Answer:
For the first question, 180 degrees equals to a half of the sphere. For the second question, you need 24 central meridians for a complete sphere, which are exactly the hours in a day.
Step-by-step explanation:
A sphere is basically a 3D circle. As a circle has 360 degrees, 180 degrees would be half of a circle. Imagine you are on a satellite over the north pole or the south pole and you have a way to cut the earth by the middle. You will get two halves of sphere.
About the second question, you may need to have in mind that a day is the time spent for the earth to rotate all 360 degrees over its own axis. British fellow, on XIX century, decided they were the center of the world. As previously, back in the days, some other people decided a day had 24 hours, they decided to draw this lines and divide the earth in 24 pieces, so they could knew which time was on every point their extense kingdom had. As I said, a circle has 360 degrees, (360 degrees)/(24 hours) equals to 15 degrees.
