AST 105. The Sky. Coordinates and Constellations

Transcription

1 AST 105 The Sky Coordinates and Constellations

2 Early evening August o N

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6 Where is it? "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 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)

7 Altitude Azimuth Coordinates

8 Latitude and Longitude

9 Celestial Sphere

10 Declination

11 Right Ascension

12 Motions of the Stars

13 Pole and Equator

14 Ecliptic Coordinates

15 Earth s Orbit

16 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

17 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

18 Orion (images from

19 Scorpius

20 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

21 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"

22 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

23 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

24 Significance of the Constellations None - except as mnemonic devices, or as position indicators. Stars in constellations: are not physically related are at different distances

25 Stories in the Sky

26 Orion and the Scorpion J. Flamsteed Atlas Coelestis (1753)

27 The Bears Hevelius Uranographia (1690)

28 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)

29 Hevelius Uranographia (1690) Pegasus Cepheus Cassiopeia Cetus Poseidon The Nereids

30 Andromeda Bayer Uranometria (1603)

31 Cetus: Hevelius Uranographia (1690)

32 Cassiopeia Hevelius Uranographia (1690)

33 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!

34 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)

35 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 Stars to about 9 th magnitude. Yale Catalog of Bright Stars HR 1 - HR The Henry Draper catalog. HD 1 - HD

36 Aliases of Betelgeuse α Orionis 58 Orionis BD HR 2061 HD GC 7451 AG GSC HIP PPM SAO GCRV 3679 FK5 224 ADS 4506 AP IRAS

37 The Age of Aquarius?

38 Precession of the Equinoxes Period of precession: 26,000 years

39 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.

40 Due to precession, γ, the intersection between the ecliptic and the equator, moves completely around the sky in 25,800 years. Why is γ called the first point of Aries?

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