A lunar calendar counts months by the Moon's phases, so each month lasts about 29.5 days and twelve of them make a year of about 354 days. A solar calendar follows the Sun and the seasons, with a year of about 365.24 days. That gap of roughly eleven days is the whole story: a purely lunar calendar, like the Islamic one, drifts through the seasons, while lunisolar calendars, such as the Hebrew and Chinese calendars, add an extra month every two or three years to stay in step with the Sun.
Three kinds of calendar
Almost every calendar in use today is one of three types:
| Type | Follows | Year length | Examples | How it handles the gap |
|---|---|---|---|---|
| Lunar | The Moon only | 354 or 355 days | Islamic (Hijri) calendar | It doesn't: dates move through the seasons |
| Solar | The Sun only | 365 or 366 days | Gregorian calendar | Months are no longer tied to the Moon at all |
| Lunisolar | Both | 12 or 13 months | Hebrew, Chinese, Hindu and Buddhist calendars | Adds a leap month every two or three years |
The Gregorian calendar, the one on most office walls, is purely solar. Its months still carry traces of the Moon, though. The word "month" comes from "moon," and the Roman calendar it descends from began as a lunar one before its months drifted free of the phases.
The eleven-day problem
The Moon's cycle of phases, the synodic month, averages 29.53 days. Twelve of them add up to about 354.4 days. The year of the seasons, from one spring equinox to the next, is about 365.24 days. The difference is just under eleven days a year, and no whole number of lunar months fits neatly into a solar year.
A worked example shows what that means. Suppose a festival falls on a fixed date in a purely lunar calendar, and one year that date lands on June 20. The next year it lands about eleven days earlier, around June 9. The year after, around May 29. After about three years it has slid more than a month, and after roughly 33 years it has traveled through every season and come back to June.
That's exactly what happens with Ramadan. The Islamic calendar has twelve lunar months and no leap month, so Ramadan arrives about eleven days earlier each year by the Gregorian calendar, falling in winter in some decades and in the long, hot days of summer in others. Each month traditionally begins with the first sighting of the new crescent after sunset, which is why the start date can differ from one country to another. Our guide to the crescent moon symbol covers that crescent's wider history.
How lunisolar calendars catch up
Lunisolar calendars keep their months tied to the Moon but stop the year drifting by adding a thirteenth month when the gap grows too large. The trick that makes this workable is an ancient discovery often named after Meton of Athens, an astronomer of the fifth century BC: 19 solar years are almost exactly equal to 235 lunar months. So if you add seven extra months spread across every 19 years, the Moon and the Sun come back into line.
Different calendars apply that idea in different ways:
- The Hebrew calendar adds a second month of Adar in seven years out of every nineteen, which keeps Passover in spring.
- The Chinese calendar inserts a leap month in a year when a lunar month passes without one of the calendar's principal solar markers, which keeps the Lunar New Year between late January and late February.
- Hindu calendars add an extra month, often called adhik maas, roughly every two and a half to three years.
Older societies did the same. The Anglo-Saxon scholar Bede wrote in the early eighth century that the English "calculated their months according to the course of the moon," and that in a year with an extra month they gave the extra one to summer, calling the year Thrilithi. In the 1760s, the explorer Jonathan Carver reported that some peoples of the North American interior counted twelve lunar months and, "when thirty moons have waned," added one more, which they called "the lost moon."
How the solar calendar got its leap days
The solar calendar has its own catching up to do, because the year isn't a whole number of days either. The ancient Egyptian civil calendar used a flat 365 days, which slipped slowly against the seasons. Julius Caesar's reform in 46 BC introduced the Julian calendar, with a leap day every four years, giving an average year of 365.25 days.
That was slightly too long, by about eleven minutes a year, and over centuries the error added up. In 1582, Pope Gregory XIII introduced the Gregorian calendar, which skips the leap day in century years unless they divide by 400. That's why 1900 wasn't a leap year and 2000 was. The average Gregorian year, 365.2425 days, is close enough that it drifts by only about a day in three thousand years or so.
The price of all that precision was the Moon. Gregorian months have nothing to do with the phases, which is why full moons and new moons land on different dates every year and why we need tables to find them. For the next twelve of each, see our full moon calendar and new moon calendar.
Holidays that still follow the Moon
Plenty of the world's festivals are set by lunar or lunisolar calendars, which is why they move around on a Gregorian calendar:
- Easter. Western churches celebrate it on the Sunday after the first ecclesiastical full moon on or after March 21, a rule worked out from tables. We explain the details in Pink Moon meaning.
- Passover. It begins on the 15th of the Hebrew month of Nisan, at or near a full moon in spring.
- Lunar New Year. It falls on a new moon between late January and late February.
- Diwali. The Hindu festival of lights falls on the new moon of the month of Kartika, usually in October or November.
- Vesak. Many Buddhists mark it on a full moon, usually in May.
- Ramadan and Eid. They move through the whole solar year over about 33 years.
The almanac names for full moons, from the Wolf Moon to the Cold Moon, come from yet another way of keeping time by the Moon, explained in full moon names by month.
Living by a lunar calendar today
You don't need to change your diary to keep time by the Moon. Many people use the phases as a second, gentler calendar alongside the working one: a check-in at each new moon and full moon, a journal that runs from one lunation to the next, a garden planted by the waxing and waning light. The astronomy is real and predictable, and the meaning you attach to it is your own. Our moon phase today page shows where the Moon is in its cycle tonight.
Questions readers ask
Is the Chinese calendar lunar or solar?
Lunisolar. Its months follow the Moon, each starting at a new moon, but it adds a leap month about every two or three years so that the year stays aligned with the Sun and the seasons.
Why does Ramadan change every year?
Because the Islamic calendar is purely lunar. Its year of twelve lunar months is about eleven days shorter than the solar year, and it never adds a leap month, so Ramadan starts about eleven days earlier each year on the Gregorian calendar.
How many days are in a lunar year?
About 354. Twelve lunar months of about 29.53 days each add up to roughly 354.4 days, so years in the Islamic calendar have either 354 or 355 days.
Which is more accurate, a lunar or a solar calendar?
They measure different things. A solar calendar is better for tracking the seasons, farming and fixed dates, and a lunar calendar is better for tracking the Moon. Lunisolar calendars do both, at the cost of years of different lengths.
