Edward M. Murphy

1
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
  Earths 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
  Earths 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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Circumpolar 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 Intermediate 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 Intermediate 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
http//brahms.phy.vanderbilt.edu/rknop/astromovie
s/
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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
  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 38o44
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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?