Activity 3B

Activity 3B:

SUNLIGHT THROUGHOUT THE YEAR

Do Now:

Print this file and staple it to the back of Activity 3B handed out in class.
Print the Image 1, Image 2, Image 3, Image 4, Image 5, and Image 6, and Image 7 Files.

Examine the three visible satellite images. These are actual images which were obtained on or near the first days of the Northern Hemisphere’s fall, winter, and summer seasons. Next, examine the small drawing to the right of each Earth image. The drawing shows the relative positions of the Earth, satellite, and rays of sunlight at the time each image was recorded. (In the small drawing, the view is from above the Earth’s Northern Hemisphere.) If you were located on the satellite, you would have seen the same view of the Earth as shown in each accompanying satellite image.
The three Earth images were acquired when sunset was occurring at the point on the equator directly below the viewing satellite (in the center of the Earth’s disk). Sunset was occurring along the dashed line passing through the sub-satellite point. The arrows to the left in each image represent incoming rays of sunlight at different latitudes.

Compare the 23 September image (Image 1) with Figure 3.9 (Image 4), showing autumnal and vernal equinox Earth/Sun relationships. Note that in both, the sunset line and the Earth’s axis line up together and are oriented perpendicular to the sun’s rays. Because the Earth rotates once in 24 hours, the period of daylight is ______ hours everywhere except right at the poles.
Now compare the 21 December satellite image (Image 2) with Figure 3.11 (Image 5). On the Northern Hemisphere’s winter solstice, the Earth’s North Pole is tilted the farthest away from the sun it ever gets during the year. Consequently, poleward of the Arctic circle, the period of daylight is ____ hours. Poleward of the Antarctic Circle, the period of daylight is _____ hours.
Compare the 21 June satellite image (Image 3) with Figure 3.10 (Image 6). On the Northern Hemisphere’s summer solstice, the Earth’s North Pole is tilted towards the sun as far as it ever gets during the year. Consequently, poleward of the Arctic Circle, the period of daylight is ____ hours. Poleward of the Antarctic Circle, the period of daylight is _____ hours.
Along with these variations in the length of daylight at various latitudes as shown in the satellite views, the intensity of incoming sunlight varies with the angle of incidence at the Earth's surface as seen in the arrows drawn for the Equator and the North (or South) Pole. (A latitude line could also be added at your latitude.) Thus, the solar energy received at a location over the course of the year depends on the solar altitude (angle of the sun above the horizon) and the period of daylight.
A satellite image was not provided in this activity for the first day of the Northern Hemisphere's spring season. If it were, it would look very much like the image for the first day of ______________.
Now return to Activity 3A visible satellite image (Image 7, 0015Z 23 SEP 1999). On this image you drew the sunset line (terminator). Compare the US visible satellite image (Image 7) to the images in this activity. The Activity 3A Image 1 depicts a portion of the full-disk visible satellite view of the Equinox: 23 September on which the terminator is also oriented in the [(north-south) (northwest-southeast)] orientation. How do you think a visible satellite image at sunset for today, Friday, 09 March 2000, a week and a half before the Equinox, would be expected to show the terminator orientation?

As we progress through the Spring to the Summer Solstice and on to the next Fall season, call up DataStreme visible satellite images near times of sunrise or sunset and observe the orientation of the terminator line. Relate this "remote" view of the Earth/Sun relationship to the path of the Sun through your local sky and the length of daylight at your location. Also note the general cooling of temperatures associated with these changes in solar altitude and period of daylight.

To estimate amounts of solar radiation received in Harrison for the various months of the year similar to that given in Figure 1 of Activity 3B, you can call up http://solstice.crest.org/renewables/solrad/data/index.html. Because this site is designed to calculate the energy from solar collectors, you need to choose the option of a "flat-plate collector at 0° tilt" along with your location from the map. Finally, the energy units of kilowatt hours per square meter per day need to be multiplied by 86.04 to obtain calories per square centimeter per day as used in this activity.