Time changes four minutes for each degree of longitude, making one hour for every fifteen degrees. This is because it takes four minutes for the Earth to turn enough for the vertical rays of the sun to sweep over one degree of longitude.
When its is noon on the Prime Meridian, it is four minutes past noon at 1 east of the Prime Meridian, and four times before noon at 1 west of the Prime Meridian. We number the hours from noon. PM refers to the hours after midday to twelve midnight, and AM refers to the hours from midnight to midday.
To avoid the confusion that would result if each place used its own local time, the United States has been divided into four time zones corresponding roughly with the 75th, 90th, 105th, and 120th West Meridians. Alaska and Hawaii have two additional time zones. Each time zone differs by one hour. Commencing with the east, they are known as Eastern, Central, Mountain, and Pacific time zones.
A person traveling from one time zone to another changes his watch only when leaving one zone and entering another. Going east the watch is set ahead one hour when crossing a dividing line and set back one hour when crossing a dividing line while traveling west. The lines between zones are irregular due to local adjustments.
Canada has the same time zones as the United States, plus two additional time zones for the extreme eastern part of the country. Standardization of time has become almost universal.
Most globes have a Time Dial at the North Pole. This is a disk that can be turned and is divided into 24 equal parts, corresponding to the Meridians shown on the Globe, and marked to represent the hours AM and PM of the Solar Day. The dial may be used to quickly show the comparative time at any two points on the Globe.