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Finish reading Chapter 2
First graded online homework:
“Homework #1” due Thursday, Sept.
10th before class. Available now.
Lecture notes are now posted on the
course website.
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M.A. Homework Notes
Reminder: Practice Homework is
ungraded. “Overdue” is not an issue.
Late Homework: 10% off per day.
Grading: 2% bonus per hint not used (use
them!). Only 3% off for wrong answers
on “sorting”-type questions.
Still need help with
MasteringAstronomy.com?
See me after class.
Next Week:
Planetarium Visit
During Class (no ticket needed).
Meet there (across the courtyard just
outside the door).
Mon Sep 8th: Last Names A-K
Wed Sep 10th: Last Names L-Z
other group: meet here as usual!
Last Time
Earth’s Rotation (once per day) causes sun/
stars/etc. to rise, transit at their highest
point at the meridian, and then set. Zenith is
the point directly overhead.
Stars you see depends on your latitude. Some
stars are always up: they are circumpolar.
Constellations: Just random groupings of
stars invented by humans.
The celestial sphere is an imaginary sphere
where we place stars on a two dimensional
grid, similar to latitude and longitude on
earth.
Last Time
1 degree=60 arcminutes = 3600 arcseconds.
The “size” of objects in the sky is measured
as an angle. Without knowing its distance,
we can’t know its true size.
From Toledo, you observe a star
rising due east. When this star
reaches its highest position above
the horizon, where will it be?
A) high in the
northern sky
B) high in the
eastern sky
C) high in the
southern sky
D) high in the
western sky
E) directly overhead
From Toledo, you observe a star
rising due east. When this star
reaches its highest position above
the horizon, where will it be?
A) high in the
northern sky
B) high in the
eastern sky
✪ C) high in the
southern sky
D) high in the
western sky
E) directly overhead
Equator Star Trails
Imagine sitting
at the equator
at night as the
Earth turns.
What would the
stars look like?
All stars rise
and set
Horizon
Polaris is
located on the
horizon
Latitude and Longitude
How did sailors
know their
location on the
Earth before
GPS?
How to find your latitude
on Earth?
Polaris
★
Horizon
at the North Pole, lat=90°,
altitude of polaris=90°
How to find your latitude
on Earth?
Polaris
at the Equator, lat=0°,
altitude of polaris=0°
★
Horizon
Star Trails in
between....
20°
Mauna Kea, Hawai’i
Altitude of
North Star =
your latitude!
How to Find Polaris?
Imagine you are located in the Northern
Hemisphere and see a star directly
overhead (zenith). In what direction will
you have to look to see this star set?
A) southwest
B) west
C) northwest
D) This star will
never set.
Imagine you are located in the Northern
Hemisphere and see a star directly
overhead (zenith). In what direction will
you have to look to see this star set?
A) southwest
B) west
✪ C) northwest
✪ D) This star will
never set.
Seasons, Annual Motions, and
Phases of the Moon
Apparent Motion of
the Sun
Annual (yearly)
motion
Earth orbits Sun
once per year
Sun seen in
front of
different
constellations
throughout year
The sun “slips” day to day on the celestial
sphere, moving from W to E relative to the
background of stars.
SOHO Observes Solar Motion
What Time is It?
Possible times: sunset, sunrise, midnight, noon
What Time is It?
6am:
Sunrise
Possible times: sunset, sunrise, midnight, noon
What Time is It?
12pm:
Noon
6am:
Sunrise
Possible times: sunset, sunrise, midnight, noon
What Time is It?
6pm:
Sunset
12pm:
Noon
6am:
Sunrise
Possible times: sunset, sunrise, midnight, noon
What Time is It?
6pm:
Sunset
12pm:
Noon
12am:
midnight
6am:
Sunrise
Possible times: sunset, sunrise, midnight, noon
The Day
1 day = time for object to return to
same point on sky (e.g. transit to
transit)
Solar day (sun)
Time from noon until next noon
Sidereal day (star)
Time for a star to return to same
point.
http://www.pfm.howard.edu/astronomy/Chaisson/AT401/IMAGES/AACHCIR0.JPG
http://www.nmm.ac.uk/server/show/nav.00500300l005001000
Annual Path of Sun
Ecliptic
path of sun around celestial sphere
shape: circle
Zodiac
Set of 12 constellations containing
Ecliptic
Sun in each constellation for about
one month
(solar) signs of the zodiac
Ecliptic on Celestial Sphere
Earth’s axis tilted 23º with respect to orbit
Locations on Ecliptic
Solstice: Sun “stops” (moving N or S)
Summer Solstice
Jun 21=1st day of summer
Sun appears farthest North (from celestial
equator)
Longest day of year
Winter Solstice
Dec 21=1st day of winter
Sun appears farthest South (from celestial
equator)
Shortest day of year
Locations on Ecliptic
Equinox: equal night and day
Vernal Equinox
~March 21 = 1st day of spring
Sun on equator (crossing from S to N)
Autumnal Equinox
~Sept 21 = 1st day of fall (autumn)
Sun on equator (crossing from N to S)
Equinoxes are intersection points of
Ecliptic and Celestial Equator
The reason for Seasons
Cause of Seasons
Changing distance to Sun?
