Lecture 4

Astronomy 1Instructor: Dr. Babar Ali

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• Please raise your hand if you are here to add the course.

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• Lectures and materials on LA mission college website.

http://www.lamission.edu/~alib

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• Use my mission college e-mail• alib@lamission.edu

Fall 2013 Astro 103 April, 2009

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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

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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

Telling time.

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North Pole

Noon Midnight

Side View

6 pm

Telling time.

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North Pole

Noon Midnight

Top View

6 pm

Time on Earth

Earth’s rotation is causing the day/night cycle

Review

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North PoleWhere is Noon, Midnight, 6am and 6pm?

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

The Small Angle FormulaFall 2014 Astron 1 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.

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You can see the constellations on this side only.

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

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.

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.

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

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