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The Sky

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The sky l.jpg

The Sky

Edward M. Murphy

Space Science for Teachers

2005

The Sky


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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.

The Sky


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Horizon and Zenith

The Sky


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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.

The Sky


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Altitude and Azimuth

The Sky


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Magnetic North Pole

The Sky


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Magnetic North

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Correction from Magnetic North to True North

The Sky


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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.

The Sky


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Measuring Angles in the Sky

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.

The Sky


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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.

The Sky


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Angular Sizes and Distances

The Sky


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The Celestial Sphere

The Sky


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The Celestial Sphere

The Sky


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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.

The Sky


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Longitude and Latitude

The Sky


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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

The Sky


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Rotation of the Earth

The Sky


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

The Sky


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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.

The Sky


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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

The Sky


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Celestial Poles

The Sky


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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.

The Sky


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Diurnal Paths of Stars at Intermediate Latitude

The Sky


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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

The Sky

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


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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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The Sky


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The Sky


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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?

The Sky


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