Post on 28-Feb-2019
Lec. 25: The Night Sky I
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The Night Sky
Lecture 25
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Outline
The night sky - overview
Why the sky changes During the night
From night to night
Some definitions Celestial sphere, ecliptic, zodiac, zenith, meridian
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Things to see in the night sky
The Moon & Planets
Stars Constellations
Asterisms (groupings of stars) e.g. the Pleiades)
Binaries
Variable Stars
Sky map – north view
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Sky map – south view
Sky map – south view
http://www.skyviewcafe.com/skyview.php
Tonight at 8:00 pm
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In-Class Question
Roughly how many stars are visible to the unaided (eye over the entire sky), that is, how many could you seeA. 6B. 600C. 6,000D. 60,000E. 600,000
27,0007.5
8,8006.5
2,8005.5
9004.5
Total # of stars
Visual Magnitude
!
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With binoculars or a telescope
Object Example
Globular cluster - M13 in Hercules
Open clusters - Pleiadesh and Perseus
Planetary Nebula - Ring Nebula
Galaxy - Andromeda
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M13 – Globular Cluster
h and Perseus
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Pleiades Star Cluster
Ring Nebula (M57)
HST Image
Ground Images
Above - rescaled
Deeper exposure
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Andromeda Galaxy (M31)
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But where to look?
Need to know where an object is on the sky (coordinates or it location relative to some reference stars or constellations)
And what time that part of the sky is viewable, e.g. the sun is not up. Time of night and Time of year
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How do we know where and when to look?
The major motions that affect the night sky are:
Rotation of the earth Causes the stars to move through the sky
at night.
Revolution of the earth around the sun Causes different stars to be available at
different times of the year.
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N
Not to scale!
Pegasus &Andromeda
Lyra &Hercules
Leo &Coma
Auriga &Orion
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Real proportions
If the sun were the volleyball: ~ 8 in = 20 cm in diameter
The earth is smaller than a pea: ~ 1.8 mm in diameter
The earth-sun distance is: 22 m (about 70 feet)
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Cycles of the Sky
A class demo, wherein the part of the sun is play by a tennis ball
while the earth is played by a volleyball,
and the students are, of course, stars.
(And I get to play Atlas!)
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N
Not to scale!
Pegasus &Andromeda
Lyra &Hercules
Leo &Coma
Auriga &Orion
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In-Class Question What causes the stars to move through the
sky at night?
A. The rotation of the sky about the Earth
B. The rotation of the Earth
C. The revolution of the Earth about the Sun
D. Magic
E. The planets drag the stars along with them
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The Celestial Sphere The celestial sphere is
the vast hollow sphere on which the stars appear fixed.
The celestial equator is define by extending the earth’s equator outward.
The N & S poles of the celestial sphere correspond to the earth’s poles.
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The Ecliptic The ecliptic is the
apparent path of the sun through the sky.
It is also the plane of the earth’s orbit about the sun on the celestial sphere.
Note: The ecliptic is tilted w.r.t. the earth’s equatorial plane by 23.5o.
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The Zodiac
The zodiac is a band of celestial sphere which represents the path of the planets, the moon and the sun.
Extends ~8o to either side of the ecliptic.
In astrology the zodiac is divided into 12 equal parts called signs, each bearing the name of a constellation.
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Astrology is NOT a science!
Propagates the claim that a person’s life is determined by the position of the sun, moon, and planets at birth.
This notion is patently false, and potentially harmful.
Astrology is neither a science nor a religion.
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Zenith
The zenith is the point on the celestial sphere that is directly above the observer.
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Meridian
The meridian is the great circle passing through the two poles of the celestial sphere and the observer’s zenith.