The Metric System - BIG
Okay, so you have gotten a look at the metric
world of the small and very small, but now it
is time to look at the metric world of the very
big.
1000 meters = 1 kilometer … or 1 km
To put this in perspective, a kilometer is
shorter than a mile … 0.62 miles to be exact.
So when you go 100 km/hr, you are only
going 62 miles/hr.
The Metric System - BIG
Kilometers are useful when measuring the
size of moons, planets, and the Sun.
The Moon has a diameter of 3476 km.
The Earth has a diameter of 12,756 km.
The planet Jupiter has a diameter of 142,800 km.
The Sun has a diameter of 1,392,000 km.
The Metric System - BIG
But kilometers are NOT useful when trying to describe
distances in space. Astronomers use a term called the
Astronomical Unit (AU) when referring to the distance to the
planets in the Solar System. One AU is defined as the
average distance from Sun to Earth … 149,600,000 km
The Earth is 149,600,000 km from the Sun … or 1 AU.
Planet Jupiter is 777,900,000 km from the Sun … or 5.2 AU.
Planet Neptune is 4,488,400,000 km from the Sun … or 30 AU.
Pluto used to be the ninth planet, but it was demoted on Aug 24, 2006
The Metric System - BIG
But Astronomical Units are not useful when
describing the distances to other stars and
galaxies in space. For that, astronomers use
a distance measure called the Light Year (ly).
It is the distance that light travels in one year.
And light can go pretty far in one year!
The Metric System - BIG
A beam of light from a flashlight travels at a
speed of 300,000 km in one second. For
those who are metrically challenged, this is
186,000 miles in one second. Light will wrap
around the Earth more than 7 times in just
one second.
But how far does light go in one year?
The Metric System - BIG
Well, there are 60 seconds in one minute.
60 x 300,000 km = 18,000,000 km in 1 light minute
There are 60 minutes in an hour.
60 x 18,000,000 km = 1,080,000,000 km in 1 light hour
There are 24 hours in a day
24 x 1,080,000,000 km = 25,920,000,000 in 1 light day
There are 365.25 days in an official year (accounting for
leap year)
365.25 x 25,920,000,000 km = 9,467,280,000,000 km in 1
light year.
This is almost 9.5 TRILLION kilometers
The Metric System - BIG
Finally, astronomers use a few even bigger
units of measuring distance.
A parsec is 3.26 light years
A megaparsec is 3.26 million parsecs
I will teach you what the parsec means at a later time in the
course.
Okay … what do we do with light years?
The Metric System - BIG
A
An artist’s image of our galaxy,
The Milky Way in cross-section
B
C
The distance from the Sun (arrow) to
the galaxy core = 26,000 light yrs.
A – the distance from the Sun to Earth = 8 light minutes
B – the distance from the Sun to Proxima Centauri (the closest star) = 4.3 light yrs
C – the distance from the Sun to Andromeda (the closest galaxy) = 2.65 million light yrs
The Metric System - BIG
A
B
C
Astronomers use a term called LookBack time to describe something truly
incredible. When we look at something
in space, we are not seeing the object in
real time, but what it looked like when the
light that we see first left the object.
So, we on Earth see the Sun the way it
looked 8 minutes ago.
We see Proxima Centauri the way it looked
4.3 years ago.
The light we see from Andromeda left the
galaxy 2.65 million years ago!!!
A – the distance from the Sun to Earth = 8 light minutes
B – the distance from the Sun to Proxima Centauri (the closest star) = 4.3 light yrs
C – the distance from the Sun to Andromeda (the closest galaxy) = 2.65 million light yrs
The Metric System - BIG
And when we look at some of the galaxies in this image, we are looking at light that
first left these objects BILLIONS of years ago. This is why using terms like kilometers
and Astronomical Units just cannot adequately describe the size of the Universe and
things like light years are needed … to describe space distance and time.
The Metric System - BIG
Let’s review:
1 meter = 1 m
1 kilometer = 1000 meters = 1x103 meters
1 Astronomical Unit (AU) = 149,600,000 meters
1 AU = 1.496x108 m = 1.4986x105 km
1 Light Year (ly) = 9,467,280,000,000 kilometers
1 ly = 9.46728x1012 km = 1.946728X1015 m
1 ly = 63,284 AU
1 Parsec (pc) = 30,863,332,800,000 kilometers
1 pc = 3.26 Light Years = 206,305 AU
1 Megaparsec (Mpc) = 30,863,332,800,000,000,000 km
1 Mpc = 1,000,000 pc = 3,260,000,000,000 ly
The Metric System - BIG
So, you can see that metric can be very small
and very big, but it always breaks down in
units of 10, 100, 1000 etc.
Now that you have finished a look at the
metric world, it is time to take a tour of the
Universe and get a better idea of the kinds
of things we will cover in this class.
Please go to the Moodle Site and click on
Tour of the Universe.