AST 101

Lecture 4

Figures in the Sky

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

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

Orion

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

Scorpius

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

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"

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 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

Significance of theConstellations

None - except as mnemonic devices, oras 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)

TheBears

HeveliusUranographia (1690)

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)

Hevelius Uranographia (1690)

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

AndromedaBayer Uranometria (1603)

Cetus: Hevelius Uranographia (1690)

Cassiopeia Hevelius Uranographia (1690)

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!

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)

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.

Aliases of Betelgeuseα Orionis58 OrionisBD +07 1055HR 2061HD 39801GC 7451AG +07 681GSC 00129-01287HIP 27989PPM 149642SAO 113271GCRV 3679FK5 224ADS 4506 APIRAS 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 polarradius (0.3% departure from sphericity)

• The gravitational force of the Moon and Sun, actingon the Earth’s equatorial bulge, drives theprecession.

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”?