A solar day is the day that we use to keep time on the planet earth. It is the time from when the sun is directly over head until the time the sun is directly over head again. This takes a rotation of about 361 degrees. It is slighty more than one full rotation due to the fact that the earth orbits the sun at the same time that it rotates. For the same point on earth to line up with the sun again it would have to over-rotate by about 1 degree. This is why a day is 24 hours long. This is the day that we base our calender system on.