By T.V. Venkateswaran | 2026-05-25

Look up on May 31. That pale disc you see, the second full moon of the month, carries a curious name. 'Blue moon'. But do not expect any color. And do not expect the whole world to agree on what it means.

How did the moon become blue? The story begins with how the moon moves, or rather, how it refuses to move at a steady pace.

On the night of the full moon, the Sun, Earth and moon are neatly lined up, with the Earth in the middle. In broad terms, the centers of the three bodies lie in a plane, which is exactly what a full moon is.

The moon takes about 27.3 days to orbit the Earth and return to the same position in the sky, but we do not get a full moon every 27.3 days.

By the time the moon finishes its 360-degree journey, the Earth has shifted a bit in its own path. So when the moon completes one orbit, the Sun-Earth-moon alignment needed for a full moon has shifted.

The moon must travel a little farther to catch up with this moving geometry. That extra chase stretches the cycle to an average of about 29.53 days.

Further, the moon does not orbit the Earth in a perfect circle, but in an elliptical orbit. Its motion along this path is not even; it moves faster when it is closer to Earth, at perigee, and slower when it is farther away, near apogee.

If a full moon occurs when the moon is near perigee, the moon could catch up with the Sun-Earth plane more quickly than when it occurs near apogee.

Because of this, the gap between two successive full moons varies, with the shortest possible gap about 29.18 days, and the longest about 29.93.

Nearly three-quarters of a day of variation may not sound dramatic. Yet these small variations shape how we experience the moon in our calendars and cultures.

Seasonal designation

From the astronomical point of view, the intervals between equinoxes and solstices are the seasons.

In the Northern Hemisphere, the period from the spring equinox (around March 20–21) to the summer solstice (June 20–21) is spring. Likewise, the period from the summer solstice to the autumn equinox is summer.

Since a solar year is about 365.24 days, each of the four seasons lasts about 91.3 days. As the average gap between full moons is 29.53 days, a season would generally span three full moons.

But because of those variations in the moon's speed, a season sometimes has four full moons.

In medieval Europe, that extra full moon created a problem for monks who kept the calendar for church festivals, as occasionally a year had 13 moons.

The phrase "once in a blue moon" had existed for centuries to mean something rare. Just as a year with 13 full moons was rare and out of the usual rhythm.

When Europeans crossed the Atlantic and settled in America, they were influenced by the knowledge of the land's First Peoples, who kept track of time by observing the seasons and lunar months.

Native American tribes gave each full moon a name, reflective of the season, with regional and tribal variations in these designations. May's flower moon and June's strawberry moon come directly from the Algonquin people, for example.

In the 1930s, the names were standardized by farmers' almanacs that blended the names used by Native American tribes with those of European origin.

Almanacs gave each full moon a name that described the season, and in the season with four full moons, the odd one out became the blue moon, thus keeping the rest of the naming cycle on track with the equinoxes and solstices.

The modern definition

The modern definition of a blue moon emerged accidentally.

In the 1940s, a popular astronomy writer misunderstood the seasonal concept of the designation, and interpreted it as the second full moon in a calendar month.

This is how the now familiar 'blue moon' designation -- the second full moon in a single calendar month -- began to take hold.

In the Gregorian calendar (*** see description below), which is mapped on the solar year, months ranged from 28 to 31 days.

Since the interval between full moons ranges from about 29.18 to 29.93 days, a full moon occurring on the first or second day of a 31-day month can leave enough room for another full moon near the month's end.

That second one is now called a blue moon, to indicate its rarity.

A fresh example will arrive soon.

On May 31, we will see a calendar month blue moon.

The first full moon of the month appeared May 1, and the second will appear May 31. It will actually be a micromoon, appearing slightly smaller because the moon sits near its farthest point from Earth.

So blue moon, in this modern sense, is not an astronomical event in itself. It is a cultural artefact created by fitting lunar rhythms into a Roman-derived civil calendar.

This becomes clearer when one looks at calendrical traditions across the world, such as those on the Indian subcontinent.

Indian calendrical traditions

In Indian lunisolar calendars, a month spans one Pūrṇimā (full moon) to the next, in one tradition; and in another, one Amāvāsya (new moon) to the next.

Islamic lunar calendars are based on sightings of the crescent moon.

In such systems, by definition, you cannot have two full moons inside one month. The month itself is defined by the lunar cycle.

A blue moon in the modern Gregorian sense simply cannot occur.

There are typically between 29 and 30 days in lunar month, with an average of 29.5. But the lunar year is only 354 days long. The rhythm of the solar year of 365.25 days and the lunar year of 354 days drift apart by roughly 11 days.

To keep them in sync, about every 2.7 years an extra month called adhika māsa is added to the Indian lunisolar calendars. That year carries 13 months and 13 full moons. The added month is treated as extra.

