Equatorial Coordinates:
Finding Astronomical Objects
Chapter 3
3-1
Coordinates - Finding Celestial Objects
3-2
Telescope Setting Circles Are Used to
Find Objects with Equatorial Mounts
Declination
Equatorial Mount
(Polar Aligned)
Right Ascension
3-3
Equatorial Coordinates
Objective
• 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
3-4
Celestial Sphere
North Celestial Pole
Northern Sky/
Constellations
Earth
Celestial equator
Northern Sky/
Constellations
independent of time and location
South Celestial Pole
3-5
Time and Location Dependant Points
and Circles on the Celestial Sphere
Zenith
Celestial Meridian
N
E
W
S
Celestial Horizon
3-6
Altitude/Azimuth Coordinate System
Zenith: Altitude = 90 degrees
Altitude - measured above the horizon
N = 0o
W = 270o
Azimuth - Measured along
celestial horizon
E= 90o
S= 180o Celestial Horizon: 3-7
Altitude=0
Problems with Altitude/Azimuth
Coordinate System
• Coordinates change with position
– Longitude and latitude
• Coordinates change with time
– Hourly changes - rotation
– Monthly changes - revolution
3-8
Earth Based Coordinate System
Latitude and Longitude
Latitude
Longitude
3-9
Equatorial Coordinate System
Projects Earth’s coordinate system onto celestial sphere
Declination
Right Ascension
3-10
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
3-11
Ecliptic - Sun’s Path on the Celestial
Sphere
The Earth has a 23.5o tilt with respect to the Sun
23.5o tilt
Sun
Summer
Winter
3-12
Earth’s Tilt is Responsible for Seasonal Changes
North Pole
Sun’s
Rays
Eq
uat
To Sun
or
South Pole
3-13
The Ecliptic
Dec. 21
Sept. 21
June 21
Mar. 21
Dec. 21
3-14
Time
• The interval between two events
• The measurement of a phenomena
- Our Clocks are synchronized to
the Rotation of the Earth.
- 1 Rotation = 24 hours
3-15
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
3-16
Sidereal Time
• Use the Vernal Equinox to measure the rotation
• 1 sidereal day is 4 minutes shorter than a solar day
3-17
Sidereal Time is a useful Tool for
Observational Astronomers
- Local Sidereal time = RA of the celestial meridian
- Local Sidereal time can be used to determine which objects
are above the horizon
RA=12 Hr
18 Hr>RA>6 Hr
N
W
RA=6 Hr
E
RA=18 Hr
3-18
The Equatorial Telescope Mount:
Fork Mount on a Wedge and Tripod
Declination Axis
Right Ascension
Polar Axis
3-19
Polar Alignment
• Must align telescope mount with equatorial
coordinate system
• Set the polar axis parallel to Earth’s axis
North Celestial Pole
Telescope polar
axis
S
3-20
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
3-21
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
3-22
Equatorial Coordinates
Summary
• 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
3-23