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sAstro notesConstellations- Reading 2.1- Patterns in the sky which hold position - Constellations – are patterns of stars that have a historical reference to a mythological figure, animal or object- The stars are usually not near each other in space: they lie in the same direction- Big dipper – 5 inner stars were created together- Many cultures have created stories based patterns in the skyo For example the stars in Orion have been seen as the hunter in ancientGreece and Romeo Osiris, the god of light in ancient Egypt - Origin of constellationso Homer and Hesiod 700 BCE mention a few of our modern constellations (ursa major, orion, Pleiades) – ancient Greeceo Most of constellations were introduced to Greece by Eudoxus C. 390-340 BCE in his phaenomena (appearances)  He likely learned the constellations from Egyptian priest Eudoxus phaenomena is lost to us- The gap of constellations in the southern sky indicates that the northern constellations were derived from a society that lived at 36 degrees northo Greece is to far north and Egypt is too far southo Constellations originated around 2000 BCE- Babylonians probably started the constellations – had a written system and was very advanced of constellations by 700 BCE- made up 60 minutes per hour and 360 degrees per circle – evidence show system originated with their Sumerian ancestors c 2000 bce. - Earliest depiction of classical constellations is statue of titan Atlas in Italyo Copy from 150 ce of a Greek original using the star atlas oh Hipparchus, showed 41 out of 48 constellations without starsModern Constellations - In 1928 the international astro inion defined the boundaries into 88 regions aka constellations in the sky- The boundaries are drawn along east west and north south lines corresponding to the constellations recognized by Europeans- Most of constellations in northern sky were derived from ancient Greek constellations- The constellations in southern sky were taken from 17th century Europe explores - Big dipper – Uursa major, thebig bear- Asterism – is a prominent pattern of stars that is smaller than a constellations for example the Big dipper inside the Ursa Major constellation- Little dipper – inside Ursa minor, small bear- Ecliptic and Ecliptic constellations o Constellations that fall near the path of the sun in our sky: earths orbitprojected out into space – zodiac signs, can see certain constellations depending on where earth is in location to sun Summary - Historical constellations are patterns formed by chance alignments of starso Varies from culture to cultureo Mostly based on Greeko 88 in totalo Occupy 100% of skyDifference between modern and classical constellations- Regions of the sky rather than pictures or figuresMotions of the earth and sky- 2.1- Proposed spherical earth: Aristotle 384- 322 BC- Evidence: curved shadow earth casts on the moon- Additional evidence: new stars seen when sailing at sea- Spherical earth widely believed since time of Aristotle – ancient Greeks - Eratosthenes 276-194 BCE – first guy to measure size of earth withoutleaving Egypt using basic trigonometryo Assumed the earth was a sphere but proved that it wasn’t flat o Measured angle sun came in at Alexandria and syenes- Washington Irving made the myth that Columbus found the earth was round. But actually everyone new it was round - Celestial sphere = map of skyo Like a globe but inside out with the earth being in the centero The celestial north pole is over earths north pole. There happens to be a bright star near which is called Polaris aka north star. o The celestial equator is the extension of the earths equator o Latitude= declinationo Longitude = right ascension- Daily motiono Each day the earth rotates once on it axis (west to east). This causes us to face different directions and see different stars. The stars daily (diurnal) motion reflects earth’s spin. o As the earth rotates, the celestial sphere appears to rotate around us.o The pivot points for the celestial sphere are the north and south poles. So from the northern hemi the sky appears to rotate around Polaris. - Yearly annual motion of eartho The earth orbits the sun each year in a path that is nearly circular o The celestial sphere is unaffected by this motiono The earth’s axis of rotation is inclined 23.5 degrees with respect to its orbital plane. o Variation in sunlight is from tilt of earth o Autumnal equinox = sept 21o Vernal equinox = mar. 21o Ecliptic: the apparent path of the sun projected out onto the celestial sphere At any given point in time the sun blocks approx. One constellationo Zodiac = the band of celestialo Precession In addition to its rotation and revolution, the earths axisalso wobble like a top. The angle between the ecliptic and the equator remains at 23.5 degrees. But the direction changes. The period of the precession of the equinoxes is about 26,000.  Consequences – move over time, the ecliptic constellations shift by one constellation per 2,000 years Over 26k years the north celestial pole is tracing out a circle  Currently it is near north poleThe Copernican Revolution - Chapter 3.1 -3.2- Geocentric model – earth is at center of universe – sun and planets orbit us- Heliocentric- sun is the center of solar system- earth and other planets revolve around it- Copernicus proposed the heliocentric view- The sun is now at the center- The planets orbit in circular paths – Copernicus but actually elliptical- His idea was not accepted because it failed to predict the positions of the planets correct - The basic problem – he assumed circular orbits- On the revolution of heavenly spheres” 1523- Tycho Brahe Measurementso Tycho brahe without a telescope made extremely accurate measurements of the positions of the stars and planets over the course of 20 years o Liked the idea of perfect cirlceso East at the center of universeo The sun travels about the earth in perfect circleo The planets around the suns in perfect circles - Proof of motion through parallax- Kepler was brahe’s assistanto Working for tycho brahe and then stealing his data, kepler had access to the most precise data on planetary positionso Keplers three laws 1st – planets move in ellipses with the sun at its focus- ellipse with extrocintiy of 0 = circle  2nd – planets in their orbits sweep out equal areas in equal times 3rd the period p of an orbit in years squared is equal to a, the semi major- solar systemo


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PSU ASTRO 001 - Constellations

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