Why Is the Hijri Year Eleven Days Shorter?

Eleven days. A small number, but it explains why your grandparents fasted Ramadan in winter cold while you fast it in summer heat, and why Eid al-Adha never settles into one Gregorian month.

Where the gap comes from

The two calendars measure different motions. The Gregorian tracks the Earth around the Sun: 365 days, 5 hours, 48 minutes and 45 seconds. That leftover fraction is exactly why a leap day is inserted every four years.

The Hijri tracks the Moon around the Earth. A lunar month averages 29 days, 12 hours, 44 minutes and 3 seconds, so twelve of them come to about 354 days, 8 hours and 48 minutes.

Subtract: 365.2422 minus 354.3671 leaves 10.875 days. That is the gap, and it is fixed, because it is the product of two independent astronomical motions.

Why wasn't an extra month added?

The standard fix is intercalation — inserting a thirteenth month every few years to pull the lunar year back into step with the solar one. The Hebrew calendar does exactly this, and pre-Islamic Arabs did it too, under the name nasi'.

The Qur'an prohibited the practice explicitly — "intercalation is only an increase in disbelief" (al-Tawbah 37) — and fixed the count at twelve months. The Hijri calendar therefore remained purely lunar, and was never anchored to the seasons.

The thirty-three-year cycle

Because the Hijri year is shorter, Islamic occasions move eleven days earlier in the Gregorian calendar each year. A Ramadan that began in March begins in late February the next year, then mid-February, and so on.

After roughly thirty-three years, Ramadan has passed through all four seasons and returned to where it started. Anyone who lives sixty-six years fasts Ramadan in every season twice.

Defect or design?

A calendar unmoored from the seasons is undeniably less useful for agriculture, and Muslim societies knew it. Throughout history they ran solar calendars alongside the Hijri one for farming and taxation — the Coptic calendar in Egypt, the Persian calendar in Iran.

The drift has a compensation, though, and many find it a considerable one: worship is never tied to a single season. A believer in the far north does not always fast a twenty-hour day, nor one in the south always a short one. Over a lifetime, ease and difficulty are distributed across everyone.

There is a second advantage, easily overlooked. A lunar calendar is visible. Anyone can look up and know roughly where they stand in the month, with no printed calendar and no device. No solar calendar offers that.

Eleven days a year sounds like a rounding error. It is the difference between a calendar you read on paper and one you read in the sky.

Sources: Qur'an, al-Tawbah 36–37; al-Biruni, "The Remaining Signs of Past Centuries"; NASA lunar synodic month data