Friday Jan. 28Syllabus and class notes are at: www.as.utexas.edugo to courses, AST301 – Introduction to Astronomy – LacyHomework is due now in the back of the room.Also pick up next week’s homework.Read Chapter 4 for Monday.Topics for this weekKnow what the magnitudes of stars tell you about their relative brightnessesDescribe the apparent motions of stars across the sky during a night as seen from various locations on EarthDescribe and explain the apparent motion of the Sunrelative to the stars during a yearExplain how the tilt of the Earth’s axis causes the seasonsDescribe how the path of the Sun across the sky during aday differs during different seasonsDescribe and explain the phases of the Moon and themotion of the Moon relative to the stars during a monthExplain how eclipses occurApparent motion of Sun during the yearThe Earth orbits the Sun once a year.This makes the Sun appear to pass in front of differentstars (the constellations of the zodiac) during a year.The zodiac does not lie on the celestial equator, but is on acircle tipped about 23o from the equator.This is because the axis of the Earth’s rotation is tippedabout 23o from the axis of its orbital motion.During a day the Sun follows the same path across the skyas the stars it is in front of, since this motion is caused bythe rotation of the Earth.But from day to day the Sun slowly moves along the zodiac(also called the ecliptic).This causes it to rise with different stars and to move northand south of the celestial equator during a year.The solar day and the sidereal dayThe motion of the Sun along the ecliptic causes the timefrom sunrise to sunrise (the solar day = 24 hours) todiffer from the time from when one star rises until when itnext rises (the sidereal day = 23 hours, 56 minutes).There are 365.24 solar days in a year.How many sidereal days are there in a year?SeasonsThe fact that the ecliptic is tipped relative to the celestialequator means that at some times of the year the Sun isin front of stars north of the celestial equator and atsome times it is south.Stars on the equator rise due east, pass 30o south ofoverhead (seen from Austin), and set due west.Stars north of the equator rise north of east, pass closer tooverhead, and set north of west.Stars south of the equator rise south of east, pass morethan 30o south of overhead, and set south of west.What path is the Sun currently following across the sky?If you don’t know, watch it.The earth’s axis of rotation always points toward the sameplace in the stars (the celestial pole), even as the earth goesaround the sun.The polar axis is tilted by 23.5 degrees with respect to theecliptic planeThe Moon: Phases, Eclipses, etc.Most questions about the appearance of the Moon can beanswered by drawing the right drawing:The Moon orbits around the Earth in the same direction asthe Earth rotates. The Sun lights up the side of theMoon (and Earth) that faces the Sun.Eclipses (but not phases!) are caused by shadows of theEarth and Moon on each other.Typical questions:What time does the Moon rise and set in each of itsphases?What phase is the Moon in just before a solar eclipse?What phase is the Moon in just before a lunar eclipse?On Tuesday the Moon was full.At what time of day was it overhead?Draw yourself standing on the Earth with the full Moonoverhead, and ask what time of day it is.At what time of day did it rise?Draw yourself on the Earth where you first see the Moon,and ask what time of day it is.QuizNext Tuesday the Moon will be in the 3rd quarter phase.At what time will it rise?Hint: Draw the picture, remembering that everything movescounter-clockwise.A. around sunriseB. around noonC. around sunsetD. around midnightQuizNext Tuesday the Moon will be in the 3rd quarter phase.At what time will it rise?A. around sunriseB. around noonC. around sunsetD. around midnightDraw the picture.QuizThe Moon will be in 3rd quarter phase next Tuesday.What phase will the Moon be in a week from next Tuesday?A. newB. fullC. 1st quarterD. 3rd quarterQuizWhat phase will the Moon be in a week from next Tuesday?A. newB. fullC. 1st quarterD. 3rd quarterThe Moon takes about 4 weeks to orbit the Earth, so it goesfrom 3rd quarter phase to new phase in a week.When will it rise?Lunar eclipseEclipsesLunar eclipses (when the Moon disappears, or at least getsdarker) occur when the shadow of the Earth falls on theMoon.Solar eclipses (when the Sun is hidden) occur when theMoon passes in front of the Sun, blocking our view.In that case, the shadow of the Moon falls on us.What is the phase of the Moon just before a lunar eclipse?A. newB. fullC. 1st quarterD. 3rd quarterEclipsesLunar eclipses (when the Moon disappears, or at least getsdarker) occur when the shadow of the Earth falls on theMoon.Solar eclipses (when the Sun is hidden) occur when theMoon passes in front of the Sun, blocking our view.In that case, the shadow of the Moon falls on us.What is the phase of the Moon just before a lunar eclipse?A. newB. full Draw the picture!C. 1st quarterD. 3rd quarterEclipses and the eclipticWhy isn’t there a lunar eclipse every full Moon?The reason is that the orbit of the Moon isn’t exactly in thesame plane as the orbit of the Earth (the ecliptic).Usually the Moon passes above or below the Earth’sshadow.There are only 1 or 2 lunar eclipses each year.And we only see an eclipse if we are on the side of theEarth facing the Moon (i.e. the Moon is up).Solar eclipseEclipses and the eclipticWhy isn’t there a solar eclipse every new Moon?It’s almost the same reason.Usually the Moon’s shadow passes above or below theEarth.Also, the Moon’s shadow is very small when it hits theEarth, and only those people in the shadow see a
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