# Question Video: Comparing Units of Distance Science

Each of the following is a unit of distance. Which one is the largest? [A] Kilometer [B] Meter [C] Light-year [D] Mile [E] Millimeter

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### Video Transcript

Each of the following is a unit of distance. Which one is the largest? Is it (A) kilometer, (B) meter, (C) light-year, (D) mile, or (E) millimeter?

Let’s start with options (A), (B), and (E), each of which includes meter. Now, option (B) just says meter, but (A) and (E) include the prefixes kilo- and milli-. It’ll be helpful to recall that the prefix milli- means one thousandth, so a millimeter is one thousandth of a meter. Another way to say this is that there are 1000 millimeters in a meter. Since the millimeter isn’t as large as the meter, we should eliminate option (E).

Moving on, the prefix kilo- means 1000, so one kilometer equals 1000 meters. Because the kilometer is larger than the meter, we should eliminate option (B). The kilometer is a unit often used in everyday life. For example, one kilometer is roughly the length of a large suspension bridge that we might drive a car over.

Moving on, option (D) gives us another unit that we might recognize from everyday life, the mile. Miles and kilometers are often used in the same context because they’re of roughly the same magnitude. A mile is equal to about 1.6 kilometers, so the mile is a little bit larger than a kilometer. Thus, we should eliminate option (A).

Now, option (C), the light-year, is a unit that we don’t really use in daily life. Rather, we use light-years when we’re talking about space. That’s because this unit of distance is incredibly large. Nothing that we interact with here on Earth even comes close to being as large as a light-year. A light-year is defined as the distance that a beam of light would travel in one year if nothing were in its way.

It can be difficult to understand how fast the speed of light truly is, so consider the following. When astronauts travel to the moon, it takes about three days for their spacecraft to complete the journey from Earth. In comparison, it takes light less than two seconds to travel the same distance. So just how far can light travel over the span of a whole year? That’s one light-year, or about six trillion miles. Therefore, we know that the light-year is the largest unit of distance listed here, so option (C) is correct.