0. Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali

1. Announcements
Please raise your hand if you are here to add the course.
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2. Announcements Lectures and materials on LA mission college website.
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5. Announcements Use my mission college e-mail alib@lamission.edu
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6. Announcements Quiz # 1 next week Covers the first 4 lectures
In class, given at the end of the lecture
20 minutes … questions
Multiple choice format
Bring scantron sheets
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7. Constellations Revisited How the Greeks measured the size of Earth
Outline
Time on Earth
Observing the sky
Constellations Revisited
How the Greeks measured the size of Earth
Precession
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8. Telling time.
North Pole
Noon
6 pm
Midnight
Side View
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9. Telling time.
Noon
Midnight
6 pm
North Pole
Top View
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10. Earth’s rotation is causing the day/night cycle
Time on Earth
Earth’s rotation is causing the day/night cycle

11. Review
Where is Noon, Midnight, 6am and 6pm?
North Pole
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12. 1-5: Astronomers use angles to denote the positions and apparent sizes of objects in the sky
Figure 1-11a Angles are measured in degrees (°). There are 360° in a complete circle and 90° in a right angle. For example, the angle between the vertical direction (directly above you) and the horizontal direction (toward the horizon) is 90°. The angular diameter of the full moon in the sky is about 1⁄2°.
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13. Figure 1-11a Angles are measured in degrees (°)
Figure 1-11a Angles are measured in degrees (°). There are 360° in a complete circle and 90° in a right angle. For example, the angle between the vertical direction (directly above you) and the horizontal direction (toward the horizon) is 90°. The angular diameter of the full moon in the sky is about 1⁄2°.
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14. Figure 1-11b The seven bright stars that make up the Big Dipper can be seen from anywhere in the northern hemisphere. The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°.
Figure 1-11c The four bright stars that make up the Southern Cross can be seen from anywhere in the southern hemisphere. The angular distance between the stars at the top and bottom of the cross is about 6°.
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15. Figure 1-11b The seven bright stars that make up the Big Dipper can be seen from anywhere in the northern hemisphere. The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°.
Figure 1-11c The four bright stars that make up the Southern Cross can be seen from anywhere in the southern hemisphere. The angular distance between the stars at the top and bottom of the cross is about 6°.
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16. Estimating Angles with Your Hand
Figure 1-12 The adult human hand extended to arm’s length can be used to estimate angular distances and angular sizes in the sky.
Estimating Angles with Your Hand
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17. The Small Angle Formula
Box 1-1 Tools of the Astronomer’s Trade: (a) Two objects that have the same angular size may have different linear (c) Relating an object's linear size D, angular size α, and distance d sizes if they are at different distances from the observer. (b) For an object of a given linear size, the angular size is smaller the farther the object is from the observer. (c) The small-angle formula relates the linear size D of an object to its angular size α and its distance d from the observer.
The Small Angle Formula
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18. Earth’s Motion Earth is also revolving around the Sun.
The time it takes to complete on revolution is called a year.
At any given time, only half the sky is visible at night.
You can see the constellations on this side only.
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19. Conversely The sun appears in a constellation on the “other side”
Zodiac: Belt around sky, ~18° wide, centered on ecliptic, w/in which we find  & planets
Root of Zodiac same as Zoo, means collection of animals
Pattern of ’s w/in zodiac belt reminded ancients of animals
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20. Eighty-eight constellations cover the entire celestial sphere
Figure 2-2: The constellation Orion is easily seen on nights from December through March. (a) This photograph of Orion shows many more stars than can be seen with the naked eye. (b) A portion of a modern star atlas shows the distances in light-years (ly) to some of the stars in Orion. The yellow lines show the borders between Orion and its neighboring constellations (labeled in capital letters). (c) This fanciful drawing from a star atlas published in 1835 shows Orion the Hunter as well as other celestial creatures. (a: Eckhard Slawik/Science Source; c: Stapleton Collection/Corbis)
Three Views of Orion
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21. Constellations
In ancient times, constellations only referred to the brightest stars that appeared to form groups.
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22. Constellations (2)
They were believed to represent great heroes and mythological figures. Each culture has its own set of constellations, usually pertaining to local beliefs.
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23. Constellations (3)
Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.
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24. Constellations (4)
The stars of a constellation only appear to be close to one another.
Usually, this is only a projection effect:
The stars of a constellation may be located at very different distances from us.
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25. Constellations (5) Orion
Stars are named by a Greek letter (a, b, g) according to their relative brightness within a given constellation + the possessive form of the name of the constellation:
Orion
Betelgeuse = a Orionis
Rigel = b Orionis
Betelgeuse
Rigel
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26. Constellations (6)
Some examples of easily recognizable constellations and their brightest stars
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27. Apparent Motion of The Celestial Sphere
Some constellations around the Celestial North Pole never set. These are called “circumpolar”.
The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”.
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28. Eratosthenes (c. 276-194 B.C.) If the Earth is round ...
How big is it?
In Syene (modern day Aswan, Egypt) the Sun casts no shadow at Summer Solstice.
In Alexandria, Egypt Sun is ~7o from Zenith.
The angle a is 7 degrees
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30. Eratosthenes’ Findings
Distance between Alexandria and Syene ~ 5,000 stadia
Angular between Alexandria and Syene ~ 7 degrees
R ~ 40,000 stadia ….. probably within 15%
,000 =
*p*R
Circumference of a circle
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31. The Sun’s gravity is doing the same to Earth.
Precession (1)
At left, gravity is pulling on a slanted top. => Wobbling around the vertical.
The Sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

32. It will be closest to Polaris ~ A.D. 2100.
Precession (2)
As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.
It will be closest to Polaris ~ A.D
There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.)
~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra.

33. Review Topics How are constellations used in modern astronomy?
How many constellations are recognized by modern astronomers?
Name three constellations.
What is the concept of ‘projection’? And, how does it relate to constellations?
What are circumpolar constellations?
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34. Review Topics
Why do the constellations we see at night change throughout the year?
What is precession? What causes it?
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35. Review For Quiz
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