The solar cycle is a 28-year cycle of the Julian calendar with respect to the week. It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28 year sequence.

This cycle also occurs in the Gregorian calendar, but it is interrupted by years such as 1700, 1800, 1900 and 2100, which are divisible by four but which are not leap years. This interruption has the effect of skipping 16 years of the solar cycle between February 28 and March 1. Because the Gregorian cycle of 400 years has exactly 146,097 days, i.e. exactly 20,871 weeks, one can say that the Gregorian so-called solar cycle lasts 400 years.

Calendar years are usually marked by Dominical letters indicating the first Sunday in a new year, thus the term solar cycle can also refer to a repeating sequence of Dominical letters.

Unless a year is not a leap year due to Gregorian exceptions, a sequence of calendars is reused every 28 years.

The name solar cycle comes from Sunday, the traditional first day of the week.

## Julian solar cycle

The allocation of the Sunday letters to the calendar years repeated in the Julian calendar every 28 years. This cycle is called the sun compass. Each of these 28 years j a sun compass is SC -called number in the following definition assigned:

SC = (j + 9)  mod 28; Result: SC = 0 *), 1, ..., 26, 27
• Instead of the unknown in ancient times zero is the divisor, here written 28th

The constant association between Julian Solar Cycle SC and Dominical Letter DL following table shows:

 SZ 0 1 * 2 3 4 5 * 6 7 8 9 * 10 11 12 13 * SB A F E D C A G F E C B A G E SZ 14 15 16 17 * 18 19 20 21 * 22 23 24 25 * 26 27 SB D C B G F E D B A G F D C B
• Every 4 years, the Dominical Letter DL moves by 2 letters in the alphabet (second shift after the leap day).

## Gregorian solar cycle

Every 4 years, the Dominical Letter DL moves by 2 letters in the alphabet (second shift after the leap day).

The Gregorian calendar is the allocation of the Sunday letters to solar cycle no longer constant. It changes whenever the full centuries that are not divisible by 400, the leap fails.Here, move every time. Every Sunday letters for a position in the alphabet forwards.

The association between Gregorian Solar Cycle SC and Dominical Letter DL shows for the years 1900 to 2099 the following table:

 SZ 0 1 * 2 3 4 5 * 6 7 8 9 * 10 11 12 13 * SB G E D C B G F E D B A G F D SZ 14 15 16 17 * 18 19 20 21 * 22 23 24 25 * 26 27 SB C B A F E D C A G F E C B A

Both the Julian and the Gregorian calendar, the Sunday letters can quite easily with the calendar by W. Bogatyrev determine - can in leap years there are only the second Sunday letter read (valid after the leap day).