Page "government" Paragraph 615

It was recognized years ago that the transition from daytime to nighttime propagation conditions, and vice versa, is not an instantaneous process, but takes place over periods of time from roughly 2 hours before sunset until about 2 hours after sunset, and again from roughly 2 hours before sunrise until some 2 hours after sunrise.

During the period of about 4 hours around sunset, skywave transmission conditions are building up until full nighttime conditions prevail ; ;

during the same period around sunrise, skywave transmission is declining, until at about 2 hours after sunrise it reaches a point where it becomes of little practical significance.

However, in this case as elsewhere it was necessary to arrive at a single standard to be applied to all situations, representing an averaging of conditions, and thus to fix particular points in time which would be considered the dividing points between daytime and nighttime conditions.

It was determined that the hours of sunrise and sunset, respectively, should be used for this purpose.

Accordingly, the 1938-39 rules adopted these hours as limitations upon the operation of daytime stations.

Class 2, stations operating on clear channels are required to cease operation or operate under nighttime restrictions beginning either at local sunset ( for daytime class 2, stations ) or sunset at the location of the dominant class 1, station where located west of the class 2, station ( for limited-time class 2, stations ).