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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