The Sun’s Effect on Climate: Content Background Document 1. Introduction As we begin exploring the Sun’s effect on Earth’s climate and seasons, take a moment to consider what you already know about this topic. You probably have a basic picture in your mind of the relationship between Earth and the Sun. For example, you may know that night and day occur because Earth spins on its axis every 24 hours, and that Earth orbits the Sun every 365.25 days. (You’re probably also aware that we make up the extra quarter day each year that it takes Earth to revolve around the Sun by having a leap year with 366 days.) And you almost certainly know that the Sun not only provides Earth with heat and visible light but also emits different kinds of energy waves, such as ultraviolet light that can lead to sunburn if you’re out too long in unfiltered sunlight. If someone were to ask you why there are seasons, you might say it has something to do with the way Earth tilts on its axis. But how deep is your understanding of the relationship between Earth and the Sun? Can you connect your understanding of the seasons to observations of the Sun’s path when it appears higher or lower in the sky at different times of the year? Can you explain why we experience summer in the Northern Hemisphere in June, July, and August while the Southern Hemisphere is experiencing the chill of winter? Can you explain why some places near the ocean have more moderate temperature patterns over the course of a day or a year, or why higher elevations experience cooler temperatures on average than lower elevations? Can you explain why daily weather is so variable when the energy we receive from the Sun, based on Earth’s orbit and spin, is so regular and predictable? This document will challenge you to broaden and deepen your understanding of the Sun’s effect on climate and seasons. It will also support and further your learning about the underlying factors that lead to seasonal temperature variations, especially how Earth’s tilt and orbit influence the intensity of the Sun’s energy and seasonal temperature patterns. Understanding these factors will help you relate these concepts to the uneven heating of Earth’s surface referred to in the Next Generation Science Standards (NGSS Lead States, 2013). The goal of this exploration is for you to develop a conceptual understanding of climate so you’ll be able to teach your elementary students more effectively. This content was written with you, the teacher, in mind. The subject matter is tied to the lessons you’ll be teaching, but the concepts are presented at a higher level to equip you with the tools and background you’ll need to guide student learning. After all, teachers should know more about science content than their students! 2. Getting Started: Understanding Celestial Motion The goal of this module is for you and your students to emerge with (1) a clear mental image of how Earth moves in relation to the Sun, (2) an understanding of why this motion causes varying amounts of © 2017 CPP and BSCS 1 RESPeCT Grade 6 The Sun’s Effect on Climate Module energy from the Sun to impact different locations on the planet in regular and predictable patterns, and (3) how this differential (uneven) heating influences Earth’s climate and seasons. Most elementary science curricula fail to connect the fundamental science concept of motion in the solar system with climatic patterns and seasons on Earth, but this relationship is essential for developing students’ understandings based on 6th-grade science standards and grade-level activities. Throughout this series of lessons, it’s assumed that most 6th-grade students have been introduced to the basic relationship between the Sun and Earth. According to Disciplinary Core Ideas for Earth and the Solar System in the Next Generation Science Standards (NGSS Lead States, 2013), expectations for 5th- grade students regarding Earth and the solar system include understanding that “the orbits of Earth around the sun and of the moon around Earth, together with the rotation of Earth about an axis between its North and South poles, cause observable patterns. These include day and night; daily changes in the length and direction of shadows; and different positions of the sun, moon, and stars at different times of the day, month, and year.” Most 6th graders will already have seen models of the solar system in which the Sun is static, and Earth not only rotates daily on its axis but also revolves annually around the Sun (see figure 1). It’s difficult, however, for students to retain this abstract image of Earth in relation to other bodies in the solar system when their personal experiences support the idea that Earth is stationary, and the Sun and Moon move across the sky. Reinforcing these experiences are references to the Sun rising in the east and setting in the west. Using such terms as sunrise and sunset seems practical because, from our earthly perspective, the Sun does appear to rise and set, but this terminology only reinforces student misconceptions that the Sun moves while Earth stands still. Courtesy of BSCS Figure 1. Every year, or 365 days, Earth makes a complete orbit (revolution) around the Sun, and every 24 hours, Earth spins on its axis, marking day and night. © 2017 CPP and BSCS 2 RESPeCT Grade 6 The Sun’s Effect on Climate Module However, by 6th grade, students are beginning to make sense of celestial motion apart from their own experiences and perceptions. Students in early elementary school typically have a geocentric, or Earth- centered, understanding of the motion of objects in space, while students in upper elementary school and middle school are ready to learn about the motion of objects in space from a heliocentric, or Sun-centered, perspective. Before you begin teaching this unit, you’ll want to make sure that both you and your students have a clear mental image of the Sun at the center of the solar system, with Earth revolving (orbiting) around the Sun every year or 365 days and rotating (spinning) on its axis every 24 hours, marking day and night. You might begin by asking your students these questions: Why do we start a new year every 365 days? Which objects moving in space cause us to experience a year on Earth? To prevent confusion, the word orbit rather than revolve has been used in this document and the RESPeCT lessons to describe Earth’s motion around the Sun. However, since many national and state standardized tests use the R words revolution and rotation, make sure your students know the difference between these terms by the end of the unit. Students should also know that Earth’s orbit is very nearly circular, even though in most textbook illustrations, it appears elliptical. Elliptical representations lead many students to believe that Earth is farther from the Sun at certain times of the year and closer at other times. As a result, students often hold a common misconception that seasons are caused by variations in Earth’s distance from the Sun at different times during its orbit. For example, many students believe that Earth is closer to the Sun during the summer and farther away during the winter. But in fact, Earth’s orbit is only slightly closer to the Sun around January 4 of each year. On this day, called Earth’s perihelion, our planet is 147.5 million kilometers from the Sun. In contrast, around July 4—Independence Day in the United States—is Earth’s aphelion, the day when our planet is farthest from the Sun at 152.6 million kilometers. The difference between Earth’s perihelion and aphelion is 5.1 million miles. If that seems like a lot of miles, remember that compared to the total distance between Earth and the Sun, this only represents about a 3% change from January to July. STOP AND THINK Based solely on Earth’s distance from the Sun, would you expect July or January to be the hottest month of the year? Why? To make sense of the relationship between Earth and the Sun, students need to envision how other bodies in the solar system compare with Earth in size and distance from the Sun. At present, there are no really effective ways of representing these size and distance relationships in the classroom, which tends to reinforce student misconceptions rather than correct them. (No doubt you’ve seen models of the solar system mapped out from one end of a school playground to the other in an attempt to demonstrate scale and size.) Based on their own observations, students might think that the Sun and the Moon are about the same size and distance from Earth. But actually, the Moon is approximately one quarter the size of Earth, while the Sun is the size of 109 Earths! They merely appear to be the same size because the distances from Earth are so different. If the diameter of Earth (about 12,756 kilometers) is used as a measuring tool, the Moon would be about 30 Earths away, while the Sun would be about 12,000 Earths away. To put it another way, the Sun is 400 © 2017 CPP and BSCS 3 RESPeCT Grade 6 The Sun’s Effect on Climate Module times bigger than the Moon, but it’s also 400 times farther away from Earth, so the Sun and Moon appear to be about the same size in the sky.