Orbit is ellipse; distance changes
Good idea, BUT...
Orbit almost circular
Trivial change in distance (3%!)
little change in heating
Closest to sun in January
So this predicts summer in January! (wrong)
Both N and S hemispheres at same distance
Predicts seasons same in N and S hemispheres
(wrong)
Cause of Seasons
✕
Changing distance to Sun?
Orbit is ellipse; distance changes
Good idea, BUT...
Orbit almost circular
Trivial change in distance (3%!)
little change in heating
Closest to sun in January
So this predicts summer in January! (wrong)
Both N and S hemispheres at same distance
Predicts seasons same in N and S hemispheres
(wrong)
Climate and Latitude
Sun’s rays
spread over large area = cooler
concentrate in small area = warmer
Warmest where sun directly overhead (at noon)
cooler
warmer
cooler
Why Then?
Due to 23.5° tilt of
the earth’s axis?
Sun up for longer,
and higher in the sky
= More sunlight =
warmer: Summer
Sun up above horizon
shorter time, and
lower in the sky =
Colder: Winter.
Why Then?
✓
Due to 23.5° tilt of
the earth’s axis?
Sun up for longer,
and higher in the sky
= More sunlight =
warmer: Summer
Sun up above horizon
shorter time, and
lower in the sky =
Colder: Winter.
Seasons: Interactive
Figure
Sunrise above the
artic circle
Workbook Time
Break up into groups of 2-3.
We’re skipping “Motions” which
covers more diurnal motion
concepts. “Motions” is a good study
exercise on your own.
Work on “Seasonal Stars” on page 7.
One night, you see the star Sirius rise at
exactly 7:36 PM. The following night it
will rise
A) slightly earlier.
B) at the same time.
C) slightly later.
One night, you see the star Sirius rise at
exactly 7:36 PM. The following night it
will rise
✪ A) slightly earlier.
B) at the same time.
C) slightly later.
What component of Earth’s motion
causes the stars to rise earlier on
successive nights?
A) its rotation about its axis
B) its orbit around the Sun
C) the tilt of its rotation axis
What component of Earth’s motion
causes the stars to rise earlier on
successive nights?
A) its rotation about its axis
✪ B) its orbit around the Sun
C) the tilt of its rotation axis
Does the Orientation of
Earth’s Axis Change with Time?
26,000 yrs!
The Earth Moves like a
Top: “Precession”
The Moon
The moon is
about 1
quarter the
size of the
Earth
It orbits the
Earth every
27.5 days
On average, it
is 380,000 km
away
Phases of the Moon
Each complete cycle
of the moon takes
29.5 days, hence the
word “month”
It is longer than the
orbital period (27.5
days) because of the
Earth’s orbital
motion: Just like
solar vs. sidereal
day!
Phases of the Moon 29.5 days
New
Crescent
}
}
First Quarter
Gibbous
Full
Gibbous
Last Quarter
Crescent
Waxing
Moon visible in the afternoon/
evening. Get “fuller” and sets
later each day.
Waning
Moon visible in the late night/
morning. Get “lesser” and rises
later each day.
What causes moon phases?
A) Moon passes into the shadow of the
earth.
B) Relative orientation of Sun, Moon,
and Earth determines the phase.
C) Moon is made of cheese, and the
north wind eats it day by day.
What causes moon phases?
A) Moon passes into the shadow of the
earth.
✪ B) Relative orientation of Sun, Moon,
and Earth determines the phase.
C) Moon is made of cheese, and the
north wind eats it day by day.
Why do we see phases
of the Moon?
One half of
the moon is
always
illuminated by
the Sun
Phases are a
result of the
Moon’s position
relative to the
Sun as it
orbits the
Earth
Doesn’t the Moon
block out the sun?
Is the Moon up in the
daytime?
Yes!
Spends half of
it’s time up
during the day.
Smkymtnman/Flickr
From Flickr: “Taken at 10:41am this morning.
Wish it had been full.”
What’s wrong with this statement?
Rotation of Moon
Always see same side
Moon rotates about once per month
Non-rotating
moon
Rotating moon
The “Dark side of the
Moon”
Is not actually Dark!
Better called the “back side of the moon”.
Had night and day just like the front side.
Take Away
Earth’s orbit about the sun once per year
controls the position of the sun and the
seasons.
The sun appears to move slowly on the
celestial sphere.
When the sun, earth, and moon are aligned,
eclipses can occur (more next week!).
Don’t Forget: Planetarium next week, HW
#1 Due next Wednesday.