Physical Geography - Version 1 Unit 5 Earth-Sun Relationships Reasons for the Seasons

Figure Fall Colors at Lake Sabrina in Bishop , California . Image by Jeremy is under a license . UNIT RELATIONSHIPS REASONS FOR THE SEASONS Goals Objectives of this unit Describe how Earth movements affect seasons and cause day and night . Identify the relationships between latitude , length of daylight , or night , as it pertains to incipient solar angles . Explain and identify the similarities and differences between solar and lunar eclipses . Describe the phases of the Moon and explain why they occur . Explain how movements of the Earth and Moon affect Earth tides . GEOGRAPHY

THE SUN AND THE EARTH SYSTEM The solar system is made up of the Sun , the planets that orbit the Sun , their satellites , dwarf planets , and many , many small objects , like asteroids and comets . All of these objects move , and we can see these movements . We notice the Sun rises in the eastern sky in the morning and sets in the western sky in the evening . We observe different stars in the sky at different times ofthe year . When ancient people made these observations , they imagined that the sky was moving while the Earth stood still . In 1543 , proposed a radically different idea The Earth and the other planets make regular revolutions around the Sun . He also suggested that the Earth rotates once a day on its axis . idea slowly gained acceptance and today we base our view of motions in the solar system on his work . We also now know that everything in the universe is moving . Positions Movements The Earth rotates once on its axis about every 24 hours . lfyou were to look at Earth from the North Pole , it would be spinning counterclockwise . As the Earth rotates , observers on Earth see the Sun moving across the sky from east to west with the beginning of each new day . We often say that the Sun is rising or setting , but it is the Earth rotation that gives us the perception of the Sun rising or setting over the horizon . When we look at the Moon or the stars at night , they also seem to rise in the east and set in the west . Earth rotation is also responsible for this . As Earth turns , the Moon and stars change position in our sky . Another effect of Earth rotation is that we have a cycle of daylight and darkness approximately every 24 hours . This is called a day . As Earth rotates , the side of Earth facing the Sun experiences daylight , and the opposite side ( facing away from the Sun ) experiences darkness or nighttime . Since the Earth completes one rotation in about 24 hours , this is the time it takes to complete one cycle . As the Earth rotates , different places on Earth experience sunset and sunrise at a different time . As you move towards the poles , summer and winter days have different amounts of daylight hours in a day . For example , in the Northern Hemisphere , we begin summer on or around June 215 . At this point , the Earth North Pole is pointed directly toward the Sun . Therefore , areas north of the equator experience longer clays and shorter nights because the northern half of the Earth is pointed toward the Sun . Since the southern half ofthe Earth is pointed away from the Sun at that point , they have the opposite effect , longer nights and shorter days . GEOGRAPHY

For people in the Northern Hemisphere , winter begins on or around December 21 . At this point , it is Earth South Pole that is tilted toward the Sun , and so there are shorter days and longer nights for those who are north ofthe equator . sun rays Figure The Earth Tilt on its Axis Leads to One Hemisphere Facing the Sun More Than the Other Hemisphere and Gives Rise to the Seasons . Image is under a Creative Commons Alike license . Energy from the Sun The earth constantly tries to maintain an energy balance with the atmosphere . Most of the energy that reaches the Earth surface comes from the Sun . About 44 of solar radiation is in the visible light wavelengths , but the Sun also emits infrared , ultraviolet , and other wavelengths . When viewed together , all of the wavelengths of visible light appear white . But a prism or water droplets can break the white light into different wavelengths so that separate colors appear . WAVELENGTHS SUN VA 290320 INA Figure Diagram Showing the Three Types of Ultra Violet Light Emitted from the Sun . Image by team is used under a license . GEOGRAPHY

