Transcript THE SKY - n Nebbe

THE SKY
CHAPTER 2 (Part 2)
REMINDER
• PLEASE TURN IN YOUR POWERPOINTS FROM
YOUR PRESENTATIONS.
– AND PLEASE, PUT YOUR NAMES ON THE FIRST
SLIDE.
• PLEASE GET A DEDICATED LAB NOTEBOOK BY
MONDAY!
Objectives
• To be able to interpret and apply the term
“brightness” to stars (finishing this objective).
• To be able to describe how the sky moves
with reference to the Earth’s rotation.
• To be able to predict the seasons and describe
what causes them.
• To be able to synthesize information on
astronomical cycles to predict Earth’s climate.
Light Intensity (Flux)
• Flux is the light energy from a star that hits
one square meter in one second.
• Why did astronomers adopt flux? – unlike
brightness (which does not depend on
distance), flux does measure distance.
– As the distance increases, flux decreases (and vice
versa). It is an inverse relationship (which you do
not have to remember).
Flux (Illustrated)
Flux
• Flux was developed because of the
subjectivity of “brightness (Apparent Visual
Magnitudes).”
• If you like, Table 2-1 in the book shows
conversions between magnitude (brightness)
and flux (I do not expect you to calculate the
conversions between brightness and flux,
unless you want to).
Example of Flux V. Magnitude
• Sirius’ light is 24.2 times more intense than
light from Polaris.
– Thus Polaris is farther away – 425 light years from
Earth – than Sirius, 8.6 light years, from Earth.
• That can be converted to magnitude – Sirius is
3.4 magnitudes brighter than Polaris.
The Sky’s Motion
• The Celestial Sphere (the sky) – An imaginary
sphere surrounding Earth to which the
planets, stars, sun, and moon seem to be
attached (conceived by early astronomers)
The Sky’s Motion
• There are a couple of things you should note:
– The celestial sphere (which is the sky) appears to
rotate westward around the Earth each day.
•
This is a result of the Earth’s Eastward rotation.
– Astronomers measure distance across the sky as
angles (See page 19 for more on this).
– What you see in the sky depends on your latitude.
• Starting at the north pole, the celestial north pole is
directly over your head.
The Sky’s Motion
• As you move south, the celestial pole moves toward the
horizon (it moves with you), and you can see farther
into the southern sky.
• The angular distance from the horizon to the north
celestial pole always equals your latitude.
• By the time you reach the equator, the north celestial
pole sinks below your northern horizon (in other words,
it disappears and you are in the southern hemisphere
(with the south celestial pole, which is expressed in
negative degrees).
• As you cross the equator and head south, you see the
southern sky with its constellations.