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2/28/2013 The Sky 1

The Sky

Edward M. Murphy

Space Science for Teachers2005

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Horizon and Zenith

It is sometimes useful to think of thesky as a great dome over our heads.

The horizon (ufuk) is where the domemeets the Earth.

The zenith is the point directly overhead.

As the Earth turns, this dome turns overour heads. It appears as if the sky is alarge hollow sphere centered on the Earth.

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Horizon and Zenith

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Defining the position of objects The are two common systems for

defining position of astronomical bodies

in the sky. The Azimuth-Altitude (Az/Alt) system, and

The Right Ascension-Declination (RA/Dec)

system.

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Altitude and Azimuth

The height of a star above the horizon iscalled the altitude.

The direction to the star as measured fromtrue north is called the azimuth.

Note: True north is not the same as magneticnorth. 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 truenorth.

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The Az/Alt System

Alt / Az system defines the position of an objectin the sky in terms of its:azimuth (0 to 359 degrees; N = 0o, E = 90o,S = 180o, W = 270o) along the horizon,

its altitude above the horizon (0 to 90degrees; 0o = horizon, 90o = zenith)

N Sa

b

E.g.

Az = a=70o

Alt = b= 60o

Observer (you)

Object inthe sky

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Altitude and Azimuth

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Estimating Az and Alt:

The Hand and Fist Method

Your hand span, closed fist and fingers canbe used as a measure of angle to estimateangles.

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Measuring Angles in the Sky

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Angular Sizes and Distances

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Measuring Angles

The Sun and Moon appear to be about degree in size.

Your finger held at arms length is aboutone degree across.

Your fist at arms length is about 10degrees.

Your outstretched hand at arms length isabout 20 degrees across.

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Altitude and Azimuth

The altitude and azimuth of a starchange 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

Angles in astronomy are measured indegrees, 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 asseen from 5 km (3 miles) or a penny asseen from 2.2 miles.

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The RA/Dec System RA / Dec system defines the position of an

object in the sky in terms of its position onthe Celestial Sphere.

The RA / Dec system is analogous to theLongitude / Latitude system of defining the

location of places on the Earth, i.e., RA issimilar to lines of Longitude and Declination issimilar to lines of latitude.

Units of RA are measured in hours, minutes

and seconds. (00h:00m:00s -->23h:59m:59s)

Units of Dec are measured in degrees northand south of the celestial equator. (-90S -->+90N)

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The North Celestial Polemark theposition of +90o degrees Dec;

The South Celestial Polemarks theposition of -90o degrees Dec; and

The Celestial equator is defined as

0odegrees Dec.These positions are analogous to the

North pole of the Earth (+90o Lat);

South Pole (-90o

Lat), and the equator(0o Lat)

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Celestial Coordinates

Just as Earth has lines of longitude andlatitude, the celestial sphere has a system ofcelestial coordinates:

Declination (dec): The north-south position of a staron the celestial sphere. Declination is measured indegrees, arcminutes, and arcseconds. The celestialequator is defined to have a declination of 0

o.

Right Ascension (RA): The east-west coordinates ofan object on the celestial sphere. R.A. is measuredin hours, minutes, and seconds of time. The springequinox is defined to be 0

h.

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Celestial Coordinates

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Celestial Coordinates

Vega

RA 18h

35m

DEC +38o44

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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 of0

o. The north pole is at 90

oN and the south

pole at 90

o

S. Longitude: Your east-west position on Earth.

An arbitrary point, the Prime Meridian inGreenwich, England marks a longitude of 0

o.

Leander McCormick Observatory is at: Longitude 78

o31 19.8 W

Latitude 38o01 58.2 N

Altitude 264 meters

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Rotation ofthe Earth

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The stars move

from east towest becausethe earth rotatesfrom west toeast.

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Classroom Exercise

Find Orion in the night sky. Is it on theequator, the ecliptic, both, or neither.

How long is Orion above the horizoneach day?

Find Sagittarius. Is it north or south ofthe equator?

How long is Sagittarius above thehorizon each day?

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Classroom Exercise

Find Taurus. Is it north or south of theequator?

How long is it above the horizon each day? Find the Sun on December 21. Whichconstellation is it in? How long is it above thehorizon each day?

Find the sun on June 21. Which constellationis it in? How long is it above the horizoneach day?