## How to determine the limits of the extreme period?

First of all, one must calculate the astronomical phenomena (i.e. twilight rise, sunrise, noon, sunset, twilight set and midnight) times for the whole year. Visually the astronomical phenomena times may be plot in a figure.

In order to determine if there is any summer extreme period and its corresponding limits, the following checks focusing on summer (or even could start from late spring to early autumn) can be done:

- Check if the sun is always above the horizon. The limit is indicated by the sunrise and sunset curves meet at midnight curve.
- Check if the sun never goes 18 degree below the horizon. The limit is indicated by the twilight rise and twilight set curves meet at midnight curve.
- Check the fasting time, i.e. the time from Fajr until sunset. If there is no Fajr (or Fajr is coincides with midnight), then it is the time from midnight until sunset. The limit is when the fasting time is equal or more than 18 hours or 20 hours.

The limit which gives the longest period of summer extreme period is selected.

In order to determine if there is any winter extreme period and its corresponding limits, the following checks focusing on winter (or even could start from late autumn to early spring) can be done:

- Check if the sun is always below the horizon. The limit is indicated by the sunrise and sunset curves meet at noon curve.
- Check the length of the day, i.e. the time from sunrise until sunset. The limit is when the length of the day is equal or less than 4 hours or 6 hours.

The limit which gives the longest period of winter extreme period is selected.