2. Knowing the Heavens
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Transcript of 2. Knowing the Heavens
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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
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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
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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
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Stonehenge: Wiltshire, England
http://www.seeyourinterest.com/2013/05/stonehenge-wiltshire-england/
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Chichén Itzá, Yucatan, Mexico
http://www.journeymart.com/holidays-ideas/festivals/chichenitza-equinox-mexico.aspx
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Casa Grande National Monument
http://www.theroamingboomers.com/wp-content/uploads/2010/07/Casa-Grande-Ruins-National-Monument.jpg
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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
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The Pleiades Asterism
http://en.es-static.us/upl/2009/07/Pleiades_1000.jpg
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Modern Orion Representation
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Ancient Orion Representation
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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
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Stars Through the Night
The night sky over Kitt Peak Observatory west of Tucson, Arizona
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Circumpolar Stars Through a Night
The South celestial pole over Siding Spring Mountain, Australia
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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
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A Model of the Celestial Sphere
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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 – 8°12’06”
• Positive: Point is north of the celestial equator
• Negative: Point is south of the celestial equator
– East-to-West direction Right ascension• Units: Time 5h 14m
32.2s
• Positive: Point is east of the vernal equinox• Negative: Point is west of the vernal equinox
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Celestial Sphere Coordinates
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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
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Changes in Sunlight Intensity
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Earth’s Axial Tilt: Constant In Space
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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 springMarch equinox
• Autumnal equinox: First day of southern springSeptember 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
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A Model of Solstices & Equinoxes
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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 celestialpole
~ 66.5° North of celestial equator• Antarctic circle ~ 23.5° North of South celestial
pole
~ 66.5° South of celestial equator
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Earth at Northern Winter Solstice
This is also known as the Southern Summer Solstice
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Earth at Southern Winter Solstice
This is also known as the Northern Summer Solstice
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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 precessioncycle
• Changing orientation of the celestial equator– Precession of the equinoxes– ~ 26,000 years for one complete first-day-of-
season cycle
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Precession: Earth A Spinning Top
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Ever-Changing Celestial North Pole
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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
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The Prime Meridian at Greenwich
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Earth’s Variable Orbital Speed
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Angelic Sundial: Chartres, France
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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
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Visualizing Sidereal Time
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Observations & the Modern Calendar• Observations
– 1.0000 sidereal year = 365.2564 mean solar days– Julian calendar Leap years every four
years• Complicated because of precession of the equinoxes
– 1.0000 tropical year = 365.2422 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
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• 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
• 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
Important Concepts