The twelve months and the stars they are named for

Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwina, Kartika, Margashirsha, Pausha, Magha, Phalguna. Most Indian families use these month names without ever being told where they come from. Every one of them is a star's name first.

Here is the idea behind it. The moon takes about 27 days to go once round the sky against the background stars. The old Indian reckoning cut that path into 27 sections called nakshatras, and named each section after the star or small group of stars sitting in it.

Once a month the moon comes to the full. When it does, it is standing somewhere along that path, in one named section. So people named the month after the star the moon was visiting on its brightest night. The full moon near the star Chitra gave the month Chaitra, and the same was done for the other eleven.

Three months keep the star's name almost unchanged: Jyeshtha, Shravana and Magha. Six stretch the first vowel: Chitra becomes Chaitra, Krittika becomes Kartika, Pushya becomes Pausha. Three are named after a pair of neighbouring stars: Ashadha, Bhadrapada and Phalguna.

The rule does not always work, and that is the interesting part. The moon speeds up and slows down as it goes, and the month from full moon to full moon is not the same length as the moon's trip round the stars. So the full moon does not land on the same star every year. Almanac-makers dealt with that by giving each month a band of two or three stars instead of one.

Eventually they gave up on the moon for this job. Today a lunar month is named by where the sun is. Each month the sun crosses into a new sign of the zodiac, and the month holding the crossing into the first sign is Chaitra. The star names stayed on as inherited labels.

An older set of month names came from the weather rather than the stars: Madhu for spring, Nabhas for the rains. Those are in the Vedic texts. The star names came later, and they are the ones that survived.