Here, for illustration purposes we pick a prominent national landmark, the Washington Monument. It doesn't matter what landmark you pick, except that it must show a shadow on the ground in the satellite image. More often than not, we pick an image of a landmark we are familiar with or interested in, like our house or a local library or school. On the left side of the image the shadows clearly point in a different direction than those on the right hand side of an image! When were these two different images taken? Students are motivated by the same question when viewing areas that are familiar and important to them. Consider the image of an area of New York shown in the first image above. A prominent indicator is the differences in shadows from one side of an image to another. For instance, most of the areas are virtually cloudless, and the seasons indicated by trees and ground cover are not always consistent. If you look at wide area mosaics in satellite map layers, you will notice they have been made from images taken at different times. The first task is to pick a landmark that you would like to date the satellite photograph of. An introductory textbook on astronomy will discuss these changes in the context of the seasons, and how they arise from the tilt of the Earth's axis and its position in its orbit around the Sun. The exact reasons for these changes and differences underlie this project, but are not described here.
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