Ford has beaten General Motors for the first time in nearly a century, in terms of number of US factory workers. Ford has 50,703 to GM's 50,300 or so.
Answers. Well, the simplest answer is that the astrolabe pretty well had to be used at night and could not be used in bad weather. The compass, which relys on the magnetic field of the Earth rather than a visualiztion of the stars, works at any time, and in any weather.
What is certain is that the overall magnetic field of the Earth has decreased in the last 150 years, and areas of opposite polarity have increased in the last 30 years. Magnetic Flux (Image taken from www.bushwalking.org.au/FAQ/FAQ_Navigation.htm Used by permission-original source Scientific American.ans Oersted (Image taken from http://www-spof.gsfc.nasa.gov/Education/whmfield.html) Prior to this, in 1600, William Gilbert was the first to suggest that the Earth exerted a magnetic field.