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The Celestial Sphere

The Celestial Sphere. Karen Meech Institute for Astronomy TOPS 2003. Latitude and Longitude. Latitude ( f ) meas from equator Longitude ( l ) point of reference – Greenwich UK Units of measure: Deg, arcmin, arcsec O ‘ “. The Horizon System.

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The Celestial Sphere

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

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

  3. 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?

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

  5. 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

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

  7. Circles of Declination

  8. 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

  9. The Ecliptic

  10. 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

  11. Time Scales • UT/Local – measured from noon to noon (movement of sun) • Earth’s orbital motion  must rotate >360o • q = 360/365.25 = 0.986o • 24 : (360+ q) = sidereal : 360 • Sidereal day = 23h 56m 04s • Start defined when ^ is on the celestial meridian

  12. 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

  13. Airmass – Coordinate Relations • Best observe @ 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)

  14. 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

  15. 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)

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

  17. 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

  18. Transparent Overlay

  19. RA / Dec Grid

  20. Elevation Guide

  21. 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 Astrolabe Exercises

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