Chapter 2: Patterns in the Sky (patterns in the motions of celestial objects across the celestial sphere)
Daily ( Diurnal ) Motion of the Sky A star’s daily path is its diurnal circle [diurnal.snf, Night sky star rotation time lapse.mp4 ( http://www.youtube.com/watch?v=Ja9Tx_gQt3g ), celestial_sphere.avi]
North and South Celestial Poles [figures 2.2, 2.3, 2.1, 2.8] the points directly above earth’s geographic poles these points have no diurnal motion Polaris is about 3/4 º from the actual celestial pole
Celestial Equator : 90 º from poles [figure2.4, 2.8] Meridian : the great circle passing through both the celestial poles and through your zenith [figure2.4]
A1c2:1
The Equatorial System : how things are located in the celestial sphere
Horizon System coordinates depends upon observer’s location! [figure 2.6] system based on great circles, the celestial poles and the celestial horizon [figure 2.5, equatorial_system.avi] Declination : angular distance above celestial equator similar to latitude for earth and altitude in horizon system Right Ascension : angular distance eastward along celestial equator from the vernal equinox. vernal equinox : the location of the sun on the celestial sphere on the first day of spring. similar to longitude/azimuth
A1c2:2 Additional Points on the Equatorial System •Horizon System coordinates depends upon observer’s location, but the Equatorial System does not! To go from Equatorial System to where in sky (Horizon System), need connections (remember: celestial sphere rotates and effect of latitude) •A star’s local hour angle is the angular distance westward around the celestial equator from the meridian to the point on the equator nearest the star. •The local hour angle corresponds to the hours since that star crossed the observer’s meridian •Sidereal Clocks keep track of the local hour angle of the vernal equinox. •Observer’s Latitude is also needed The Equatorial System provides an excellent means for creating star charts and catalogs
A1c2:3
The Apparent Motion of the Sun
Daily motion: east to west relative to the horizon [sun_motion_daily.snf]
Yearly motion: west to east across the celestial sphere [sun_motion_sidereal.snf , fig 2-17] path is through the zodiacal constellations
[figure 2-10, sun_motion_sidereal.snf with constellations, zodiac.avi]
ecliptic : path of the sun through the zodiac [figure 2-11] different constellations are visible at night at different times of year summer/winter solstices : sun’s declination is most northerly/southerly [figure 2-12,13, meridian.avi, meridian_seasons.avi] Stonehenge, Bighorn Medicine Wheel, etc. contain marks to denote the solstices [figure 14,15, 16] autumnal/vernal equinoxes: sun is at the point(s) where the ecliptic crosses the celestial equator [figure 2-11,chichenitza.jpg, pyramid-serpent.gif ] Changing number of daily hours over course of the seasons. Tilt of Earth's rotation Axis relative to its orbit [ seasons.avi, seasons_daylight_hours.mov ]
A1c2:4 The Sun’s Motion and Time: sidereal day : time for one rotation of the celestial sphere (stars) 23 h 56 m 4 s : earth makes one complete rotation apparent solar day : from noon to noon
crossing meridian [Figure 2.17, meridian.avi] On a sundial, the raised part which casts the pointing shadow is the gnomon . The gnomon keeps local apparent time, and casts the shortest shadow of day at apparent noon The apparent solar day varies earth orbit is elliptical+rotation axis tilted 23.5°
use mean solar day (average) = 24 hours to keep time [annalemma.snf] ⇒mean solar time (“normal” clocks) differs from apparent solar time (hour angle of the sun plus 12 hours) [Figure (old) 2ex1,bsfig6.jpg] http://www.analemma.com tropical year : vernal equinox to next vernal equinox not exactly 365 days (365.242199) A1c2:5 adjusted with leap years/minutes/seconds
The Moon
Phases [figure 2.19,21,22] Full phase (gibbous) => quarter phase (half lit, waning)=> crescent => new =>crescent=>quarter (waxing)... Moon’s Motion
daily east to west relative to horizon [moon1.snf] The Month: moon moves west to east relative to celestial sphere sidereal month : time to make one circuit relative to celestial sphere (=> laps the earth) 27.3 days [figure2.24] synodic month : time to come back to same position relative to the sun 29.5 days
path is close to ecliptic (5 º tilt) [figure2.25,26] nodes : where moon’s path intersects ecliptic, points where eclipses may occur It takes about 18.6 years for Moon’s orbit to slide around the ecliptic [ movement_moons_nodes.mov ] A1c2:6 Motion of the Planets
daily east to west relative to horizon [mars1.snf] prograde motion : usual west to east relative to celestial sphere [mars2.snf] retrograde motion : temporary east to west relative to celestial sphere [mars2.snf] [figure2.27,28] retrograde motions occur at conjunctions with Sun for Venus and Mercury, oppositions with Sun for Mars, Jupiter and Saturn conjunction : two objects appear close together in the sky retrograde motions occur at oppositions for other planets oppositions : two objects are 180 degrees apart in the sky all paths are “close” to ecliptic [retrograde_motion.swf]
A1c2:7