Prior Knowledge Required. Before starting these "Thought Questions", you should know the meaning of these terms:
- axis of the earth's rotation [an imaginary line running through the North and South Poles, around which the earth rotates]
- circle of illumination [the boundary between the sunlight and dark halves of the earth]
- down [the direction in which gravity pulls, toward the center of the earth]
- equinox [either of the two times of year when the length of daylight (and hence) night is the same everywhere on the earth (12 hours)]
- horizontal, and horizon [perpendicular to "down"; and any point in the very far distance in a horizontal direction, often where the sky "meets" the earth]
- latitude, and latitude circle [the angle between the equator and any particular place on the earth, measured from the center of the earth; and the collection of all locations with the same latitude and hence the same distance from the equator, forming a circle centered on the axis of rotation]
- length of daylight [the number of hours of daylight in a day]
- midlatitudes [latitudes between 30° and 60°, in both the N. and S. Hemispheres]
- solar noon, and solar midnight [the time of day when the sun is highest in the sky; and 12 hours later/earlier]
- solstice [either of the two times of year when the earth's axis of rotation is tilted most directly toward or away from the sun]
- sun angle [the angle between the sun and the horizon]
- tilt of the axis of rotation [the angle (23.5°) between the plane of the earth's orbit around the sun and the earth's axis of rotation]
- Arctic Circle, Antarctic Circle, and the polar regions [66.5°N and 66.5°S, and the latitudes between them and the poles, respectively]
- Equator [0° latitude]
- North Pole and South Pole [90°N and 90°S, respectively]
- Tropic of Cancer, Tropic of Capricorn, and the tropics [23.5°N, 23.5°S, and the latitudes between them, respectively]
Learning Objectives. When you are done responding to these questions and discussing them in class, you should be able to use figures such as Figure 2-5 to:
- Describe how the length of daylight and sun angle at solar noon vary over the course of a year, at any particular latitude.
- Describe how sun angle at solar noon and length of daylight vary with latitude at the times of the solstices and equinoxes.
- Explain why length of daylight and sun angle at solar noon vary with time of year and with latitude the way they do.
- Explain the significance of the Tropics of Cancer and Capricorn and the Arctic and Antarctic Circles.
- Describe how the seasons differ betwen polar regions, midlatitudes, and the tropics.
For the questions below, refer to the accompanying Figure 2-5.
- In Figure 2-5,
how does the sun angle (the angle between the sun's rays and a horizontal
surface) at solar noon generally vary with increasing latitude? What does this imply about the intensity
of sunlight at high latitudes vs. low latitudes?
- Several globes are positioned around the room, simulating the position of the earth as it orbits the sun. (The sun is represented by an overhead or slide projector at the center of the orbit.) Notice that globes (should be) positioned so that the axis of
rotation of each globe points in the same direction, just as it does (pointing
toward Polaris, the North Star) when the real earth obits the sun.
- What time of year is it at the position of each globe? How can you tell?
- In Figure 2-5,
which figure (a, b, or c) corresponds to each globe? Where do you have to position yourself next to each globe to see it as it is shown in its corresponding figure?
- At each time of year shown, how does the length of daylight seem to vary with latitude? How can
- Note the length of daylight (more than 12 hours, about 12 hours, or less than 12 hours) at San Francisco's latitude at each time of year shown. Does it seem to vary with time of year? If so, describe the variation.
- Note the sun angle at solar noon at San Francisco's latitude at each time of year shown. Does it seem to vary with time of year? If so, describe the variation.
- The seasons are defined in terms of variations in (a) length of daylight and (b) sun angle at solar noon. Why do these two quantities vary over the course of the year? At what latitudes does
variation in length of daylight seem to be the greatest? What are the seasons like there, compared
to here? (Note: seasonal variations in solar noon sun angle at different latitudes don't vary as simply, but they do contribute to seasonal variations in solar radiation, too.)
- Bonus Questions (there are many more such questions—the topic is very rich!):
- In Figure
2-5, in each of the figures (a), (b), and (c), when does the
sun set (before 6
pm, at 6 pm, or after 6 pm)? When does it rise? How can you tell?
- In each figure, in what direction
(on the horizon) does the sun appear at sunrise and sunset?
Does this vary with latitude? [Hint: east is always in a direction
along a latitude circle; north is along a longitude circle toward the N.