2. Knowing the Heavens

Transcription

1 2. Knowing the Heavens Ancient naked-eye astronomy Eighty-eight constellations The sky s ever-changing appearance The celestial sphere Celestial coordinates Seasons: Earth s axial tilt Precession of Earth s rotational axis Position & time in astronomy Observations & the modern calendar Ancient Naked-Eye Astronomy Cultural context Most people lived in rural areas near few people Most people were too poor to afford fuel for light Most people had lots of time to observe the sky People were extremely familiar with diurnal sky motion People were extremely familiar with lunar phases People were extremely familiar with planetary motion People used their imagination Constellation arrangements & names Astrology and its presumed impact on human affairs Scientific context People devised accurate measurement equipment People tried to predict future events Conjunctions of planets with each other & with stars Eclipses of the Sun & Moon Ancient Positional Astronomy Definition Study of the location in the sky of celestial objects Sky location is inextricably tied to observation time Examples Moon s position Monthly cycles Sun s position Annual cycles Planets positions Synodic period cycles Instruments Stonehenge England Chichén Itzá Yucatan Peninsula Casa Grande Arizona Stonehenge: Wiltshire, England Chichén Itzá, Yucatan, Mexico Casa Grande National Monument

2 Eighty-Eight Constellations Constellations Names given to irregular areas of the night sky Based on recognized patterns Similar to cloud patterns Constellation sizes vary tremendously Ursa Major is a very large constellation The Southern Cross is a very small constellation Stars in constellations are usually far apart Analogous to aligned nearby trees & distant mountains Asterisms Easily recognized parts of constellations Big Dipper Pleiades Orion Trapezium The Pleiades Asterism Modern Orion Representation Ancient Orion Representation Sky s Ever-Changing Appearance Diurnal motion The daily motion of the stars Caused by the Earth s very regular axial rotation Observational evidence The motion of the Sun across the sky every day The motion of the stars across the sky every night Monthly Moon One cycle per moonth Annual motion The yearly motion of the Sun Caused by Earth s slightly irregular orbital revolution Observational evidence The annual cycle of changing Sun elevation at noon The annual cycle of changing constellations at midnight Irregular motion Planets One cycle persynodic year Stars Through the Night The night sky over Kitt Peak Observatory west of Tucson, Arizona

3 Circumpolar Stars Through a Night The South celestial pole over Siding Spring Mountain, Australia The Celestial Sphere Unassisted human vision Earth appears to be at the center of a huge sphere Diurnal motion requires at least one celestial sphere Annual motion requires at least one additional sphere Irregular motion requires at least two additional spheres Many celestial bodies are beyond closest sphere These spheres must be made of transparent crystal No compelling evidence for any other model Assisted human vision The Moon Obvious mountains & craters The planets Jupiter has four large moons Saturn has rings Venus has phases like the Moon A Model of the Celestial Sphere Celestial Coordinates Foundations Two natural points: The celestial poles One natural surface: The celestial equator Characteristics Spherical coordinate system Coordinate lines always intersect at right angles North-to-South direction Declination Units: Angle Positive: Point is north of the celestial equator Negative: Point is south of the celestial equator East-to-West direction Right ascension Units: Time 5 h 14 m 32.2 s Positive: Point is east of the vernal equinox Negative: Point is west of the vernal equinox Celestial Sphere Coordinates Earth s Axial Tilt Causes Seasons Facts The Earth is a massive spinning top The axis points in nearly the same direction in space Precession seen as a very slow wobbling of Earth s axis Cause Earth s rotational axis is tilted to its orbital plane ~ 23.5 Sun angle varies ~ 47 summer to winter Result: Sun intensity varies ~ 30% summer to winter Patterns Opposite in northern & southern hemispheres Repeated every year Summer Sun high at noon & days longest of the year Winter Sun low at noon & days shortest of the year

