Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali
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Transcript of Lecture 4 Astronomy 1 Instructor: Dr. Babar Ali
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Lecture 4
Astronomy 1Instructor: Dr. Babar Ali
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• Please raise your hand if you are here to add the course.
Announcements
Fall 2014 Astro 1
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• Lectures and materials on LA mission college website.
http://www.lamission.edu/~alib
Announcements
Fall 2014 Astro 1
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• Quiz # 1 next week• Covers the first 4 lectures• In class, given at the end of the lecture• 20 minutes … 15-20 questions• Multiple choice format• Bring scantron sheets
Announcements
Fall 2014 Astro 1
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Outline
I. Time on EarthII. Observing the skyIII. Constellations Revisited
IV. How the Greeks measured the size of Earth
V. Precession
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Telling time.
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North Pole
Noon Midnight
Side View
6 pm
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Telling time.
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North Pole
Noon Midnight
Top View
6 pm
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Time on Earth
Earth’s rotation is causing the day/night cycle
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Review
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North PoleWhere is Noon, Midnight, 6am and 6pm?
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1-5: Astronomers use angles to denote the positions and apparent sizes of objects in the sky
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Estimating Angles with Your HandFall 2014 Astron 1 17
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The Small Angle FormulaFall 2014 Astron 1 18
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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.
Fall 2013 Astro 103 April, 2009
You can see the constellations on this side only.
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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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Three Views of Orion
Eighty-eight constellations cover the entire celestial sphere
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Constellations
In ancient times, constellations only referred to the brightest stars that appeared to form groups.
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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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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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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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Constellations (5)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
Rigel
Rigel = b OrionisBetelgeuse = a Orionis
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Constellations (6)
Some examples of easily recognizable constellations and their brightest stars
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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
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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.
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The angle a is 7 degrees
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Eratosthenes’ Findings
• Distance between Alexandria and Syene ~ 5,000 stadia• Angular between Alexandria and Syene ~ 7 degrees
• R ~ 40,000 stadia ….. probably within 15%
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7 5,000 = 360 2*p*R
Circumference of a circle
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Precession (1)
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.
At left, gravity is pulling on a slanted top. => Wobbling around the vertical.
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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. 2100.
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.
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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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Review Topics
• Why do the constellations we see at night change throughout the year?
• What is precession? What causes it?
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Review For Quiz
Fall 2014 Astro 1