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Astronomy & Astrophysics
Ingrid L.Stefanovic

General Information • Earth is in the Milky Way galaxy • Andromeda is the closest galaxy to ours • Remarkable fact about other galaxies is that they are all moving away from ours • Planets that were visible to ancients – Mercury, Venus, Mars, Jupiter, Saturn • Range in planet sizes: Mercury (tiniest), Jupiter (11 times larger than earth) • Sun is 109 times earth’s diameter • The calendar we use is called Gregorian calendar • Entire celestial sphere is divided into 88 regions (constellations) • We now use Copernican’s model that came 1800 years after Aristarchus proposed the Heliocentric model. • A telescope has magnification (objects look bigger) and light gathering (can see dimmer objects) Definitions: • Galaxy – a group of gravitationally linked stars • Milky Way Galaxy – the group of few hundred billion stars of which our sun is part of • Constellation – an area of the sky containing a patter of stars named for a particular object, person or animal • The constellations through which the sun passes as it moves along the ecliptic are called constellations of “Zodiac”. • Celestial sphere – imaginary sphere of heavenly bodies that seem to centre on the observer • Celestial pole – point on celestial sphere directly above the earth’s pole • Celestial equator – equator of celestial sphere that is in line with earth’s equator • Angular separation – an angle between the objects as seen from earth • 1 degree = 60 x arcminutes = 60 x 60 x arcseconds • Ecliptic – the apparent path that the sun takes among the stars in a year • Equinox – position of Sun is exactly along the celestial equator • Vernal Equinox – when the sun crosses the celestial equator moving north on March 21 st • Autumnal Equinox – when the sun crosses the celestial equator moving south on Sept 22 nd • Angle of Elongation – angle of moon (or planet) from the sun in the sky • Stellar Parallax – the apparent annual shifting of nearby stars with respect to background stars, measured as the angle of shift. (the thumb and blinking eye experiment) • Perihelion – the nearest point of a planet to the Sun (moves fastest) • Aphelion – the farthest point of a planet to the Sun (moves slowest) • Right Ascension = angle east of the point where the Sun crosses the celestial equator at the vernal equinox (longitude) • Declination = degrees above the celestial equator (latitude) • Fact = observations that occur repeatedly • Law or principle = well established theory • Eccentricity of an ellipse is the distance between the focii divided by the longest distance across the ellipse (major axis). Eccentricity ranges from 0 for circle to 1 for a completely flat ellipse. • Pythagoreans - group of Greek mystics/philosophers who believed Mathematics contained religious truths Scientific Methods • How did we get to the understanding that the universe is expanding: (1) astronomers observed facts about the universe (2) they used this to create a theory (3) they used the theory to make predictions (4) the predictions were either confirmed by observation or modified to fit data • Three criteria for judging Scientific Theory (i) The model must fit the data (ii) The model must make testable predictions (iii) Occam’s razor The Sun • Sun – sphere made up almost entirely of hydrogen & helium (energy comes from nuclear fusion) • Each star we see is similar to our sun as in the same energy producing process • Time of Sun’s cycle through the stars is 1 year • Sun and the Seasons: For Northern Hemisphere 1. Sun rises and sets farther north in summer than in winter 2. Sun is in the sky longer each day in summer than in winter (reason for seasons) 3. Sun reaches a higher point in the sky in summer than in winter • Earth is closest to the Sun in January & farthest in July • Everyday, the Sun moves slightly East relative to stars • Path of the ecliptic relates to seasons as well • June 21 – sun is at northern most position [Summer Solstice – sun stops climbing and starts going down] st • Dec 21 – sun is at southern most position [Winter Solstice – sun stops going down and starts climbing] • March 21 & Sept 22 – Sun crosses the celestial equator • Every location on earth experiences equal periods of day & night when the sun crosses the celestial equator • Sometimes the Earth passes through the shadow of the Moon resulting in a solar eclipse • If the sun is completely blocked by the moon – from Earth you can see the Sun's corona • Annular Eclipse – moon covers sun but moon is too far away from earth to completely block it The Moon • Moon – solid planet-like object about ¼ the size in diameter of earth • Moon – interesting fact is that moon’s rotation rate is equal to it’s revolution rate around earth • This is the reason why we see the same face of moon every time • Moon circles the earth completing one orbit in slightly less than a month • The Moon rises in the East and sets in the West due to Earth’s rotation relative to celestial sphere • The Moon rises about 1 hour later everyday • “Waxing Crescent” moon – when moon is going from a new moon to a quarter moon • “First-quarter” moon – ¼ the way around orbit • “Waning Crescent” – from third-quarter moon to new moon • Waxing – increasing Waning – decreasing • “New” moon – cannot see the moon (moon is directly blocking sun) • “Siderial Period” – time interval between two successively similar alignments between Earth, Moon & distant stars (27.3 days) • “Synodic Period” – time interval between two successively similar alignments between Earth, Moon & Sun (29.5 days) – Referred to as Lunar Month • “Lunar Eclipse” – when the Moon enters the Earth’s shadow • “Umbra” – portion of a shadow that receives no direct light from light source • “Penumbra” – Moon receives full light from Sun • Reason why Lunar Eclipse does not occur at each Full moon: Moon’s plane of revolution is tilted relative to the Earth’s plane of revolution around Sun • Lunar Eclipse can only occur 1-3 times a year Mercury & Venus • Mercury and Venus have small elongation and that is why they can be seen when the sun is just below horizon • Venus appears in the east shortly before sunrise and in the west shortly after sunset, and it is never seen overhead at night. • Venus’s orbit has the smallest eccentricity and hence closest to being a perfect circle Aristotle’s Model • Geocentric view – placed Earth on the centre to all celestial objects, Earth does not move or rotate, stars are fixed in the sky • Aristotle’s argument against Heliocentricity - if the earth is moving, why don’t we see the apparent movement of the stars in the sky • Aristotle concluded that the earth does not move because stellar parallax was hard to observe due to not being able to see the shifts of stars with the naked eye. • Aristotle used three observations to argue that the earth is spherical: (i) only on a sphere will all falling bodies seek the centre (ii) a traveler going north will see more of the northern sky while the southern stars sink below the horizon (iii) during lunar eclipses, the edge of the earth’s shadow is always circular. • Aristotle said that matter on terrestrial was different from celestial (things on earth fall down whereas things in space move in circles, earthly things come to a stop whereas celestial things never stop moving) • This gave two sets of rules for earthly things and celestial things • The only problem with Aristotle’s Geocentric model was that it couldn’t explain retrograde motion and varying planetary brightness. Ptolemy’s Model • Ptolemy agreed with Aristotle’s Geocentric model, but added epicycles to each planet to explain retrograde motion and varying planetary brightness. • Ptolemy’s argument against Heliocentricity is that if the earth is moving, why don’t we feel a massive wind • Ptolemy was the most important contributor to Geocentric
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