Post on 09-Feb-2022
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Equatorial Coordinates:Finding Astronomical Objects
Chapter 3
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Coordinates - Finding Celestial Objects
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Telescope Setting Circles Are Used to Find Objects with Equatorial Mounts
Right Ascension
Declination
Equatorial Mount(Polar Aligned)
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Equatorial CoordinatesObjective
• Revisit the celestial sphere• Cover coordinate systems:
– Altitude and azimuth– Earth-based coordinate system– Equatorial coordinate system (celestial sphere)
• Learn how to use the coordinate system:– Laboratory exercise
• Path of the Sun and seasons• Sidereal time
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Celestial Sphere
independent of time and location
Northern Sky/Constellations
Northern Sky/Constellations
Celestial equator
North Celestial Pole
South Celestial Pole
Earth
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Time and Location Dependant Points and Circles on the Celestial Sphere
N
W
S
E
Zenith
Celestial Horizon
Celestial Meridian
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Altitude/Azimuth Coordinate System
Zenith: Altitude = 90 degrees
Celestial Horizon:Altitude=0
N = 0o
W = 270o
S= 180o
E= 90o
Altitude - measured above the horizon
Azimuth - Measured along celestial horizon
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Problems with Altitude/Azimuth Coordinate System
• Coordinates change with position– Longitude and latitude
• Coordinates change with time– Hourly changes - rotation– Monthly changes - revolution
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Earth Based Coordinate SystemLatitude and Longitude
Latitude
Longitude
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Equatorial Coordinate SystemProjects Earth’s coordinate system onto celestial sphere
Declination
Right Ascension
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Right Ascension and Declination
Right Ascension - measured in hours, min, sec.– Measured along the celestial equator– 1 hour = 15o
– Vernal equinox = 0 hour (in Pisces)– Measured Eastward, from 0-24 hours
Declination - measured in degrees, min, sec.– Measured above and below the celestial
equator– Celestial equator = 0o
– NCP = 90o and SCP = -90o
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Ecliptic - Sun’s Path on the Celestial Sphere
23.5o tilt
WinterSummer
The Earth has a 23.5o tilt with respect to the Sun
Sun
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Earth’s Tilt is Responsible for Seasonal Changes
North Pole
South Pole
Sun’sRays
To SunEquator
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The Ecliptic
Dec. 21 Sept. 21 June 21 Mar. 21 Dec. 21
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Time
• The interval between two events• The measurement of a phenomena
- Our Clocks are synchronized tothe Rotation of the Earth.- 1 Rotation = 24 hours
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Solar Time
• We use the Sun’s position to measure rotation• 24 hours = time it takes the Sun to appear in the
same part of sky• Problems with Solar Time
– Sun does not appear in same part of sky– Earth’s revolution ignored
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Sidereal Time
• Use the Vernal Equinox to measure the rotation
• 1 sidereal day is 4 minutes shorter than a solar day
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Sidereal Time is a useful Tool for Observational Astronomers
N
W E
- Local Sidereal time = RA of the celestial meridian- Local Sidereal time can be used to determine which objectsare above the horizon
RA=12 Hr
RA=6 Hr RA=18 Hr
18 Hr>RA>6 Hr
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The Equatorial Telescope Mount:Fork Mount on a Wedge and Tripod
Right Ascension Polar Axis
Declination Axis
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Polar Alignment• Must align telescope mount with equatorial
coordinate system• Set the polar axis parallel to Earth’s axis
Telescope polar axis
North Celestial Pole
S
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Polar Alignment Procedure
• Level tripod• Set the declination = 90o
• Move telescope mount in altitude and azimuth until NCP is in FOV
• Tighten Azimuth and Altitude adjustments
Elevation adjusted on wedge
Azimuth adjusted by rotating tripod
Elevation angle = latitude
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Setting Circle Calibration Procedure
• Polar align telescope mount• Center a star with known RA in FOV• Set RA circle to RA of star
– This is done by manually rotating the circle• Turn on the electric clock drive• Drive will maintain sidereal time and track object
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Equatorial CoordinatesSummary
• Celestial Sphere - equator, zenith, horizon, SCP, and NCP
• Altitude/Azimuth coordinate system• Earth based coordinate system
– Longitude and latitude• Equatorial coordinate system
– - Right ascension and declination• Ecliptic - Solstices and equinoxes• Polar alignment and setting circle calibration• Solar and sidereal time