The Sky

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# The Sky - PowerPoint PPT Presentation

Horizon and Zenith

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## The Sky

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### The Sky

Edward M. Murphy

Space Science for Teachers

2005

The Sky

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

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

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

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

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

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

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

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

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

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.

The Sky

Celestial Sphere Movie

The Sky

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.

The Sky

Celestial Coordinates

Vega

RA 18h35m

DEC +38o44’

The Sky

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?

The Sky

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