1. Karen Meech Institute for Astronomy TOPS 2003
The Celestial Sphere
Karen Meech
Institute for Astronomy
TOPS 2003

3. Latitude and Longitude
Latitude (f) meas from equator
Longitude (l) point of reference – Greenwich UK
Units of measure:
Deg, arcmin, arcsec
O ‘ “

4. The Horizon System
Altitude (h) – angle measured from the horizon to Zenith (Z)
Azimuth – the angle measured from NE along horizon
Problem as a celestial system?

5. Celestial Sphere Imaginary sphere where stars reside
Extension of Earth’s equator, poles
Celestial Equator
Celestial poles
Zenith & Nadir

6. Great Circles Circles covering the largest diameter on sphere
NCP altitude = f
Celestial Meridian – CM great circle through Z and NCP
Hour Angle – angular distance / time from CM
HA = 0 on CM
“-” indicates rising
“+” indicates setting

7. Declination & Right Ascension
Declination = Latitude
Celestial Equator d = 0
Latitude, NCP elevation
Units: deg, arcmin, ‘’
The Celestial Meridian
Great circle going through zenith & NCP
Right Ascension = Longitude
Units: hh:mm:ss
360o = 24 hr (1 hr = 15o)
Where to start RA?

8. Circles of Declination

9. The Ecliptic & Seasons
Obliquity – tilt of Earth’s orbital axis (23.5o)
Ecliptic – path of the Earth around the sun
Apparent path of the sun & planets in the sky
Traces a great circle on the celestial sphere
Intersects at 2 points: ^ and d (vernal & autumnal equinox)
^ is visible at midnight on CM in September

10. The Ecliptic

12. Right Ascension Starting Point
Longitude system: Prime Meridian
Two intersections between CE & ecliptic
Vernal Equinox
Autumnal Equinox
Units of measure:
Hours, min, sec
Measure Eastward from ^ (RA = 0)
RA increases to E

13. Time Scales UT/Local – measured from noon to noon (movement of sun)
Earth’s orbital motion  must rotate >360o
q = 360/ = 0.986o
24 : (360+ q) = sidereal : 360
Sidereal day = 23h 56m 04s
Start defined when ^ is on the celestial meridian

14. Relation between ST and RA
HA = ST – RA
ST at night = RA of object on CM
^ is on the CM at midnight at d
Observing tip
RA = 0 on CM in Sep
Advances 2 hr / mo

15. Airmass – Coordinate Relations
Best HA = 0
Airmass – amt of atm
Extinction = absorption & scattering
c = sec(ZD)
Spherical Trig – law of cosines
cos(s1) = cos(s2)cos(s3) + sin(s2)sin(s3)cos(A1)

16. Effect of Airmass c = sec(z) = sin(d) sin (f) + cos(d) cos(f) cos(HA)
Higher airmass = more extinction
Higher airmass = more refraction
Higher airmass = poorer seeing

17. Summary Coordinates: a, d CM – passes thru Z and NCP a increases to E
Altitude of NCP = f
HA = ST – a
^ is on CM at d
Best obs at small HA (small c)

19. The Astrolabe 2-D model of csphere Greek origins: astron + lambanien
Ancient laptop!
Oldest about 900 BC (Hipparchus)
Middle Ages
Arabian astronomers

20. Astrolabe Functions View of night sky Position of stars
Rise/set of sun, stars
Altitude of object
Measure time of year
Measure time of night

21. Transparent Overlay

22. RA / Dec Grid

23. Elevation Guide

24. Astrolabe Exercises The Sky Tonight When an object rises or sets
Sunset for 6/20/02
RA = 05:58:31
Dec = +23:26:18
04:56 UT
Determine the time of year
The Astrolabe timepiece