announcements/ assignments - jd smithtir.astro.utoledo.edu/jdsmith/class/a1010_f08/lecture3.pdf ·...
TRANSCRIPT
Announcements/Assignments
Need syllabus/tickets/schedule? See me after class.
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.
Cell Phones off please!
M.A. Homework NotesReminder: 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-KWed Sep 10th: Last Names L-Zother 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
Polaris is located on the horizon
Horizon
Latitude and Longitude
How did sailors know their location on the Earth before GPS?
Horizon
at the North Pole, lat=90°, altitude of polaris=90°
Polaris
How to find your latitude on Earth?
★
How to find your latitude on Earth?
Horizon
at the Equator, lat=0°, altitude of polaris=0°
Po
lar
is
★
Star Trails in between....
Mauna Kea, Hawai’i
20°
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.
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✪
Seasons, Annual Motions, and Phases of the Moon
Apparent Motion of the SunAnnual (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?
Possible times: sunset, sunrise, midnight, noon
6am: Sunrise
What Time is It?
Possible times: sunset, sunrise, midnight, noon
12pm: Noon
6am: Sunrise
What Time is It?
Possible times: sunset, sunrise, midnight, noon
6pm: Sunset
12pm: Noon
6am: Sunrise
What Time is It?
Possible times: sunset, sunrise, midnight, noon
6pm: Sunset
12pm: Noon
6am: Sunrise
12am: midnight
The Day1 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
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
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 SeasonsChanging distance to Sun?
Orbit is ellipse; distance changesGood idea, BUT...
Orbit almost circularTrivial 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 distancePredicts seasons same in N and S hemispheres (wrong)
Cause of SeasonsChanging distance to Sun?
Orbit is ellipse; distance changesGood idea, BUT...
Orbit almost circularTrivial 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 distancePredicts seasons same in N and S hemispheres (wrong)✕
Climate and LatitudeSun’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
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Does the Orientation of Earth’s Axis Change with Time?
26,000 yrs!
The Earth Moves like a Top: “Precession”
The MoonThe 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
Waning
Moon visible in the afternoon/evening. Get “fuller” and sets later each day.
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 MoonAlways 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 AwayEarth’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.