ast101 lect 4 - stony brook university · constellations •there are about 6000 stars visible to...
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AST 101
Lecture 4
Figures in the Sky
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Analemma
The position of theSun at civil noon(standard time).This demonstrates:•The inclination of theecliptic•The equation of time•The non-circularity ofEarth’s orbit
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Constellations
• There are about 6000 stars visible tothe naked eye under good conditions
• About 2000 are visible at any one time• Far fewer are visible from urban locales
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Orion
(images from http://www.astro.uiuc.edu/~kaler/sow/const.html)
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Scorpius
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Patterns in the SkyMany societies have identified constellationsSumer (4000 BCE): 6 constellations:•Bull (Taurus)•Crab (Cancer)•Maiden (Virgo)•Scorpion (Scorpius)•Sea Goat (Capricorn)•Fishes (Pisces)Rest of the Western Zodiac codified in Babylon (2350 BCE)
Also: Chinese, Koreans, the Mayans, American Indians,and various African tribes
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Greek Constellations
•366 BCE: Eudoxos publishes "Phaenomena", describing45 Egyptian constellations.•240 BCE: Eratosthenes records 42 constellations.•150 CE: Hipparchus catalogs 1080 stars in 49constellations.•~150 CE: Ptolmey records 48 constellations in the"Almagest"
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Modern Constellations
• 88 recognized by the IAU• 48 classical constellations (mythological)
e.g., Camelopardalis, Scutum, Hercules, Canes Venaticorum
• Southern constellations named in 17th
and 18th centuries (animals, machines)e.g., Tucana, Horologium, Fornax, Musca, Doradus
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Zodiac
• 12 (13) constellations containing theecliptic
• Western zodiac: Aries, Taurus, Gemini, Cancer, Leo,Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, andPisces
• Sun spends 18 days in Ophiuchus; only 7 in Scorpius.• Planets also pass through Cetus, Corvus, Crater, Hydra, Orion,
Pegasus, Scutum, and Sextans
• Chinese zodiac: Tiger, Horse, Dragon, Rat, Hare,Ram, Serpent, Ape, Cock, Dog, Boar, and Ox
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Significance of theConstellations
None - except as mnemonic devices, oras position indicators.
Stars in constellations:• are not physically related• are at different distances
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Stories in the Sky
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Orion and the Scorpion
J. Flamsteed Atlas Coelestis (1753)
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TheBears
HeveliusUranographia (1690)
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Perseus: AGreek Soap
Opera• King Acrisius of Argos• Danae• Zeus• Perseus• Dictys• King Polydectes• Medusa• Hermes• Athena• the Graeae• The GorgonsDoppelmayr's Atlas coelestis (1742)
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Hevelius Uranographia (1690)
•Pegasus•Cepheus•Cassiopeia•Cetus•Poseidon•The Nereids
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AndromedaBayer Uranometria (1603)
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Cetus: Hevelius Uranographia (1690)
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Cassiopeia Hevelius Uranographia (1690)
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Today• Perseus, Andromeda, Pegasus, Cetus,
Cepheus, and Cassiopeia are all to be foundin the fall evening sky.
• Perseus holds the head of Medusa (thevariable star Algol - the Ghoul - is her eye).
• As punishment for her vanity, QueenCassiopeia, as a circumpolar constellation, iscondemned to hang upside down half theyear, a most undignified position!
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Names of the Stars
Few stars have proper namesMost are of Arabic origin
•Aldebaran: The Follower (rises after the Pleiades)•Algol: The Ghoul (the demon star)•Antares: Rival of Ares (Mars)•Betelgeuse: Armpit of the central one (Orion)•Fomalhaut: Mouth of the Southern Fish•Rigel: left leg (of Orion)
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Numbers of the StarsClassic catalogs:Ptolmey’s Almagest (~150 CE)•The Bayer catalog. Stars named alphabetically (in Greek)e.g., α Orionis (Betelgeuse) β Orionis (Rigel), γ Orionis (Bellatrix)•The Flamsteed Catalog. Stars listed numerically from west to east by constellation e.g., 1 Tauri, 2 Tauri, 3 Tauri•The Bonner Durchmusterung (1855). Stars listed numerically in latitude bands around sky. e.g., BD+48o 3456. Stars to about 9th magnitude.•Yale Catalog of Bright Stars HR 1 - HR 9110.•The Henry Draper catalog. HD 1 - HD 229000.
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Aliases of Betelgeuseα Orionis58 OrionisBD +07 1055HR 2061HD 39801GC 7451AG +07 681GSC 00129-01287HIP 27989PPM 149642SAO 113271GCRV 3679FK5 224ADS 4506 APIRAS 05524+0723
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The Age of Aquarius?
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Precession of the Equinoxes
Period of precession: 26,000 years
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Physics of Precession• Precession is caused by asymmetric forces.• Earth is not perfectly spherical
– The equatorial radius is about 22km larger than the polarradius (0.3% departure from sphericity)
• The gravitational force of the Moon and Sun, actingon the Earth’s equatorial bulge, drives theprecession.
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Due to precession, , the intersection between theecliptic and the equator, moves completely aroundthe sky in 26,000 years.
Why is called the “first point of Aries”?
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