The Celestial Sphere Karen Meech Institute for Astronomy

The Celestial Sphere Karen Meech Institute for Astronomy TOPS 2003

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

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

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

Great Circles l l 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 l l l HA = 0 on CM “-” indicates rising “+” indicates setting

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

Circles of Declination

The Ecliptic & Seasons l Obliquity – tilt of Earth’s orbital axis (23. 5 o) l Ecliptic – path of the Earth around the sun l l 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

The Ecliptic

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

Time Scales l l UT/Local – measured from noon to noon (movement of sun) Earth’s orbital motion must rotate >360 o l l q = 360/365. 25 = 0. 986 o 24 : (360+ q) = sidereal : 360 Sidereal day = 23 h 56 m 04 s Start defined when ^ is on the celestial meridian

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

Airmass – Coordinate Relations l l l Best observe @ HA = 0 Airmass – amt of atm Extinction = absorption & scattering c = sec(ZD) Spherical Trig – law of cosines cos(s 1) = cos(s 2)cos(s 3) + sin(s 2)sin(s 3)cos(A 1)

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

Summary l l l l 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)

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

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

Transparent Overlay

RA / Dec Grid

Elevation Guide

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