Earth-Sun Relationships - Reasons for the Seasons PDF

Figure 5.23 Fall Colors at Lake Sabrina in Bishop, California. Image by Jeremy Patrich is under a CC-BY 4.0 license. UNIT 5: EARTH-SUN RELATIONSHIPS: REASONS FOR THE SEASONS Goals & Objectives of this unit Identify the relationships between latitude,...

Figure 5.23 Fall Colors at Lake Sabrina in Bishop, California. Image by Jeremy Patrich is under a CC-BY 4.0 license. UNIT 5: EARTH-SUN RELATIONSHIPS: REASONS FOR THE SEASONS Goals & Objectives of this unit 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 Ea 66 | P H Y S I C A L G E O G R A P H Y 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 of the year. When ancient people made these observations, they imagined that the sky was moving while the Earth stood still. In 1543, Nicolaus Copernicus proposed a radically different idea: The Earth and the other planets make regular revolutions around the Sun. He ea 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. If you 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 of the Sun rising or setting over the horizon. When we look at the Moon or the stars at night, this. As Earth turns, the Moon and stars change position in our sky. 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 day-night 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 21st pointed directly toward the Sun. Therefore, areas north of the equator experience longer days and shorter nights because the northern half of the Earth is pointed toward the Sun. Since the southern half of the Earth is pointed away from the Sun at that point, they have the opposite effect, longer nights and shorter days. 67 | P H Y S I C A L G E O G R A P H Y For people in the Northern Hemisphere, winter begins on or around December 21st. At this longer nights for those who are north of the equator. Figure 5.24 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 Attribution-Share Alike 3.0 Unported license. Energy from the Sun The earth constantly tries to maintain an energy balance with the atmosphere. Most of the % 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. Figure 5.25 Diagram Showing the Three Types of Ultra Violet Light Emitted from the Sun. Image by Trudi Radtke team is used under a CC BY 4.0 license. 68 | P H Y S I C A L G E O G R A P H Y Of the solar energy that reaches the outer atmosphere, UV wavelengths have the greatest energy. Only about 7% of solar radiation is in the UV wavelengths. The three types are: UVB: the second-highest 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 UVC, most UVB and some UVA from incoming sunlight. Oxygen, carbon dioxide, and water vapor also filter out some wavelengths. THE GREENHOUSE EFFECT The 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 Figure 5.26 The Greenhouse Effect (NASA).Image is in the public domain. 69 | P H Y S I C A L G E O G R A P H Y Greenhouse gases include CO2, H2O, methane, O3, nitrous oxides (NO and NO2), and chlorofluorocarbons (CFCs). All are a normal part of the atmosphere except CFCs. 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 CO2 molecule. One CFC-12 molecule (a type of CFC) traps 10,600 times as much heat as one CO2. Still, CO2 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 2 ½ times higher as a result of human activity. Carbon dioxide has increased by more than 35%. CFCs 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. 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 23.5° volution around the Sun. This results in one part of the 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. 70 | P H Y S I C A L G E O G R A P H Y  Figure 5.27 Seasons Diagram, Note the Tilt and Circle of Illumination for Each Season. Image is in the public domain. Northern Hemisphere Summer The more directly in summer. At the summer solstice, which is around June 21st or 22nd rays hit the Earth most directly along the Tropic of Cancer (23.5° N); that is, 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 21st or the 22nd 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. 71 | P H Y S I C A L G E O G R A P H Y Equinox hine most directly at the equator, called an equinox equinox happens on or around September 22nd or the 23rd and the vernal or spring equinox happens on or around March 21st or 22nd in the Northern Hemisphere. Analemma In astronomy, an analemma is a diagram showing the position of the 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 north-south component of the analemma results from the change in the Sun's declination due to the tilt of Earth's axis of rotation. The east-west component results from the nonuniform rate of change of the Sun's right ascension, governed by combined effects of Earth's 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 Sun's declination against the equation of time. The resulting curve resembles a long, slender figure- 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 24-hour 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, e.g., about 47°. The component along this axis of the Sun's 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 of time, 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 time-difference is related to the angle at the rate of 15° per hour, e.g., 360° in 24 hours. The difference in the size of the lobes of the figure-eight form arises mainly from the fact that the perihelion and aphelion occur far from equinoxes. They also occur a mere couple of weeks after solstices, which in turn causes a slight tilt of the figure eight and its minor lateral asymmetry. 72 | P H Y S I C A L G E O G R A P H Y Figure 5.28 Diagram or Earth during the Aphelion (Away) & Perihelion (Near). Image by Trudi Radtke is used under a CC-BY 4.0 license. 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 solstices. 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 east-west line along the celestial equator. Figure 5.29 The Three Variation of Analemmas, Eccentricity, Obliquity & Combined. Image by Anthony Flores is used under a CC-BY-4.0 license. The north-south component of the analemma shows the Sun's declination, its latitude on the celestial sphere, or the latitude on the Earth at which the Sun is directly overhead. The east- 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 73 | P H Y S I C A L G E O G R A P H Y 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's 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, closely spaced, dates throughout the year. North 16 14 12 10 8 6 4 2 0 2 4 6 8 10 12 14 16 10 15 20 25 30 5 Tropic of 5 10 Cancer 30 15 25 20 20 25 30 15 10 5 5 10 30 15 25 20 20 25 5 10 10 5 15 30 20 25 25 20 Equator 30 15 5 10 10 5 15 20 25 20 25 15 30 10 5 5 10 30 15 25 20 25 20 30 15 5 10 Tropic of 10 30 5 15 20 25 Capricorn South Figure 5.30 The Analemma. Image by Jeremy Patrich is used under a CC-BY-4.0 license. 74 | P H Y S I C A L G E O G R A P H Y