Introduction to Astronomy
Introduction to Astronomy NSCI 111
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Date Created: 10/05/15
MOTIONS OF THE EARTH AND MOON The Earth 1 The earth quotspinsquot on its axis which accounts for the daynight The period of this daynight is 24 hours by definition The earth spins counterclockwise as viewed above the north pole which accounts for the sun rising in the east and setting in the west 2 The earth quotorbitsquot the sun which accounts along with its tilt for the seasons The period ofthe earth39s orbit is 365242 days a 365 days 1 normal year b leap year adds one day every four years or 1A day per year or 25 days per year for a total of 36525 c we subtract one day every century or 01 days per year for a total of 36524 no leap year on years divisible by 100 d we add two days every 1000 years or add one day every 500 years or 002 days per year for a total of 365242 keep leap year on years divisible by 500 3 The earth has a tilt of 23 2 which along with its orbit accounts for the seasons a The tilt ofthe earth is thought to be changing between 22 and 24 This is technically called quotnutationquot This may affect the appearance and disappearance of ice ages since a bigger angle will cause the seasons to be more extreme b The tilt ofthe earth 39Wobblesquot or more technically precesses in a clockwise manner so that the earth39s seasons appear a little bit earlier relative to the stars each year It takes about 26000 years for the earth to precess once Thus it takes about 2000 years for the earth to precess far enough that the sun appears in a different constellation ofthe zodiac on the spring equinox Also Polaris the North Star now appears almost straight above the north pole but it will not always be so since the axis will precess away from that alignment 4 The earth moves not in a perfect circle but in an ellipse although close to a circle a Perihelion is when the earth is closest to the sun aphelion is when the earth is farthest from the sun b The earth is actually closest to the sun on about Jan 3 Since it moves close to a circle this has little to do with the seasons c However it may contribute to the appearance and disappearance of the ice ages since the day of closest approach moves forward relative to the seasons very slowly it rotates through the seasons with a period of about 115000 years The Moon 1 The moon quotorbitsquot the earth with a synodic period synodic means relative to the earthsun rather than to the stars of 29 2 days Due to the orbiting ofthe earth around the sun this is longer than the sidereal period which is 273 days 2 The moon quotspinsquot about its axis causing daynight on the moon with a period of 29 2 days same as the period of its orbit about the earth Thus a quotdayquot on the moon lasts about 15 earth days ie 24 hours and a quotnightquot on the moon also lasts about 15 earth days 3 The moon39s orbit is not a perfect circle but is in fact an ellipse The point of closest approach to the earth is called perigee the point farthest away is called apogee 4 The moon39s orbit is not along the ecliptic but is inclined 5 2 relative to the ecliptic The intersection ofthe plane of the ecliptic with the plane of the moon39s orbit defines a line called quotthe line of nodesquot NSCI 111 Study Guide for Part 111 page Eclipses 1 Because the basic alignment ofthe earth and moon a solar eclipses can happen only near new moon and b lunar eclipses can happen only near full moon 2 The quotumbraquot is the area behind an object that is totally in shadow The penumbra is the area behind an object only partially in shadow a The umbra for the earth extends 1384000 km behind it b The umbra for the moon extends 373400 km behind it c The moon39s distance from earth at perigee is 356412 km at apogee it 39s 406686 km 1 Since the earth39s umbra extends much farther than the moon even at apogree the moon can pass through the earth39s shadow umbra regardless of apogee or perigee 2 But since the moon39s umbra extends to a distance between the moon39s perigee and apogee distance the earth can pass through the moon39s shadow umbra only near perigee total solar eclipse only near perigee otherwise partial or annular solar eclipse if near agogee 3 Because ofthe tilt of the moon39s orbit from the ecliptic its distance from the earth and its own size there can be eclipses only when the line of nodes is approximately lined up with the sun a e quotseason for eclipsesquot is 31 days long for solar including total annular and partial eclipses where part ofthe earth goes through the moon39s umbra Note not all parts ofthe earth will see every solar eclipse Total eclipses at any one spot on the earth are extremely rarel b The quotseason for eclipsesquot is 24 days long for lunar eclipses where at least part ofthe moon goes through the earth39s umbra This is shorter since the diameter of the earth39s umbra is only 72 ofthe earth39s diameter as it crosses the moon39s orbit c Since the line of nodes slowly moves the quotseason for eclipsesquot happens not simply twice a year once every six months but instead happens once every 58 months