Download Astronomy 111 – Lecture 2

Astronomy 111 – Lecture 2
Orientation in the Sky
and
on the Earth
Getting to know the Heavens
• Outline for next lectures :
– Naked Eye Astronomy  Positional Astronomy, The Celestial Sphere
– Description of objects and their motion in sky :
•
•
•
•
•
Stars
Sun
Moon
Planets : Mercury, Venus, Mars, Jupiter, Saturn
Notice order : Increasingly complicated !
– Mapping the Sky
– Mapping the Earth
– Understanding the motion of the objects 
Major Scientific Revolutions :
Geocentric  Heliocentric Model
Newton’s Theory of Gravity
• Important premise for today’s class :
– Pretend we have no prior knowledge about Astronomy.
– Just describe what we see in the sky.
– Aims :
• Develop proper terminology for accurate description.
• Find hypotheses and test them.
Getting to know the Heavens
• First and surprisingly obvious observation :
– There is a time when it is light. = Day
– There is a time when it is dark. = Night
• Collect more data !
The Starry Night
• Number of visible stars : 6000 (only?)
• Distribution :
– Bright and faint stars
– Groups and voids
– Positions fixed Constellations
Constellations
• People have seen patterns and drawn figures in the sky by
connecting the bright stars for ages.
• These figures of stars are the Constellations.
• All cultures have populated the night sky with constellations.
• Most constellations are composed of bright stars that stand out from the
others.
• Many are named after their appearance.
• Peoples greatly separated in distance and/or time often made the same
connections. A few common examples:
– Orion depicted as a male human
– Scorpius called a scorpion by most peoples
– Celestial Rivers and Snakes
Classical Constellations
•Oldest constellations known (Leo, Taurus, Scorpius)
• cuneiform tables 3000 BC (potentially older)
• Greek constellations and star lore:
• Phaenomena (Eudoxus of Cnidos (c. 366 BC)), likely derived from Babylonian
(Assyrian?) lore of c. 1100 BC.
• Greek Ptolemy :
• catalog of 48 "classical" constellations (2nd century AD).
• All of the classical constellations visible from the middle latitudes of the
northern hemisphere.
Populating the Sky
• 16th & 18th century travelers to the southern hemisphere filled in the rest
of the sky.
• Map makers invented new constellations to add to the Classical 48 of
Ptolemy:
– Mix of classical and modern themes from mythology to air pumps and hot air
balloons.
– Up to 150 constellations at one time.
– Many defunct constellations (e.g., Felis the Cat)
Modern Constellations
•Now
88 constellations
• Precise boundaries and figures
•Defined by International Astronomical Union (IAU)
•completed in 1888
Reinventing the Sky ?
• A few Europeans even tried replacing the "pagan" classical
constellations with updated figures depicting Biblical or
Christian figures.
– Julius Schiller (1627):
• New Testament in the Northern Hemisphere
• Old Testament in the Southern Hemisphere
Names of Stars
• Brightest stars all have been given proper names. In western tradition,
these names are mostly Arabic & Greek.
• Revealing the chain of transmission of the western astronomical tradition:
– Mesopotamia -> Classical Greece -> Roman Empire -> Islam -> Renaissance
Europe -> Today
• Other cultures have also named some of the brightest stars.
Names of Stars
• Common names of stars in use today are Arabic, but there is a mix of
Greek and Latin for a few famous stars.
– Examples:
•
•
•
•
Arabic: Rigel, Alberio, Deneb
Greek: Sirius (bright "dog star")
Latin: Polaris (Northern Pole Star)
Unknown: Betelgeuse (thought to be corrupted Arabic)
Modern System
• The Modern naming convention :
– is more systematic.
– was worked out in the 18th century.
– The star name consists of two parts :
• Greek letter to indicate brightness, in order of brightest to faintest.
• Genitive (possessive) form of the constellation name
– Examples: Orion
•
•
•
•
Betelgeuse = Alpha Orionis (brightest star)
Rigel = Beta Orionis (2nd brightest star)
Bellatrix = Gamma Orionis (3rd brightest star) etc.
And so forth. After you run out of Greek letters (24 letters), stars are assigned "Flamsteed
Numbers" (e.g., 61 Orionis).
Outlook
•
•
•
•
Introducing the Celestial Sphere
How stars move
The Idea of a Spherical Earth
The Size of the Earth