The Sky
Edward M. Murphy
ASTR 1210
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• Linear measurement
– Actual size of an object
– Measured in meters, kilometers, etc.
2
• Angular mesurement
– Apparent size of an object
• Angular size of an object
• Measured in degrees
– No indication of actual size
– Measured in degrees
• Apparent size depends on distance
– Closer is larger
– Farther is smaller
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Measuring Angles in the Sky
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Measuring Angles
• Angles in astronomy are measured in
degrees, arcminutes and arcseconds.
– 1 degree = 60 arcminutes
– 1 arcminute = 60 arcseconds
– 1 degree = 3600 arcseconds
– 1 arcsecond is the size of a U.S. quarter as
seen from 5 km (3 miles) or a penny as
seen from 2.2 miles.
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Measuring Angles
– The Sun and Moon appear to be about ½
degree in size.
– Your finger held at arms length is about
one degree across.
– Your fist at arms length is about 10
degrees.
– Your outstretched hand at arms length is
about 20 degrees across.
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Angular Sizes and Distances
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Small Angle Approximation
• Imagine a long skinny triangle:
angle
distance
size
The small angle approximation states that :
size
Angle Subtended  57.3 
distance
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Horizon and Zenith
• It is sometimes useful to think of the
sky as a great dome over our heads.
– The horizon is where the dome meets the
Earth.
– The zenith is the point directly overhead.
– As the Earth turns, this dome turns over
our heads. It appears as if the sky is a
large hollow sphere centered on the Earth.
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Horizon and Zenith
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Altitude and Azimuth
• The height of a star above the horizon is
called the altitude.
• The direction to the star as measured from
true north is called the azimuth.
–
–
–
–
True north is at an azimuth of 0 degrees.
East is 90 degrees
South is 180 degrees
West is 270 degrees
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Altitude and Azimuth
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Altitude and Azimuth
• The altitude and azimuth of a star
change during the course of night as
the star rises and sets.
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The Motion of the Stars
• Just like the Sun and Moon the stars
rise and set due to the rotation of the
Earth.
– They rise in the east and set in the west
because Earth rotates from west to east.
1/24/2011
The Fall Constellations
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Daily
(Diurnal)
Motion of
the Stars
1/24/2011
The Fall Constellations
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N
The
Milky
Way
E
W
S
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Number of stars in the
Universe
• Under dark skies, there are
approximately 6000 stars visible to the
naked eye.
– All of these stars are in our Milky Way
Galaxy
– They are a small fraction of the few
hundred billion stars in the Milky Way
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Apparent Brightness
• The apparent brightness of a star
depends on the luminosity of the star
and its distance from Earth.
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Magnitude System
• Around 150 B.C. the Greek astronomer
Hipparchus compiled a catalog of nearly 1000
stars listing their positions and apparent
brightness.
– The brightest stars were first magnitude stars.
– The next brightest were second magnitude stars.
– The faintest stars he could see were sixth
magnitude stars.
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Magnitude System
• Today, the magnitude system is defined
such that a difference of 5 magnitudes
is exactly a factor of 100 times in
brightness.
– One magnitude is a difference in
brightness of about 2.5 times.
– Two magnitudes are 2.5x2.5=6.3 times.
– Three magnitudes are 2.5x2.5x2.5=15.9
times.
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Magnitude System
• Unfortunately, it turns out that many
objects are brighter than first
magnitude.
– These have been assigned magnitudes
smaller than 1, including negative
numbers.
• Sirius, the brightest star in the sky, has an
apparent magnitude of –1.5.
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Magnitude System
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Apparent Magnitudes
• The apparent magnitude measures the
brightness of a star as seen in the night
sky on Earth.
– The apparent magnitude depends on the
luminosity and distance to the star.
– Fainter objects have larger apparent
magnitudes.
– Brighter objects have smaller apparent
magnitudes.
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Magnitude System
• The Sun has a magnitude of –26.2.
• Your eye can easily see the full moon
(magnitude about –13) and the faintest stars
(magnitude 6). This is a difference of nearly
8
20 magnitudes or a range of 10 .
• The magnitude system is only used in visual
astronomy. All other areas of astronomy
define brightness in terms of energy per
second per area received here on Earth.
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Constellations
• The constellations are groupings of
stars that have an historical reference
to a mythological figure, animal, or
object.
• An asterism is a small part of a
constellation that has an interesting
shape.
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Ursa Major, The Big Bear (Big Dipper)
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29
30
31
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33
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Constellations
• The stars in a constellation are usually
at widely varying distances and have no
relation to one another.
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101 LY
~80 LY
124 LY
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Constellations
• Many cultures have created stories and
figures in the sky.
• For example, the stars in Orion have been
seen as
–
–
–
–
–
–
–
Orion the hunter in ancient Greece and Rome
Osiris, the God of Light, in ancient Egypt
The great man Zilikawai by the Taulipang people in Brazil
The military commander Tsan, defender of farmers, in China
An animal trap by the Dayak of Borneo
A thief and mischief maker by the Chimu Indians of Peru.
As the Canoe of Tamarereti by the Maoris of New Zealand.
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Orion, The Hunter
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• Naming of stars
– This is a good time to explain star names.
– Not all stars have a proper name. In fact,
only about 300 of the 6000 stars visible
with the naked eye have a name.
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Ptolemy’s Star Catalog
The bright star
in the left foot in
common with
the water
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Rigel
• Our modern star names are a mix of
mistranslated and mistaken Arabic names and
Greek and Roman names.
– This explains the prevalence of star names beginning
with Al-.
– Many other Arabic words appear frequently in star
names.
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Meissa
Betelgeuse
Bellatrix
Mintaka
Alnilam
Alnitak
Rigel
Saiph
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