August 29, 2011: 10am Class On-line Quiz #1 on d2l: Please complete before class on Friday, Sept. 2 HW 2 on line (d2l and boojum): Create a Constellation.

August 29, 2011: 10am Class

• On-line Quiz #1 on d2l: Please complete before class on Friday, Sept. 2

• HW 2 on line (d2l and boojum): Create a ConstellationDue Wednesday, Sept. 7, in class.

• Honors Section 3H: Please complete doodle poll (see Megan’s email and class web site)

• Today: Precession, Phases of the Moon, Eclipses, Seasons

Precession

A spinning top “precesses”

http://en.wikipedia.org/wiki/Precession

Precession• The Earth’s axis of rotation precesses, so makes a

big circle, with a period of 26,000 years (so it goes once around every 26,000 years)

• The Earth’s pole moves about one degree in 72 years.

• The precession is a result of the fact that the Earth’s axis of rotation is tilted with respect to the ecliptic, and with respect to the orbit of the Moon.

The Moon’s and Sun’s gravity tug on the Earth and cause the pole to precess.

Currently the North Celestial Pole is very near the North Star, Polaris. In 3000 BC, the pole star was Thuban, in the constellation Draco.

But it is moving away from Polaris:

in 14000 AD, it will be pointed towards Vega, as it was in 12000 BC

Polaris will be the North Star again in 27800 AD.

Direction of the North CelestialPole as a function of year, AD

Direction of the South CelestialPole as a function of year, AD

Hipparchus of Rhodes (190 BC-120BC)• Precession was first observed by Hipparchus

• Also was “father” of trigonometry

• Made one of the first star catalogs

Phases and Eclipses

We always see the SAME SIDE of the Moon because the moon orbits the Earth and rotates on its axis with EXACTLY the same period (29 ½ days).

The “DARK SIDE OF THE MOON” was not seen by any human until spacecraft were sent to the moon and photographed it, in the 1950s.

Rotation of the Moon Applet Animation

24 hour cycle of the Earth

PHASES

Phases of the Moon youtube.com rap.

The Moon appears to go through a series of PHASES during each month: new moon, crescent, gibbous, full, gibbous, crescent, new. 

Lunar Eclipse = Earth's shadow falls on the Moon

Solar Eclipse = Moon's shadow falls on the Earth

Cause of Eclipse Animation

Path of some Solar Eclipses on the Earth

Solar Eclipses:

Total

Partial

Annular

Examples of Beautiful Eclipses!

Why don’t we have a lunar and solar eclipse every month?

(1) The Moon’s orbit is tilted with respect to the ecliptic (orbit of the Earth around the Sun) by about 5 degrees

(2)Also, the Moon’s orbit PRECESSES: the period of precession is about 18.6 years

There is a long cycle for the eclipses, called the Saros Cycle.

Tilt of the Moon’s Orbit

PRECESSION of the Moon’s Orbit

Summary: Key Concepts

1. We always see the SAME SIDE of the Moon because the moon orbits the Earth and rotates on its axis with EXACTLY the same period

2. Phases of the Moon: New, Waxing Crescent, Quarter, Waxing Gibbous,

Full, Waning Gibbous, Quarter, Waning Crescent, New,

3. Eclipses: Penumbra and Umbra4. Solar Eclipses: Total, Partial, and

Annular5. Saros Cycle

Why Are There Seasons??

http://livewithpurposecoaching.com/wp-content/uploads/2010/04/seasons1.jpg

NOT NOT NOT because the Earth is closer to the Sun in summer and farther in winter.  OK????

Why are there Seasons?

The Reasons for the Seasons1. The Earth's axis is tilted, and during the year

the axis points basically in the same direction (except for tiny motion or precession).

2. During summer in the northern hemisphere, the northern part of the Earth points TOWARDS the Sun.

sunlight is direct, more intense, heats the earth

days are longer

sunlight has less atmosphere to go through so is more effective in warming the ground

and vise versa for winter.

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Earth on June 22

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Earth on December 22

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Equinoxes and Solstices

• The Vernal (Spring) Equinox (about March 21): The location where the Sun crosses the equator when going from south of the equator to north of the equator.– Position of the Sun: R.A. 0h, Dec 0o

• The Summer Solstice (about June 21): The location where the Sun is at its furthest north.– Position of the Sun: R.A. 6h, Dec +23.5o

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Equinoxes and Solstices

• The Autumnal (Fall) Equinox (about September 21): Where the Sun crosses the equator when going from north to south.– Position of the Sun: R.A. 12h, Dec 0o

• The Winter Solstice (about December 21): The location where the Sun is at its furthest south.– Position of the Sun: R.A. 18h, Dec –23.5o

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Tropics

Note: When it's summer in the northern hemisphere, it's winter in the southern hemisphere, and vise versa.  

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The Annual Path of the Sun

• On the summer solstice:– the Sun will appear directly overhead to someone at 23.5

degrees north latitude. This latitude is called the Tropic of Cancer.

– The Sun does not set for people within 23.5 degrees of the North pole (above the Artic circle)

– The Sun does not rise for people within 23.5 degrees of the South pole (below the Antarctic Circle).

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Earth on June 21

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Standing on the North Pole

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Midnight Sun

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Standing on the Tropic of Cancer

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The Annual Path of the Sun

• On the autumnal equinox:– The Sun will appear directly overhead to someone on the

equator at 0 degress latitude.– The Sun sets at the North Pole, ending 6 straight months

of day and beginning 6 straight months of night. – The Sun rises at the South Pole ending 6 straight months

of night and beginning 6 straight months of day.

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Standing on the Equator

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The Annual Path of the Sun

• On the winter solstice:– The Sun will appear directly overhead to someone at 23.5

degress south latitude, the Tropic of Capricorn.– The Sun does not set for people within 23.5 degrees of the

South Pole (below the Antarctic Circle)– The Sun does not rise for people within 23.5 degrees of

the North Pole (above the Arctic Circle)

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Earth on December 21

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The Annual Path of the Sun

• On the vernal equinox:– The Sun will appear directly overhead to someone on the

equator at 0 degress latitude.– The Sun rises at the North Pole, ending 6 straight months

of night and beginning 6 straight months of day. – The Sun sets at the South Pole ending 6 straight months of

day and beginning 6 straight months of night.