MurphyCelestial.ppt 1518KB Jun 23 2011 10:31:22 AM

The Sky
Edward M. Murphy
Space Science for Teachers
2005
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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.
– Note: True north is not the same as magnetic
north. The magnetic north pole is not located

in the same place as the true north pole.
• On maps, the legend will show you how to correct
from magnetic north, as measured by a compass, to
true north.

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Altitude and Azimuth

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Magnetic North Pole

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Magnetic North

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Correction from Magnetic North to True North

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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.
• Angles are measured using
degrees, minutes of arc, and
seconds of arc.

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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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The
Celestial
Sphere

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The
Celestial
Sphere

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The Celestial Sphere
• North Celestial Pole: The point directly over
the Earth’s true north pole.
– The north star, Polaris, is near the North Celestial
Pole, but not exactly at the pole. It is currently
about 1 degree away from the pole.

• South Celestial Pole: The point directly over
the Earth’s true south pole.
• Celestial Equator: The equator of the Earth

projected onto the celestial sphere.
• Meridian: A line from due north to due south
that passes straight overhead.
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Longitude and Latitude

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Longitude and Latitude
• Latitude: Your north-south position on Earth.
The equator is defined to have a latitude of

0o. The north pole is at 90oN and the south
pole at 90oS.
• Longitude: Your east-west position on Earth.
An arbitrary point, the Prime Meridian in
Greenwich, England marks a longitude of 0o.
• Leander McCormick Observatory is at:
– Longitude 78o 31’ 19.8” W
– Latitude 38o 01’ 58.2” N
– Altitude 264 meters

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Rotation of
the Earth

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The stars
move from
east to west
because the
earth rotates
from west to
east.
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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.
– Stars near the celestial poles do not rise or
set. Instead they circle the poles and are
called circumpolar.
• In the northern hemisphere, the stars circle the
pole in a counterclockwise direction.

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Daily
(Diurnal
) Motion
of the
Stars

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Circumpol
ar Stars

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Diurnal Paths of Stars

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Diurnal Paths of Stars

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Diurnal
Paths of
Stars at
Intermediat
e Latitude

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Celestial Poles

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North Celestial Pole
• In the northern hemisphere, the altitude of
the north celestial pole is equal to your
latitude on Earth.
• This is useful for navigation.
– If you measure the altitude of the north celestial
pole, you can determine your latitude on Earth.

• In the southern hemisphere, it is difficult,
but not impossible, to find the location of
the south celestial pole.
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Diurnal
Paths of
Stars at
Intermediat
e Latitude

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Celestial Sphere
• To find due north, drop straight
down from the North Celestial Pole
to the horizon.
• The celestial equator meets the
horizon at due east and due west.

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Diurnal Paths of Stars

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Celestial Sphere Movie

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http://brahms.phy.vanderbilt.edu/~rknop/astromovies/

Celestial Coordinates
• Just as Earth has lines of longitude and
latitude, the celestial sphere has a system
of celestial coordinates:
• Declination (dec): The north-south position of a
star on the celestial sphere. Declination is
measured in degrees, arcminutes, and arcseconds.
The celestial equator is defined to have a
declination of 0o.
• Right Ascension (RA): The east-west coordinates
of an object on the celestial sphere. R.A. is
measured in hours, minutes, and seconds of time.
The vernal equinox is defined to be 0h.

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Celestial Coordinates

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Celestial Coordinates
Vega
RA 18h35m
DEC
o
+38 44’

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Classroom Exercise
• Find Orion in the night sky. Is it on
the equator, the ecliptic, both, or
neither.
• How long is Orion above the horizon
each day?
• Find Sagittarius. Is it north or south
of the equator?
• How long is Sagittarius above the
horizon each day?
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Classroom Exercise
• Find Taurus. Is it north or south of the
equator?
• How long is it above the horizon each
day?
• Find the Sun on December 21. Which
constellation is it in? How long is it above
the horizon each day?
• Find the sun on June 21. Which
constellation is it in? How long is it above
the horizon each day?
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