lec3

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` Adding Class The following students may add the class. Please see me for a permit number after class If your name is not on this list, there is not room for you to add this class…. sorry!

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` Adding Class: Sec 2 The following students may add the class. Please see me for a permit number after class If your name is not on this list, there is not room for you to add this class…. sorry!

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` Switch Section? If any students in Sec. 1 wish to switch to Sec. 2, please let me know.

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` Finish Ch. 1, 2 Start Chapter 3 Reading

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` Celestial Sphere: Locations in the sky Magnitudes: how bright are the stars? Cycles of the Sun Summary

Slide6: 

Creating A Model Whenever we try to understand a new concept, it’s useful to create a model. A model is a representation of the real world which is easy to visualize. Models can be created using paper, computers or our mind. A model can be an accurate miniature version, or a useful simplification. Our model for space is called the Celestial Sphere

Slide7: 

The Celestial Sphere The Celestial Sphere is an imaginary sphere around the Earth. We can point out the location of a star or planet on this sphere.

Slide8: 

The Celestial Sphere In addition to North, South, East and West we have: Zenith: The “Highest Point” directly above Nadir: The “Lowest Point”, directly below.

Slide9: 

Three points on the Celestial Sphere: North Celestial Pole: Point in space just above the North Pole of Earth Celestial Equator: a circle in space above the Equator South Celestial Pole: Point in space just “above” the South Pole of Earth (The North Star is near the North Celestial Pole)

Slide10: 

p.21c If we lived at the North Pole

Slide11: 

p.21d If we lived in Alaska…

Slide12: 

p.21e Latitude for San Francisco: 38 o

Slide13: 

p.21f If we lived at the Equator…

Slide14: 

Which star is brightest? Which is faintest? How much brighter? Orion Measuring Stars Constellations help us find our way in the sky, but we want to know more…

Slide15: 

Ancient people put stars into 5 classes: “First Magnitude” stars were the brightest “Fifth Magnitude” stars were the faintest This system is still in use today Measuring Stars: Magnitudes

Magnitudes: 

Magnitudes A star’s intensity is the amount of light we receive from it. Intensity is measured on the Magnitude Scale Lower magnitudes = brighter stars The scale is logarithmic (like the Richter Scale for Earthquakes): A difference of 5 magnitudes corresponds to a star which is 100 times brighter.

The Magnitude Scale: 

The Magnitude Scale Originally there were only magnitudes 1-5 However a very bright object can even have a negative magnitude. Example: the planet Venus’ magnitude is -4.0 A very faint (low intensity) object can have a magnitude above 5. Example: A distant galaxy has a magnitude of 20.

Slide19: 

Examples of Magnitudes

The Magnitude Scale: 

The Magnitude Scale A difference of 5 magnitudes corresponds to a star which is 100 times brighter. Example: Star A’s magnitude is 2.0, Star B’s magnitude is 7.0 The difference in magnitudes is 7.0-2.0 = 5 Mags. So, Star B is 100 times fainter than Star A mA - mB = 2.5 log(IA/IB) I = intensity; m = magnitude

The Magnitude Scale: 

The Magnitude Scale Example: The star Rigel has a magnitude of 0.12. What is the magnitude of Star X, which is 10,000 times fainter than Rigel? Solution: The Intensities of the stars are: IRigel, and IX. IRigel = 10,000 IX mX - mRigel = 2.5 log(IRigel/IX) = 2.5 log (10,000) = 2.5 log(104) = 2.5 * 4 = 10 mx = mRigel + 10 = 10.12

Cycles of the Sun: 

Cycles of the Sun In addition to rising and setting every day, the Sun undergoes yearly changes. It is in the sky more in Summer and less in Winter The Sun rises and sets from different locations throughout the year. This fact was carefully observed by ancient astronomers, and used to predict the Seasons

An Ancient Observatory: 

An Ancient Observatory Stonehenge Built ~2500 BC in England

Slide24: 

Mayan Observatory Built ~600 AD at Chichen Itza, Mexico