# 2.4. What is the Solar Cycle?

## Description

This article is from the Calendars FAQ, by Claus Tondering claus@tondering.dk with numerous contributions by
others.

# 2.4. What is the Solar Cycle?

In the Julian calendar the relationship between the days of the week

and the dates of the year is repeated in cycles of 28 years. In the

Gregorian calendar this is still true for periods that do not cross

years that are divisible by 100 but not by 400.

A period of 28 years is called a Solar Cycle. The "Solar Number" of a

year is found as:

Solar Number = (year + 8) mod 28 + 1

In the Julian calendar there is a one-to-one relationship between the

Solar Number and the day on which a particular date falls.

(The leap year cycle of the Gregorian calendar is 400 years, which is

146,097 days, which curiously enough is a multiple of 7. So in the

Gregorian calendar the equivalent of the "Solar Cycle" would be 400

years, not 7*400=2800 years as one might be tempted to believe.)

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