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Update TOC with 2nd Activity
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“Stars at a Glance – Stellar
Parallax”
Because of Light Pollution, 70% of Americans cannot see the Milky Way.
Flagstaff Dark Skies Coalition:
 Their mission is to “celebrate, promote, and protect the glorious dark
skies of northern Arizona”.
 Believe people should be able to experience “seeing the stars”
 Helping to set regulations for building lights
 Fewer
 Glare-free
 Lower wattage
International Dark Sky Association:
 Developed to protect our views of the night-skies.
 How are we doing in North Bend-Snoqualmie?
http://www.blue-marble.de/nightlights/2010
Do: Fonzie Thumb Shift
Do: Mini-Lab Parallax
Answer: Reflect & Connect p 466: # 1
Watch Parallax Simulation:
Read & Take Notes p 467: Stellar Parallax
Answer: Reflect & Connect p 468: # 1a and 1c
Read/Notes p 469: Monster Units
Answer: Reflect & Connect p 470: # 2, 3
Why Does the Moon Follow You Around?
Driving down a road with mountains in the distance, you'll notice that the
mountains don't seem to move much at all, while the farm houses closer to the
road move much faster, but still a lot slower than the mailboxes blurring past you
right beside the road.
This is the same situation as the moon ‘following you’. The Moon is simply a
much bigger ‘mountain’, and it's much further away. This is called parallax. The
angle between you and a nearby object changes much more rapidly as you pass
them than the angle does for far away objects.
Take a flashlight into a dark room and stand next to a wall. Start wiggling the
flashlight, and you'll see the bright spot on the wall moving a little. Back away
from the wall, and keep wiggling the flashlight just as much as before. The spot
will move faster and further as you increase your distance from the wall. If you
get far enough away, you can try to hold the light still, but the spot will still
wiggle a bit just because your hand is not perfectly steady.
Also remember this - the Moon is about a thousand miles wide, and you would
therefore have to go 1000 miles just to have it move 1 of its widths across the sky.
How long does it take you to drive a thousand miles?
Light-Years
One Light-year [ly] is the distance light can travel in one year.
Speed of light in a vacuum (‘empty space’) ≈ 3 x 108 meters/second
In one year, light travels ≈ 9.5 x 1012 meters
Proxima Centauri (the nearest star to Earth, excluding our Sun) is 3.99 x 1016 meters
from Earth. We can’t actually see this star… it is too far, and too ‘dim’.
Using Dimensional Analysis, we find the Proxima Centauri = 4.22 ly away.
---------------------------------------------------------------------------------------------Astronomical Units
Astronomical Unit [AU] is the average distance from the Earth to the Sun.
1 AU ≈ 1.5 x 108 kilometers.
1 AU < 1 ly.
Astronomers use AU to measure distances within our solar system, and ly to
measure larger distances.
AU allow us to compare ‘relative distances’.
The distances, below, are approximate mean distances.
It has to be taken into consideration that the distances between
celestial bodies change in time due to their orbits and other factors.
The Moon is 0.0026 ± 0.0001 AU from the Earth
The Earth is 1.00 ± 0.02 AU from the Sun
--------------------------Mercury is 0.39 ± 0.09 AU from the Sun
Venus is
0.72 ± 0.01 AU from the Sun
Mars is
1.52 ± 0.14 AU from the Sun
Jupiter is
5.20 ± 0.25 AU from the Sun
Saturn is
9.58 ± 0.53 AU from the Sun
Uranus is 19.23 ± 0.85 AU from the Sun
Neptune is 30.10 ± 0.34 AU from the Sun
Proxima Centauri is ~268,000 AU from the Sun
Luminosity
Luminosity is the power output of a star
(CORRECTION to the text: it is NOT a measure of energy output)
Sun luminosity ≈ 3.8 x 1026 Watts
Rather than use such large numbers, we can compare a star’s luminosity relative to
the Sun.
The Sun has luminosity = 1 Lsun
Proxima Centauri has luminosity = .000138 Lsun
Proxima Centauri is MUCH less luminous in addition to being 4.2 ly away. Even if
it were only 1 AU, we would barely be able to see it because puts forth too little
power.
* We will be using a similar comparison for the masses of celestial bodies, compared
to the mass of the sun.
Homework Tonight:
Scientific Notation WS #2:
Do all ODD problems
*If student achieves 90%or better, they will not
have any further scientific notation homework!