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.

✪

✪

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

✪

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.