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Questions on Light Year Reading
Directions: Please answer the following questions using complete sentences with
part of the question in the answer.
1. Why are light years used a form of measurement?
2. Why is time used to measure distance?
3. What is the definition of a light year?
4. What is faster than the speed of light?
Light Year Reading
The fact that the Earth is the only inhabitable planet in our solar system is well established.
There are, however, over 200 billion other stars just in our very own Milky Way galaxy. Like our
Sun, many of these stars in our galaxy have planets orbiting them. So far scientists have
discovered about 1,714 of these planets orbiting distant stars. These planets are called
exoplanets because these planets, and the stars they orbit, are not in our solar system.
The distances to the other stars in our galaxy are immense and the light from these distant
stars takes many years to reach us here on Earth. When scientists study these stars one of the
fundamental facts to be determined is their distance from the Sun. Typical units such as miles
and kilometers are awkward to measure such distances; therefore, a unit known as the light
year is used.
Which number is easier to understand and work with: 5,840 days or 16 years? Both represent
the same amount of time. Most people would choose 16 years because smaller numbers are
easier to understand. Distances in the universe are more difficult to comprehend or imagine
when they are measured in units such as miles or kilometers because the numbers are just too
large. This led scientists to develop a new unit of measurement called the light year. Light
years make astronomical distances easier to work with.
The concept of a light year is sometimes difficult to understand partly because of the words
themselves. The term light year uses what is normally thought of as a unit of time, the year, to
measure a distance. But time units are often used to talk about time distances. People often
talk about how far it is to some destination by describing how long it takes to get there. As an
example, it takes about four hours to fly from the West Coast to the East Coast. Although a
time unit is being used, distance is actually being represented.
A light year is defined as the distance light travels in one Earth year. Light moves extremely
fast, 300 thousand kilometers per second (186,000 miles per second). In one second, light can
travel around the Earth almost 4 times. Nothing travels faster; light is the limit for speed! In
31 million seconds—or one year—light will travel a distance of 9.46 trillion kilometers, or 240
million times around Earth.
Distances in the universe are so large that it helps to express them in the scale of light years.
Consider the distance to the star Sirius, one of the closer stars to the Earth. It is almost 85
trillion kilometers from Earth. That is a huge number to comprehend. Using the light year as a
unit of measurement, the distance becomes only 9 light years, which is much easier to work
with. The light year is an appropriate unit to measure vast distances.
The speed of light makes everything appear to happen instantly in our everyday experience.
When we watch a soccer game and see someone kick the ball, we assume that the ball was
kicked right then, not five minutes before we saw it. For all practical purposes, we are safe in
this assumption because light travels so fast. In order for us to see the soccer ball, we must
see the light that is being reflected from it, and it does take time for light to travel to the ball
to our eyes. However, the amount if time is far too short for anyone to notice. If the distance
from the ball to our eyes were 10 meters, the light reflecting off the ball would take only 300
millionths of a second to reach our eyes.
Stars are millions and millions of kilometers away. When you see a star in the night sky, that
star’s light must travel across space to our eyes. The time required for light to travel such a
huge distance is easily measured. If the star is 5 light years away, then the light we are seeing
from that star took five years to travel to our eyes. It also means that what we see happening
at that star is actually what happened five years ago, not what’s happening to that star in the
star’s present. If we see a flare from the star’s surface, we are seeing an event that happened
five years ago.
Imagine that there is a planet with life forms on it that is 50 light years away from Earth. This
life form has an extremely powerful telescope and can actually make out the details of what is
happening on Earth. If they aim their telescope at Europe, they will see World War II being
fought since it was happening about 70 years ago. They will not see the events happening today
on Earth until 2064.
This presents a challenge for the astronomers who study stars. They can never see what is
going on in a star’s present, only its past. In a sense, they are detectives trying to solve a crime
without all the clues, and what evidence they do have is years out of date.