Of the solar energy that reaches the outer atmosphere , UV wavelengths have the greatest energy . Only about of solar radiation is in the UV wavelengths . The three types are the highest energy ultraviolet , does not reach the planet surface at all . the energy , is also mostly stopped in the atmosphere . UVA the lowest energy , travels through the atmosphere to the ground . The remaining solar radiation is the longest wavelength , infrared . Most objects radiate infrared energy , which we feel as heat . Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases . Ozone completely removes , most and some UVA from incoming sunlight . Oxygen , carbon dioxide , and water vapor also filter out some wavelengths . THE GREENHOUSE EFFECT The exception to Earth temperature being in balance is caused by greenhouse gases . But first , the role of greenhouse gases in the atmosphere must be explained . Greenhouse gases warm the atmosphere by trapping heat . Some of the heat radiation out from the ground is trapped by greenhouse gases in the troposphere . Like a blanket on a sleeping person , greenhouse gases act as insulation for the planet . The warming of the atmosphere because of insulation by greenhouse gases is called the greenhouse effect . Greenhouse gases are the component of the atmosphere that moderate Earth temperatures . The Effect Some solar radiation is reflected by the Some of the infrared radiation Earth and the passes through the atmosphere . Some is absorbed and in all directions by greenhouse gas effect of this is to warm the Earth surface and the lower atmosphere . Most radiation is absorbed by the Earth Infrared radiation and warms I , is emitted by the Earth surface . GEOGRAPHY

Greenhouse gases include , methane , nitrous oxides ( NO and ) and ( All are a normal part ofthe atmosphere except . The table below shows how each greenhouse gas naturally enters the atmosphere . Different greenhouse gases have different abilities to trap heat . For example , one methane molecule traps 23 times as much heat as one molecule . One molecule ( a type of ) traps times as much heat as one . Still , is a very important greenhouse gas because it is much more abundant in the atmosphere . Human activity has significantly raised the levels of many greenhouse gases in the atmosphere . Methane levels are about times higher as a result of human activity . Carbon dioxide has increased by more than 35 . have only recently existed . What do you think happens as atmospheric greenhouse gas levels increase ?

More greenhouse gases trap more heat and warm the atmosphere . The increase or decrease of greenhouse gases in the atmosphere affect climate and weather the world over . EARTH SEASONS It is a common misconception that summer is warm , and winter is cold because the Sun is closer to Earth in the summer and farther away from it during the winter . Remember that seasons are caused by the tilt of Earth axis of rotation and Earth yearly revolution around the Sun . This results in one part ofthe Earth being more directly exposed to rays from the Sun than the other part . The part tilted away from the Sun experiences a cool season , while the part tilted toward the Sun experiences a warm season . Seasons change as the Earth continues its revolution , causing the hemisphere tilted away from or towards the Sun to change accordingly . When it is winter in the Northern Hemisphere , it is summer in the Southern Hemisphere , and vice versa . GEOGRAPHY

September Autumnal Equinox December Winter Solstice June Summer Solstice March Vernal Equinox Figure Seasons Diagram , Nate the Tilt and Circle of Illumination for Each Season . Image is in the public domain . Northern Hemisphere Summer The North Pole is tilted towards the Sun and the Sun rays strike the Northern Hemisphere more directly in summer . At the summer solstice , which is around June 215 or 22 , the rays hit the Earth most directly along the Tropic of Cancer ( that is , the angle of incidence of the sun rays there is zero ( the angle of incidence is the deviation in the angle of an incoming ray from straight on ) When it is the summer solstice in the Northern Hemisphere , it is the winter solstice in the Southern Hemisphere . Northern Hemisphere Winter The Winter solstice for the Northern Hemisphere happens on or around December 215 or the 22 . The tilt of Earth axis points away from the Sun . Light from the Sun is spread out over a larger area , so that area is heated as much . With fewer daylight hours in winter , there is also less time for the Sun to warm the area . When it is winter in the Northern Hemisphere , it is summer in the Southern Hemisphere . Equinox Halfway between the two , the Sun rays shine most directly at the equator , called an The daylight and nighttime hours are exactly equal on an equinox . The autumnal equinox happens on or around September 22 or the 23 and the vernal or spring equinox happens on or around March 215 or 22 in the Northern Hemisphere . GEOGRAPHY

