pismp the skysains
TRANSCRIPT
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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?