RESULT We can have 1 or 2 solar including partial annular and total solar eclipses once every 58 months and we can have 0 or 1 lunar eclipse once every 58 months Total solar eclipses happen somewhere on the earth only about once a year and happen at any specific location vem rarely STUDY GUIDE FOR PART 2 Astronomy Light and Tools INTRODUCTION Ancient Astronomers tried to make geometrical models of the universe that would be able to predict as accurately as possible the motions ofthe astronomical objects through the sky They did not try to explain WHY the motions occurred only WHAT those motions would be Since Newton formulated his laws of motion and gravitation people have tried to explain the why of those astronomical motions in terms of more basic natural laws like the law of gravity assuming that the astronomical objects were composed of material like that of earth and obeyed the same natural laws that apply to the earth These attempts have met with tremendous success At the end of Part 1 we covered Newton39s laws of motion and his law of gravitation They concerned matter and motion But there is yet another basic element of nature that we must try to understand if we are to investigate the universe under the so very successful assumption that the astronomical objects obey the same natural laws as apply on earth This other basic element of nature is LIGHT Its importance is immediately obvious since light is the means by which we obtain information about the astronomical objects In Section A we investigate the properties of light Only by knowing the properties of light can we fully extract all the information the light contains The light can not only tell us in what direction an astronomical object lies but also its surface temperature the composition of its atmosphere its speed toward or away from us and possibly something ofthe space that lies between it and us This will be the major source ofthe knowledge that will be covered in the remaining three parts ofthe course In Section B we use our knowledge of the properties of light from Section A to understand the basic tools ofthe astronomer These are the tools that will be used to first gather the light and then to extract the information from the light Of course the basic astronomical tool is the telescope Both the basic workings ofthe telescope and the types of telescopes will be covered In addition the spectroscope and the photomultiplier will be briefly described In Section C we cover the earth Having covered the basic ideas of gravity and motion in Part1 and the idea of light in Sections A and B of this Part 2 we now considerthe earth as a physical body We need to do this in order to have a basis for understanding the astronomical objects themselves Only by comparing and contrasting other objects to the earth can we really begin to get an idea of what is really out up there when we cover Parts 3 4 and 5 A LIGHT AND THE ATOM text Ch 3 Sections 14 Ch 4 all OUTLINE 1 The speed oflight a for reference speed of sound in air is m 350 ms m 750 mph sound moves relative to medium such as air b speed of light in vacuum 300000000 ms m 670 million mph light moves relative to what 2 The nature of light a a form ofenergy b way this energy is transmitted two theories 1 as waves Electromagnetic oscillations 2 as particles photons 3 The properties of light color wavelength or frequency ofwaves energy of photon re ection both waves and photons particles re ect refraction bending waves predict correct behavior photons don39t diffraction shadows waves spread out photons don39t electricity theory light as electromagnetic waves 39D 00 cm vvvvv NSCI 111 Study Guide for Part II page 2 f energy theory photon emission correct waves fail g interactions with matter photon collide and are absorbed waves fail 4 Heat light and color a heat and light the continuous spectrum 1 total power output depends on temperature and area 2 power output peaks at certain color wavelength depending on temperature b atoms discrete emission amp absorption spectra 5 The Doppler effect frequency depends on speed towards or away Study Questions for Part A 1 What properties of light can be explained by thinking of it as a particle as a wave both 2 Know the approximate wavelengths for the colors of violet blue green yellow orange and red 3 Be able to list the following in order of increasing or decreasing energy andor increasing or decreasing wavelength gamma rays y xrays ultraviolet UV visible infrared IR microwaves radio 4 Know at what approximate wavelength the intensity of light from the sun peaks and what its surface temperature is 5 By knowing at what color the intensity of a star peaks be able to tell whether that star39s surface is hotter or cooler than the sun39s 6 Know qualitatively how the luminosity of a star depends on its temperature and size 7 What is an emission spectrum an absorption spectrum 8 Know what redshift and blueshift refer to B THE TOOLS OF THE ASTRONOMER text Ch 3 Sections 514 OUTLINE 1 The telescope a construction 1 properties ofa lens focal length f diameter d 2 the telescope in its simplest form has two lenses b properties