http://www.teachastronomy.com/astropedia/article.aspx?qid=55
http://www.teachastronomy.com/astropedia/article.aspx?qid=56 http://www.teachastronomy.com/astropedia/article.aspx?qid=57 http://www.teachastronomy.com/astropedia/article.aspx?qid=58
http://www.teachastronomy.com/astropedia/article.aspx?qid=59 http://www.teachastronomy.com/astropedia/article.aspx?qid=60 http://www.teachastronomy.com/astropedia/article.aspx?qid=61
http://www.teachastronomy.com/astropedia/article.aspx?qid=64 http://www.teachastronomy.com/astropedia/article.aspx?qid=65
http://astro.unl.edu/classaction/renaissance.html