The sky lrg

© Grunt Productions 2000
The Sky
A brief by Lance GrindleyA brief by Lance Grindley

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.

Horizon and Zenith

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.

Magnetic North Pole

Magnetic North

Correction from Magnetic North to True North

• 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.

Measuring Angles in the Sky

Measuring Angles
• Angles in astronomy are measured in
degrees, arc minutes and arc seconds.
– 1 degree = 60 arc minutes
– 1 arc minute = 60 arc seconds
– 1 degree = 3600 arc seconds
– 1 arc second is the size of a 50p as seen from 5
km (3 miles) or a penny as seen from 2.2 miles.

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.

Angular Sizes and Distances

The
Celestial
Sphere

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.

Longitude and Latitude

Longitude and Latitude
• Latitude: Your north-south position on Earth. The
equator is defined to have a latitude of 0
o
. The
north pole is at 90
o
N and the south pole at 90
o
S.
• Longitude: Your east-west position on Earth. An
arbitrary point, the Prime Meridian in Greenwich,
England marks a longitude of 0
o
.

Rotation of
the Earth

The stars move
from east to west
because the earth
rotates from west
to east.

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.

Daily
(Diurnal)
Motion of
the Stars

Circumpolar
Stars

Diurnal Paths of Stars

Diurnal Paths
of Stars at
Intermediate
Latitude

Celestial Poles

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.

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.

Celestial Sphere Movie
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, arc
minutes, and arc seconds. The celestial equator is defined
to have a declination of 0
o
.
• 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 0
h
.

Vega
RA 18h
35m
DEC +38o
44’

Celestial
Coordinates

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?

The sky lrg

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