of a telescope 1 magni cation M fofe 2 light gathering ability proportional to d2 3 resolving ability 456 arcsec d in inches 0 relations between properties 1 maximum useful magni cation 60 Xin d in inches 2 brightness field of view and magni cation d types oftelescopes 1 refractors two lenses objective and eyepiece 2 re ectors objective mirror eyepiece lens 1 prime focus inside telescope 2 Newtonian focus light comes out the side 3 Cassegranian focus light comes out the back 3 Schmidt one correcting lens objective mirror eyepiece lens 4 other radio IR UVy ray NSCI 111 Study Guide for Part II page 3 2 Other tools a spectroscope b photomultiplier Study Questions for Part B 1 What are the three major properties ofa telescope 2 What is the smallest angle a 5 inch telescope can theoretically resolve 3 Be able to determine how much more or less light will be gathered by one telescope than by another given the diameters of the objective lenses of each telescope 4 Be able to determine the magnifying power of a telescope given the focal length of the objective lens and the focal length ofthe eyepiece Also given the focal length of the objective lens and the desired magnifying power be able to specify the focal length of the needed eyepiece 5 Name one advantage and one disadvantage of a reflecting over a refracting telescope 6 What is an equatorial telescope mount 7 What advantage does an equatorial telescope mount have over an altazimuth mount 8 Be able to describe each ofthe following types of re ector telescopes Newtonian Cassegranian and prime focus 9 What is a Schmidt telescope C THE EARTH AS A PLANET text Ch 5 Sections 15 Ch 1 Sections 45 OUTLINE 1 Physical properties a size and shape roughly a sphere 25000 miles z40000 km in circumference around equator a little longerthan around the poles b mass 0 interior 1 plate techtonics 2 heat due to cooling down amp radioactive elements determine age using these volcanoes result d surface highest mountain m 5 miles 8 km deepest trench m 10 mi e atmosphere troposphere m 8 miles 14 km 79 N2 20 02 09 Ar 01 other including 03 C02 1 absorbs light UV protection ozone IR warming quotgreenhousequot effect 2 re ects light 25 of sunlight reflected off atmosphere amp clouds 3 bends light we see sun earlier at sunrise amp later at sunset stars twinkle f radiation belts 2 Motion a rotation about its axis b rotation about the sun Study Questions for Part C 1 What is the approximate diameter and the approximate circumference of the earth in kilometers 2 What fraction of the earth39s circumference is the distance from Memphis to St Louis about 400 km NSCI 111 Study Guide for Part II page 4 3 How have we determined the mass of the earth 4 Is the earth a perfect sphere lfnot how does it differ from a perfect sphere 5 What is the age of the earth according to the best scienti c estimates How is this age determined 6 What are the main forces that affect the earth39s surface 7 How high does the troposphere extend 8 Does the sun actually rise earlier or later than it appears to What causes this 9 How does the atmosphere affect light from astronomical bodies Give one example for each ofthe three major ways 10 What are the Van Allen radiation belts 11 Because ofthe earth39s rotation about its axis which way do the winds seem to de ect in the norther hemisphere Study TRUEFALSE questions for all of Part 2 questions similar to these may appear on the test a Red light has a wavelength larger than blue and consists of photons ofenergy less than those of blue b A star with a temperature higher than the sun and not moving will have its energy peak in the red rather then in the yellow like the sun c A spectral line normally in the red will be shifted toward the yellow in a star that is going away from the earth at a speed close to the speed of light d A starthat has a higher temperature and is the same size as the sun will have a luminosity higher than the sun e A telescope whose objective lens has a diameter of 5 inches and a focal length of 100 cm will collect about half as much light as a telescope whose objective lens has a diameter of 10 inches and a focal length of 200 cm f A telescope whose objective lens has a diameter of 5 inches and a focal length of 100 cm will only be able to resolve detail half as small ie twice as large as a telescope whose objective lens has a diameter of 10 inches and a focal length of 200 cm g The circumference ofthe earth is within 10 about 40000 km h The sun actually appears to rise later and set earlier than it would ifthere were no atmosphere i The winds in the Northern Hemisphere rotate clockwise around a low pressure center because the earth is rotating about its axis 39 Because of the earth39s atmosphere the earth is warmer on average than it would be without its atmosphere k Except for mountains and deep sea trenches 1 15 km the earth is a perfect sphere NSCI 111 Study Guide for Part II page 5 I The age of the earth is about 412 billion years old according to science39s best estimate and this age is determined to a large extent by radioactive dating
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