Collect Gregorian calendars (the regular calendar you use every day) for the last five years with the help of your family members or teachers or the internet. For each year, look for the dates on which the festivals Eid-ul-Fitr and Diwali were celebrated and list them year wise in a tabular form. Do you notice that the date of Eid-ul-Fitr moves earlier each year—by about 11 days? If you have a corresponding lunar calendar at home or on the internet, check that the month and the day for Eid-ul-Fitr according to the lunar calendar remains the same. Does Diwali follow the same steady pattern, or are there some sudden jumps? Based on your chart, try to guess which year might have included an intercalary month (Adhikamaasa).
Eid-ul-Fitr dates shift approximately 11 days earlier each year in the Gregorian calendar because it follows a purely lunar calendar (354 days per year vs. 365 days in solar year).
The date remains the same in the Islamic lunar calendar.
Diwali, being based on a luni-solar calendar, also shifts but with occasional 'jumps' when an intercalary month (Adhikamaasa) is added to synchronize with the solar year.
Years showing sudden larger shifts in Diwali dates likely included an intercalary month.
Explanation
The chapter explains: 'In contrast, purely lunar calendars do not account for this difference. Any festival celebrated according to the phases of the Moon, such as Eid-ul-fitr, therefore can occur in different months of the Gregorian calendar year after year.' For luni-solar calendars: 'every few years, an extra month (called Adhika Maasa or intercalary month) is added to the year... This keeps the solar year and the lunar cycle in step.'
Solution Steps
Eid-ul-Fitr dates shift approximately 11 days earlier each year in the Gregorian calendar
This occurs because Eid follows a purely lunar calendar (354 days per year vs. 365 days in solar year)
The date remains the same in the Islamic lunar calendar
Diwali is based on a luni-solar calendar and also shifts, but with occasional 'jumps'
The 'jumps' occur when an intercalary month (Adhikamaasa) is added to synchronize with the solar year
Years showing sudden larger shifts in Diwali dates likely included an intercalary month