The arrows show the direction in which the earth turns (from west to east). It is always noon at the place which is directly under the sun. Call it Sunday noon at Greenwich, at the top of the circle; then it is 10 A.M. Sunday at a point 30° west and 2 P.M. Sunday at a point 30° east, and so on. Exactly opposite to the noon point it is midnight. By common consent we change the name of the day, and the date, at midnight; consequently it is Sunday midnight just east of the vertical line at the bottom of the circle and Monday morning just west of it. If we cross that line going westward we shall pass directly from Sunday to Monday, and if we cross it going eastward we shall pass directly from Monday to Sunday. Since, by convention, this is a fixed line on the earth's surface, the same change will take place no matter what the hour of the day may be. ]