3-1

Equatorial Coordinates:Finding Astronomical Objects

Chapter 3

3-2

Coordinates - Finding Celestial Objects

3-3

Telescope Setting Circles Are Used to Find Objects with Equatorial Mounts

Right Ascension

Declination

Equatorial Mount(Polar Aligned)

3-4

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

3-5

Celestial Sphere

independent of time and location

Northern Sky/Constellations

Northern Sky/Constellations

Celestial equator

North Celestial Pole

South Celestial Pole

Earth

3-6

Time and Location Dependant Points and Circles on the Celestial Sphere

N

W

S

E

Zenith

Celestial Horizon

Celestial Meridian

3-7

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

3-8

Problems with Altitude/Azimuth Coordinate System

• Coordinates change with position– Longitude and latitude

• Coordinates change with time– Hourly changes - rotation– Monthly changes - revolution

3-9

Earth Based Coordinate SystemLatitude and Longitude

Latitude

Longitude

3-10

Equatorial Coordinate SystemProjects Earth’s coordinate system onto celestial sphere

Declination

Right Ascension

3-11

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

3-12

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

3-13

Earth’s Tilt is Responsible for Seasonal Changes

North Pole

South Pole

Sun’sRays

To SunEquator

3-14

The Ecliptic

Dec. 21 Sept. 21 June 21 Mar. 21 Dec. 21

3-15

Time

• The interval between two events• The measurement of a phenomena

- Our Clocks are synchronized tothe Rotation of the Earth.- 1 Rotation = 24 hours

3-16

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

3-17

Sidereal Time

• Use the Vernal Equinox to measure the rotation

• 1 sidereal day is 4 minutes shorter than a solar day

3-18

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

3-19

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

3-21

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

3-22

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

3-23

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