AST 105
The Sky
Coordinates
and
Constellations
Early evening
August 2014
40o N
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)
• 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)
• 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+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”?