4 Changes in Sunlight Intensity Earth s Axial Tilt: Constant In Space Hallmarks on the Celestial Sphere The ecliptic One circle around the sky The apparent path of the Sun through the sky The projection of the Earth s orbit onto the sky The equinoxes Two points on the ecliptic Derivation: Equal night & day hours Intersection points of ecliptic & celestial equator Vernal equinox: First day of northern spring March equinox Autumnal equinox: First day of southern spring September equinox The solstices Two points on the ecliptic Derivation: North south Sun motion ceases Points halfway between the two equinoxes Summer solstice: First day of northern summer Winter solstice: First day of southern summer A Model of Solstices & Equinoxes The Tropics & Circles The celestial tropics The farthest pole-ward the Sun is directly overhead Tropic of Cancer ~ 23.5 North of the celestial equator Tropic of Capricorn ~ 23.5 South of the celestial equator The celestial circles The farthest equator-ward daylight lasts 24 hours Arctic circle ~ 23.5 South of North celestial pole ~ 66.5 North of celestial equator Antarctic circle ~ 23.5 North of South celestial pole ~ 66.5 South of celestial equator Earth at Northern Winter Solstice This is also known as the Southern Summer Solstice

5 Earth at Southern Winter Solstice This is also known as the Northern Summer Solstice Earth s Rotational Axis Precession All spinning tops Abundant angular momentum Unbalanced forces cause rotation axis to wobble Directly proportional to angular momentum Circular motion of the axis projected into space Earth as a spinning top RPMs are low but mass is high Sun & Moon cause rotation axis to wobble Circular motion of the axis projected into space ~ 26,000 years for one complete precession cycle Changing orientation of the celestial equator Precession of the equinoxes ~ 26,000 years for one complete first-day-of-season cycle Precession: Earth A Spinning Top Ever-Changing Celestial North Pole Position & Time in Astronomy Apparent solar time Time based on the Sun s position in the sky Earth s orbital revolution varies due to its elliptical orbit Earth orbits the Sun slowest when distance is greatest Earth orbits the Sun fastest when distance is least Earth s axial rotation does not vary Local solar noon occurs at Sun s meridian transit The Sun is at its highest point in the sky This is the basis for sundial time Mean solar time Apparent solar time averaged over an entire year This does not vary This is the basis for clock time The Prime Meridian at Greenwich

6 Earth s Variable Orbital Speed Angelic Sundial: Chartres, France Sidereal & Synodic Time Definitions Sidereal time siderus = star Time based on relationship to the stars Synodic time synod = meeting Time based on relationship to the Sun Causes Earth rotates one time relative to the stars 360 of rotation Sidereal day Astronomers use sidereal time Telescope time Earth revolves one degree around the Sun ~ 361 of rotation Synodic day Everyone else uses synodic time Clock time Visualizing Sidereal Time Observations & the Modern Calendar Observations sidereal year = mean solar days Julian calendar Leap years every four years Complicated because of precession of the equinoxes tropical year = mean solar days Julian calendar in error by 11 min 14 sec per year The modern calendar Gregorian calendar Pope Gregory XIII intervenes Concern about the progressive shift of the date of Easter He arbitrarily dropped 10 days (October 5 to 14, 1582) He modified the system of leap years Only century years divisible by 400 are leap years The year 2000 was therefore a leap year The year 2100 will not be a leap year Positional astronomy Patterns in the sky Constellations Asterisms Movements in the sky Diurnal Stars Monthly Moon Annual Sun Irregular Planets The celestial sphere The celestial poles & equator The celestial grid Right ascension [time] Declination [angle] Earth s axial tilt The ecliptic Vernal & autumnal equinoxes Winter & summer solstices Important Concepts Tropics of Cancer & Capricorn Arctic & Antarctic circles Precession of the equinoxes Time in astronomy Apparent & mean solar time Sidereal & synodic time The calendar and astronomy Julian & Gregorian calendars