Historically, this extra month occupied an unusual social and ritual position.

In some traditions, it was considered unsuitable for initiating major activities, such as marriages. Sometimes people did not pay rent or interest for that 'extra' month.

There is an even more intriguing South Indian variation.

Tamil and Malayalam solar calendars define months by the Sun's movement along the ecliptic, roughly the time taken to traverse 30 degrees.

Because Earth's orbit is elliptical, the Sun's apparent motion is not perfectly uniform. These solar months do not all have equal lengths. Some may be 28 days; some may stretch even to 32 days.

Here, it becomes possible for a single month to hold two full moons. Such a month is called mala mādam or malamāsa.

Mala means excreta, unclean. Not a flattering name. People shunned this month for undertaking new activities, starting ventures, and celebrating auspicious occasions.

When the solar month is sufficiently long, it can contain two full moons. So the idea is not entirely alien to Indian thought. But it arises through a different calendrical logic.

Full moon festivals

Full moons anchor festivals across the subcontinent.

Take the biggest annual Tamil ceremony, Thaipusam. This date falls on the full moon of the Tamil solar month Tai (mid-January to mid-February). It is celebrated across Southeast Asia with remarkable fervor.

The wooden idols of the famous Jagannātha temple of Purī -- the central deities of the shrine -- are replaced during a special full moon, one in which two full moons occur in the month of Āṣāḍha, adhikamāsa.

One of the largest tribal communities in India is the Santal. Full moon in their month of Sohrae, which corresponds to Kārtika Pūrṇimā in the Hindu lunisolar calendar, triggers a mass pilgrimage.

Thousands travel to the sacred Luguburu hills in Bokaro, Jharkhand, to honor their rich ancestral heritage under that full moon.

And for the Ādi-dharma, the primal religion of the Oraon and Munda tribal communities in central India, the full moon in the lunisolar month of Bhādra (mid-August to mid-September) is one of their three main festivals.

Sharad Purnima takes place in the lunisolar month of Ashwin (September/October). During this full moon, rice kheer (sweet rice) is cooked, and families place it in the moonlight.

Traditionally, it is believed that during this full moon, which is the brightest full moon of the year, the moon's rays carry Amrit, the nectar of immortality. Eating kheer is thought to nurture health.

Jain and Buddhist traditions also attach significance to full moon days. Buddha Purnima, celebrated on the full moon day of the Hindu/Buddhist lunar month of Vaisakha (usually in April or May), is the most sacred festival for Buddhists.

Tamil Sangam literature, dating back 2,000 years, describes women lighting clay lamps in their courtyards during the full moon of the solar Karthigai month.

In the ancient Chola kingdom, the rain god festival, or Mazhai Vizhavu, or Indira Vizha, commenced during the full moon of the month Ootrai (now Chithirai).

This festival lasted one month, during which people worshipped Indra, the rain god, performing prayers, music, and dances for rain.

The celebration cycle was also described in the post-Sangam epics of Silappadikaram and Manimekai.

The month of Chithirai Pournami (full moon in the solar month of Chithirai) still invokes the presence of the deity Murugan.

Different traditions

Come May 31, when the second moon of the month rises, nations using the Gregorian calendar, including countries across Europe and North America, will call it a blue moon.

It is indeed a rare event: this full moon occurs when the moon is near its apogee, making it appear smaller than usual.

But in large parts of the Indian subcontinent, nobody will call it anything special. The lunisolar calendars simply do not permit two full moons in one month. The concept does not translate.

And in Tamil Nadu, if by some calendar quirk the phenomenon did occur, the word would be malmāsam, a month to wait out, not to celebrate.

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*** European colonization led much of the world to adopt the Gregorian calendar, which is widely used for international transactions, even in countries that have not officially adopted it.

The Gregorian calendar evolved from an ancient Roman calendar, which divided a 304-day year into 10 months, beginning in March, named after Mars.

This calendar was routinely manipulated at the whim of various rulers, eventually leaving it out of sync with the seasons.

Julius Caesar's reform in 46 BCE revised the messy Roman lunar calendar. He shifted the calendrical base to solar, with a year normally having 365 days, with an extra day inserted every fourth year.

By the time of Augustus, the now familiar pattern of months, some having 30, a few 31 and February with 28 days in ordinary years and 29 days during leap years, had already largely settled.

Adding one full day every four years was slightly longer than a full year, so over time, the Julian calendar and the seasons became out of step.

Pope Gregory XIII in 1582 adjusted the Julian calendar to better align with the solar year and seasons. We still live by that patched-up Roman legacy.

T.V. Venkateswaran is a Visiting Professor at the Indian Institute of Science Education and Research, Mohali, Punjab. The views expressed are personal.