Analemma In astronomy , an analemma is a diagram showing the position ofthe Sun in the sky , as seen from a fixed location on Earth at the same mean solar time , as that position varies over the course of a year . The component of the analemma results from the change in the Sun declination due to the tilt of Earth axis of rotation . The component results from the nonuniform rate of change of the Sun right ascension , governed by combined effects of Earth axial tilt and orbital eccentricity . An analemma can be traced by plotting the position of the Sun as viewed from a fixed position on Earth at the same clock time every day for an entire year , or by plotting a graph of the declination against the equation of time . The resulting curve resembles a long , slender eight with one lobe much larger than the other . This curve is commonly printed on terrestrial globes , usually in the eastern Pacific Ocean , the only large tropical region with very little land . It is possible , though challenging , to photograph the analemma , by leaving the camera in a fixed position for an entire year and snapping images on intervals . The long axis of the figure , the line segment joining the northernmost point on the analemma to the southernmost , is bisected by the celestial equator , to which it is approximately perpendicular , and has a length of twice the obliquity of the ecliptic , about . The component along this axis of the Sun apparent motion is a result of the familiar seasonal variation of the declination of the Sun through the year . The width of the figure is due to the equation oftime , and its angular extent is the difference between the greatest positive and negative deviations of local solar time from the local mean time when this is related to the angle at the rate of 15 per hour , in 24 hours . The difference in the size of the lobes ofthe form arises mainly from the fact that the perihelion and aphelion occur far from . They also occur a mere couple of weeks after , which in turn causes a slight tilt of the figure eight and its minor lateral asymmetry . January July Perihelion Aphelion million million million mi million mi Figure Diagram or Earth during the Aphelion ( Away ) Perihelion ( Near ) Image by is used under a license . GEOGRAPHY

Three parameters affect the size and shape of the analemma , which are eccentricity , obliquity , and the angle between the apse line and the line of . Viewed from an object with a perfectly circular orbit and no axial tilt , the Sun would always appear at the same point in the sky at the same time of day throughout the year and the analemma would be a dot . For an object with a circular orbit but significant axial tilt , the analemma would be a figure eight with northern and southern lobes equal in size . For an object with an eccentric orbit but no axial tilt , the analemma would be a straight line along the celestial equator . Eccentricity . Obliquity . 25 ii iii ii iii 25 ii iii ill . 20 20 15 15 10 10 ' Cu ) OJ ( ii i ill ii i 12 12 12 Equation of Time , min ) Equation of Time , min ) Equation of Time , min ) Figure The Three Variation , Eccentricity , Obliquity Combined . Image by Anthony is used under a license . The component of the analemma shows the Sun declination , its latitude on the celestial sphere , or the latitude on the Earth at which the Sun is directly overhead . The west component shows the equation of time or the difference between solar time and local meantime . This can be interpreted as how fast or ' slow ' the Sun ( or a sundial ) is compared to clock time . It also shows how far west or east the Sun is , compared with its mean position . The analemma can be considered as a graph in which the Sun declination and the equation of time are plotted against each other . In many diagrams of the analemma , a third dimension , that of time , is also included , shown by marks that represent the position of the Sun at various , fairly closely spaced , dates throughout the year . GEOGRAPHY

North 16 14 12 10 10 14 16 ' 20 Of 22 , Cancer Equator Tropic of Capricorn Figure The . Image by Anthony is used under a license . GEOGRAPHY SUMMARY As the Earth rotates on its axis and revolves around the Sun , several different effects are produced . The summer solstice is the longest day of the year , and the winter solstice is the shortest . The equinox is the time when the day and night are the same numbers of hours . The reasons for the seasons are beyond the length of daylight , it also includes the intensity and duration of daylight , as well as Earth physical distance from the Sun . The analemma is caused by the tilt of the Earth axis as it rotates and the elliptical shape of Earth orbit around the Sun . There are measurable distances between the Earth and Sun throughout the year , identified as the perihelion and aphelion . GEOGRAPHY