An elderly man who lives in the northern United States (above 43 degrees north) likes to walk to and from work each day, year-round. He goes to work at 10:00am and comes home before dark in the evening. His workplace and home are on the same street, which runs east to west through a typical residential neighborhood. There are two story houses along both sides of the street. Throughout most of the year, he varies which side of the street he walks on from day to day, but between late October and mid-December he always walks on the north side of the street. This is also true between mid-March and the end of April. Why does he walk exclusively on the north side of the street during those two periods?