There are two real problems. The first real problem is that the tropics remain undiscovered. The second real problem is that the temperate remain re-undiscovered. These two real problems are observable, because of the fact that both use of Ethiopian calendar only in the current Ethiopia and Gregorian calendar in the tropics are unrealistic.
Use of Ethiopian calendar only in the current Ethiopia is unrealistic, because circumference of current Ethiopia cannot cover 24 hours (half part day and other half night). Use of the Gregorian calendar months in the tropics is unrealistic, because longer variations of day and night and extreme seasons of the temperate which are revealed by the Gregorian calendar months cannot recur in the tropics.

Thus, although people failed to recognize that Ethiopia’s calendar months are belong to the tropics, we discovered that unity of Ethiopian calendar months and tropics and rediscovered unity of Gregorian calendar and temperates are the unities of theories and practices that demand practices.

Therefore, discovering the tropics and rediscovering the temperates are very simple, because the knowledge of realities have no power hiding themselves. For example, today Nehase 7 of 2004 of Ethiopian calendar recurs in the tropics, when August 13 of 2012 of Gregorian calendar recurs in the temperate.
The above discovery and rediscovery do assert that use of Ethiopian calendar in the tropics and use of Gregorian calendar in the temperates are natural and scientific.