough to travel around the earth's circumference about 7.5 times in a second), and is as good as infinite within the reference frame of our earth.

When considering the scale of the cosmos, however, we deal with absolutely gargantuan distances. For example, the distance from the earth to Jupiter is 588 million kilometers ($5.88 \cdot {10}^{11}$ meters). This is where we have to appreciate that light does, in fact, have a finite velocity.

Since these distances are so massive, our common units for length become impractical to use: the thought of trying to size up distances between galaxies using kilometers, for example, is laughable. This is where we use light itself as a unit of length. You may have heard of planets being light years away, which just means that it takes light itself a certain number of years to reach that place.

The sun is 149.6 million kilometers from the earth ($1.5 \cdot {10}^{11}$ meters). Setting up a simple constant velocity problem yields that it will take light roughly 500 seconds to reach earth, which equates to 8.33 minutes.

This is why we say the sun is 8.33 light minutes away from earth.

Jan 26, 2018

A light minute is the distance that light travels in one minute. It takes 8.3 minutes for light to travel the distance between the sun and the Earth.

#### Explanation:

The sun to Earth distance is about $149 \cdot {10}^{6} \text{km}$.

The speed of light is $2.99792 \cdot {10}^{5} \frac{\text{km}}{s}$.

At that speed, the time it takes for light to get here from the sun is
$t = \left(149 \cdot {10}^{6} \frac{\text{km")/(2.99792*10^5 "km}}{s}\right) \cong 497 s$