The sun is so distant from us that all the light rays that strike our planet approach us along essentially parallel lines. to see why, look at figure 2.10. take a source of light near earth-say, at position a. its rays strike different parts of earth along diverging paths. from a light source at b, or at c (which is still farther away), the angle between rays that strike opposite parts of earth is smaller. the more distant the source, the smaller the angle between the rays. for a source infinitely distant, the rays travel along parallel lines. to sun a b earth to sun figure 2.10 light rays from space. the more distant an object, the more nearly parallel the rays of light coming from it. we saw in class thursday that the size of my finger's shadow on the sidewalk cast by the sun, did not change as my finger was moved closer and farther from the sidewalk. the shadow got "fuzzier" (for a reason we'll discuss in connection with eclipses later in the semester), but not noticeably bigger. repeat these observations yourself: distance finger width of shadow (include half of fuzz) to sidewalk 5 cm 10 cm 20 cm this is the case of the blue lines in fig. 2.10. turn the arrows around so the arrows represent rays of light from the sun that just graze the edge of the earth (re-label "earth" as "finger" in the picture). the finger will then cast a shadow off to the left (out of frame in fig. 2.10)