Summary: Why do we have day and night? What makes it so we aren't in darkness all the time? What causes us to have a sunset and sunrise? How can it be midnight in Beijing, China when it is noon in North Carolina? To find out the answers to these questions students will manipulate their personal Earth they constructed in the last activity The Earth is Round. Specifically, they will study the effects on our experience here on Earth of the fact that the Earth spins about its axis from West to East, completing a rotation once a day. They will understand that this causes celestial objects, as viewed by observers on Earth, to appear to be moving around the Earth from East to West. When applied to the Sun, this explains not only the apparent motion of the Sun in the sky that was found in "The Sun Moves in the Sky," but also the daily cycle of light and darkness - Day and Night.
This activity will synthesize understandings from previous lessons. In part, this is where we have been going. The concepts of sunrise, sunset, midday, and midnight and their relation to the location of Earth, Sun, and observer were introduced in "Sunrise and Sunset on a Tilting Plate." Here, these will be combined with the effects of Earth's shape as discussed in "What's Up, Earth" to understand the apparent motion of the Sun as observed in "The Sun Moves in the Sky." Combined with our insights on the motion of light and of shadows, from "Light and Darkness in Space," this will also explain the progression of light and darkness on Earth. Because this activity brings together the insights collected thus far, it is a natural place to stop, assess students' comprehension, and help them to work through surviving misconceptions and confusions.
In the science notebooks and recorded on your chart paper the students' responses to the opening Motivation/Challenge.
After this activity, students should be able to:
Computer generated illustrated model of the Sun's path with written explanation using computer graphics software (i.e. KidPix) to illustrate and answer the following questions:
Much of the background for this activity has already been laid out in previous ones. At any given time, exactly one-half of the surface of the Earth is illuminated by the Sun, simply because the other half of the sphere that is Earth is in fact inside the Earth's shadow. People on the dark half cannot see the Sun as they would need to look down to look in its direction, and the Earth lies in the way. Thus also sunlight cannot reach them. As the Earth rotates about its axis a fixed point on Earth is swept into and out of the illuminated half. In this activity, we will hold the Earth so that the axis is vertical, while the direction from Earth to Sun us horizontal. They are thus perpendicular to each other. (This is not quite accurate, as we will see in later activities "Your World is Tilted" and "The Seasons and the Orbit," but it is a reasonable approximation and makes the relevant discussion for this activity a bit simpler.) This means that other than the poles, each point on Earth is in the illuminated half of the sphere precisely one-half of the time. Given our previous experience, it should come as no surprise that our world is dark about half of the time - when we are in the Earth's shadow. That is what we call night. The other half of the time, we are out of the shadow which means we can see the Sun (remember that being in the shadow is the same as not having a line of sight to the light source). That the entire world then seems filled with light is because we are surrounded by objects that reflect or scatter the sunlight. Even the atmosphere, because of impurities like dust, ice crystals, or tiny drops of water, scatters enough light to appear bright blue rather than dark.
This is the essential point. To complete the discussion, we want to make as many connections as possible between the observations students make with the model they manipulate and the experiences of people living on the Earth.
If we follow a fixed point on Earth as it goes through a full rotation, we can reproduce the experience of a person living at that point through a full 24-hour period including a day and a night. Let's begin with the moment in the rotation at which our observer is just entering the illuminated half of the Earth. This is the transition from Night to Day - morning. The Sun is just becoming visible to our observer. On closer inspection, we notice that when the Sun first becomes visible, the line of sight from the observer to the Sun just grazes the Earth. The observer emerging from the shadow will see the Sun emerging from being obscured by the Earth, and visible just above his or her Eastern horizon.
As the Earth rotates, the fixed point with our observer moves deeper into the illuminated half of the Earth's surface. From the point of view of the observer, the Sun is moving higher in the sky, reaching its highest point when the observer is in the middle of the illuminated half, along the line of longitude closest to the Sun. At one point on this line, at the equator, the Sun is directly overhead, while north of the equator it appears to the South and south of the equator to the North. The farther from the equator our observer, the lower the Sun will appear at midday.
As the Earth continues to spin, our observer will be carried toward the Eastern edge of the illuminated half of Earth, and the Sun will approach the horizon in the West, finally disappearing when the observer crosses the line into the dark half.
Following through a rotation we see that the observer spends precisely half of the time (12 hours out of each day) in the dark and the other half in the light, except at the poles where a perrenial twilight occurs, with the Sun on the horizon always. Of course this is not the true state of things on Earth; that is due to the tilt of Earth's axis and will be discussed later. But it is the state of things in our current model.
In terms of our study of the apparent motion of stars in various parts of the sky in the previous Activity, the Sun is just one such star (object outside Earth) which lies above the Earth's equator (once more, this is not precisely true, we will correct that later).
Group Size: Whole class and small group recording teams
Each student needs:
Part 1: Science Notebook Intro
Shadow Tracking: Track the Sun's movement by tracking a stick's shadow each hour. Much like the Sun tracking groups each hour have a group track the stick's size, shape and direction each hour. Check out
http://hea-www.harvard.edu/ECT/Stick/stick.html#intro for a great lesson that helps your students record the smooth arced motion of the Sun across the southern horizon.
A helpful site at locating constellations for different times of year is at http://www.astro.umass.edu/~arny/constel/night_sky_learn.html
Students can draw the constellations and place them next to their birthday drawings in the Earth Orbit Calendar. (see Additional Activities section)
Find exact times throughout the world at http://www.timeanddate.com/worldclock/
Find live webcams to prove that it truly is midnight near Beijing , China and near Perth, Australia when it is noon in NC at: German School in Shanghai, China http://www.ds-shanghai.org.cn/webcam/webcam.html
Adelaide, Australia http://abc.net.au/adelaide/adelwebcam.htm
K-6 Astronomy activities from Harvard-Smithsonian Center for Astrophysics' Everyday
Language arts and Astronomy connections at Stanford's Solar Center at
Earth, Sun, and Stars Teacher's Guide by LHS GEMS
Lawrence Hall of Science University Of California at Berkeley
Axis:the imaginary line through the Earth's center about which the Earth rotates once a day. The Earth's axis "points" in a fixed direction in space.
East, West, North, South: the four cardinal directions on the compass
Spin: A rotation of an object about its axis. The Earth spins about its axis from West to East, completing a rotation in about 24 hours or 1 day.
Orbit: The path taken by an object as it moves in space. The Earth moves in an orbit which is almost precisely a circle with the Sun at its center. Earth completes an orbit, circling the Sun and returning to its starting point, in one year. Axis:the imaginary line through the Earth's center about which the Earth rotates once a day. The Earth's axis "points" in a fixed direction in space.
Horizon:The imaginary line along which sky and Earth appear to meet as seen by an observer on Earth. At sea or in flat, featureless terrain, this is a horizontal circle. The line is imaginary in that there are no points at which sky and Earth meet, but it is very real as a collection of directions in space.
Sunrise:The time of day when the Sun first appears above the horizon in the East. At any instant the Sun is rising along one line of longitude on Earth.
Sunset:time of day when the Sun disappears below the horizon in the West.
Noon or Midday:12:00 midday is the time when PM starts directly after at 12:00:01 and the Sun is near its highest altitude or height in the sky
Midnight:12:00 midnight is the time in the night when AM starts immediately after at 12:00:01 people who live in Durham are fully in the earth's shadow and in darkness.
Clockwise:movement in the same direction as the hands of a clock
Counterclockwise: movement in the opposite direction as the hands of a clock