AST 105

The Sky

Coordinates and

Constellations

Early evening August 2014 40o N

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

Altitude – Azimuth Coordinates

Latitude and Longitude

Celestial Sphere

Declination

Right Ascension

Motions of the Stars

Pole and Equator

Ecliptic Coordinates

Earth’s Orbit

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 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

Orion

(images from http://www.astro.uiuc.edu/~kaler/sow/const.html)

Scorpius

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 17th and 18th 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)

Hevelius Uranographia (1690)

• Pegasus • Cepheus • Cassiopeia • Cetus • Poseidon • The Nereids

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+48o 3456. Stars to about 9th 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 FK5 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 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.

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”?