Some Common SI Units of Length
Adults are between 1 and 2 meters, so the meter is a good unit of length for measuring
things at the human being scale like items at a picnic on a grassy field. The meter is the
basic or “one” unit of the metric system of length. Recall that the metric system is based
on the number 10 and that the metric system is part of the SI (International System)
system used world-wide by scientists. As we journey out by powers of 10, when we
reach 1000 meters it becomes easier to measure things in terms of another common
metric unit: the kilometer (km). The prefix kilo- means “1000” so a kilometer is 1000
meters. A kilometer is about two thirds of a mile, and is good for measuring things like
distances between towns and cities or the heights of mountain ranges. Continuing our
powers of 10 journey, we recede beyond Earth into space and need another unit to
measure such vast distances. The light year is a unit of distance that spans an incredible
10,000,000,000,000 km! This is about 1200 times the distance to Pluto and reaches far
beyond our Solar System. In fact it is about one quarter of the way to the nearest star!
Light years are good units for measuring the vast distances throughout our Milky Way
galaxy and beyond into the Greater Universe. Don’t be fooled by the name light year. It
is not a measure of time. It is defined as the length that light will travel in one year.
Light is the fastest moving thing in the entire Universe and moves at an incredible rate of
300,000,000 meters per second! Light takes about 8 minutes to reach Earth from the Sun,
so imagine how much farther that light travels in one year (365.25 days)! Finally there is
another unit worth knowing for measuring distances within our Solar System: the
Astronomical Unit or AU. The Astronomical Unit is a measure of the distance from the
Sun to the Earth and is 150,000,000 km. In the British system of measurement (not an SI
system!) that equates to 93,000,000 miles! The Astronomical Unit is abbreviated as AU
and is a common way to measure the distance to the other planets in our solar system as
well as comets, asteroids and meteors and distant space craft.
Answer the questions below:
1. If the average distance of Pluto from the Sun is 50 times the distance of the Earth to
the Sun how many AU’s is Pluto form the Sun?
2. How many meters is 100 km (remember there are 1000 meters in every km)?
3. What does kilo stand for? How many kilometers is 10,000 meters?
4. If one light year is about one quarter of the way to the nearest star how far is that star
(Proxima Centauri by the way…) in light years?
5. The tallest mountains on Earth are all over 8000 meters. How tall are those mountains
in km?
6. The Voyager 1 spacecraft was launched in 1977 and is the farthest human artifact in
the Universe. Recently it transmitted back its position to about 100 AU from Earth. How
many times farther is Voyager 1 than Pluto from the Sun? Is that a relative or absolute
measurement (relative means "compared to" and absolute means measured exactly)?