sky - Stony Brook University · Stars in constellations: • are not physically related • are at...
Transcript of sky - Stony Brook University · Stars in constellations: • are not physically related • are at...
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AST 105
The Sky
Coordinates and
Constellations
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Early evening August 2014 40o N
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Where is it?
The sky appears as a 2-dimensional sphere • Alt-Az Coordinates (reference: local)
• Celestial Coordinates (reference: Earth)
• Ecliptic Coordinates (reference: Solar System)
• Galactic Coordinates (reference: Galaxy)
•
"What good are Mercator's North Poles and Equators Tropics, Zones, and Meridian Lines?" So the Bellman would cry, and the crew would reply "They are merely conventional signs"
L. Carroll -- The Hunting of the Snark
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Altitude – Azimuth Coordinates
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Latitude and Longitude
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Celestial Sphere
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Declination
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Right Ascension
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Motions of the Stars
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Pole and Equator
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Ecliptic Coordinates
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Earth’s Orbit
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Analemma
The position of the Sun at civil noon (standard time). This demonstrates: • The inclination of the ecliptic • The equation of time • The non-circularity of Earth’s orbit
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Constellations
• There are about 6000 stars visible to the 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 Sky Many societies have identified constellations Sumer (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", describing 45 Egyptian constellations. • 240 BCE: Eratosthenes records 42 constellations. • 150 CE: Hipparchus catalogs 1080 stars in 49 constellations. • ~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 the ecliptic
• Western zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces
• 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 the Constellations
None - except as mnemonic devices, or as 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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The Bears
Hevelius Uranographia (1690)
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Perseus: A Greek Soap
Opera • King Acrisius of Argos • Danae • Zeus • Perseus • Dictys • King Polydectes • Medusa • Hermes • Athena • the Graeae • The Gorgons Doppelmayr's Atlas coelestis (1742)
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Hevelius Uranographia (1690)
• Pegasus • Cepheus • Cassiopeia • Cetus • Poseidon • The Nereids
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Andromeda Bayer 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 found in the fall evening sky.
• Perseus holds the head of Medusa (the variable star Algol - the Ghoul - is her eye).
• As punishment for her vanity, Queen Cassiopeia, as a circumpolar constellation, is condemned to hang upside down half the year, a most undignified position!
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Names of the Stars
Few stars have proper names Most 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 Stars Classic 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 Betelgeuse α Orionis 58 Orionis BD +07 1055 HR 2061 HD 39801 GC 7451 AG +07 681 GSC 00129-01287 HIP 27989 PPM 149642 SAO 113271 GCRV 3679 FK5 224 ADS 4506 AP IRAS 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 polar radius (0.3% departure from sphericity)
• The gravitational force of the Moon and Sun, acting on the Earth’s equatorial bulge, drives the precession.
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Due to precession, γ, the intersection between the ecliptic and the equator, moves completely around the sky in 26,000 years.
Why is γ called the “first point of Aries”?
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