- Slides: 27
AST 101 Lecture 4 Figures in the Sky
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
Constellations • There 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
Orion (images from http: //www. astro. uiuc. edu/~kaler/sow/const. html)
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
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"
Modern Constellations • 88 recognized by the IAU • 48 classical constellations (mythological) e. g. , Camelopardalis, Scutum, Hercules, Canes Venaticorum • Southern constellations named in 17 th and 18 th centuries (animals, machines) e. g. , Tucana, Horologium, Fornax, Musca, Doradus
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
Significance of the Constellations None - except as mnemonic devices, or as position indicators. Stars in constellations: • are not physically related • are at different distances
Stories in the Sky
Orion and the Scorpion J. Flamsteed Atlas Coelestis (1753)
The Bears Hevelius Uranographia (1690)
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)
• Pegasus • Cepheus • Cassiopeia • Cetus • Poseidon • The Nereids Hevelius Uranographia (1690)
Andromeda Bayer Uranometria (1603)
Cetus: Hevelius Uranographia (1690)
Cassiopeia Hevelius Uranographia (1690)
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!
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)
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+48 o 3456. Stars to about 9 th magnitude. • Yale Catalog of Bright Stars HR 1 - HR 9110. • The Henry Draper catalog. HD 1 - HD 229000.
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 FK 5 224 ADS 4506 AP IRAS 05524+0723
The Age of Aquarius?
Precession of the Equinoxes Period of precession: 26, 000 years
Physics of Precession • Precession is caused by asymmetric forces. • Earth is not perfectly spherical – The equatorial radius is about 22 km 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.
Due to precession, , the intersection between the ecliptic and the equator, moves completely around the sky in 26, 000 years. Why is called the “first point of Aries”?