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The Sky
Edward M. Murphy
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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
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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.
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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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Measuring Angles
• Angles in astronomy are measured in
degrees, arcminutes and arcseconds.
– 1 degree = 60 arcminutes
– 1 arcminute = 60 arcseconds
– 1 degree = 3600 arcseconds
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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.
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The
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The
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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.
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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
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The stars
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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.
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Daily
(Diurnal
) Motion
of the
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Diurnal
Paths of
Stars at
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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.
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Diurnal
Paths of
Stars at
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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
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Celestial Sphere Movie
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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
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Celestial Coordinates
Vega
RA 18
h35
mDEC
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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?
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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
