Astronomy, orbit and rotation of the earth, seasonal changes in the position of the sun and day length, orbit of the moon, effects of sunlight on the climate and the appearance of the moon and stars
An exposed light or any object to represent the sun; room for students to circle around the “sun”; tags or marks on the different walls to represent stars; a globe and a darkened room is helpful as well if the “sun” is a light. To extend the lesson to understand phases of the moon and eclipses, you really want a dark room and a central light source, and you will also want each student to hold a small sphere to represent to moon. It’s best if the “moon” is mounted on a stick to avoid shadows from the hand holding it. It’s also nice if the “moon” held at arm’s length is at least as big as the “sun” appears from the viewing distance.
Decide, either on your own or in consultation with the students, if “north” should be up or down. Maps and globes are usually made so that north is up, but that is because their customers are usually north of the equator. If north is up, all circling will be counterclockwise as viewed from above. If south is up, all circling will be clockwise.
Place the object representing the sun at eye level in the center and stand the students in a ring around it. The students each represent the earth in orbit around the sun. (They will probably stand facing the “sun”, but do not require them to do that.)
Tell the students that they each represent the earth and that the central object (light) is the sun. Direct them to turn around in place 360°, right side leading (counterclockwise) if north is up and left side leading (clockwise) if south is up. Point out that as they turned, the “sun” “set” once and “rose” once; when they could see the “sun” was their “day” and when their backs were to the sun was their “night.” So one complete rotation is one “day”. When they are facing the sun is their “noon.” Also point out that “noon” is a different direction for each of them.
But it’s not that simple. The axis of the earth’s spin isn’t exactly perpendicular to its orbit around the sun. That means that the students shouldn’t be standing straight up. They have to tilt a little: 23.5° to be just like the earth, but that’s a bit much to try if you don’t want to fall over! And the direction of the axis doesn’t change as the earth orbits the sun, so everyone in the room needs to tilt in exactly the same direction. On one side of the circle, students lean their heads toward the sun; on the opposite side, they lean their heads away from it. The students in between lean more or less to the side. And as they turn, they must keep the same direction of tilt. So, for instance, the students with their heads into the circle keep their heads into the circle as they rotate. Have the students practice this a few times.
They should see that the sun probably doesn’t pass right in front of their faces as they rotate: if they are tilting away from the sun, they have to look down to see the sun at “noon;” if they are tilting toward it, they have to look up. For those tilting in, their heads are in summer and their feet are in winter; for those tilting out, their heads are in winter and their feet are in summer. Those right in between are in spring or autumn, depending on which way they are going. That’s what comes next.
Of course, the earth doesn’t just spin on its axis; it also circles around (orbits) the sun. So we need to do that too. The yearly motion is in the same direction as the daily spin. So, as students turn for a “day,” they also advance a little bit around the circle around the sun. (If north is up, they move to the direction that is forward when the sun is on their left; if south is up, they move in the opposite direction: forward when the sun is on their right.) When they do this, they have to spin on their axes a little bit more than 360° from one “noon” to the next. As they do this, which season are they moving into? Which season are they moving out of?
If you have a globe, place or hold it at the same level as the light. If the globe is in a stand, its axis is 23.5° from vertical. If north is up, hold or set the globe so that its stand is level; if south is up, hold the globe upside down. You can see where the sun shines on the earth. Identify the part of the earth where it is daytime, where it is night, and where the sun is rising and setting. Spin the globe slowly if it is in a stand. (Spin it so that west follows east; counterclockwise looking onto its north pole.) Point out that as the earth spins, some of the earth moves into day, some moves into night, and that at all times some place experiences sunrise and sunset. As the globe orbits the “sun”, the solstices are when the subsolar point is as far north or south as it gets; the latitudes of the solstice subsolar points are the tropics. At these orbital points, note that the arctic circle experiences endless daylight and the Antarctic circle experiences endless night, or vice versa. The equinoxes are the points in between, when the subsolar point lies on the equator. At these orbital points, the whole earth experiences 12 hours of daylight and 12 hours of night.
If the students have “moons”, they can model them orbiting their heads, again in the same direction as their spin and their orbit around the sun. The phase of the moon depends on the sun–moon–earth angle. The plane of the moon’s orbit around the earth is angled a bit from the plane of the earth’s orbit around the sun, so most of the time the sun, earth, and moon don’t line up perfectly during the new moon and full moon. When they do, however, eclipses can occur: solar eclipses during new moon and lunar eclipses during full moon. Can students explain why lunar eclipses can be seen by half the earth at a time, while solar eclipses can only be seen by a lucky few?
Copyright © 2004, Richard Barrans
Revised: 21 December 2016; Maintained by Richard Barrans.