Why Diwali can fall on different days in different cities, and how we compute it
A festival is kept on the day its tithi holds at a particular moment, so sunrise and sunset decide the date. Here is how Parallel Earth works out every city's festival calendar.
Every October someone asks the family group which day Diwali is this year. The answer is not a lookup. It is astronomy, and it depends on where you are.
A festival belongs to a tithi
A tithi is a lunar day: the time it takes the Moon to gain 12 degrees on the Sun. Tithis do not line up with midnight-to-midnight days. One can begin at 3 in the afternoon and end at 1 the next afternoon. So a festival needs a rule for which calendar day it lands on, and the rule is its karmakala, the moment of the day when its tithi must be running.
- Most festivals look at sunrise: whatever tithi is current at sunrise names the day.
- Diwali's Lakshmi Puja and Dhanteras look at pradosh, the time just after sunset.
- Karwa Chauth looks at moonrise, because the fast ends when the moon is seen.
- Bhai Dooj looks at the afternoon, and Chhath's evening offering at sunset.
Why cities disagree
Sunset in Kolkata comes about an hour before sunset in Ahmedabad, which is 16 degrees further west. When a tithi changes close to dusk, one city can see the new tithi at pradosh and the other cannot. Same night sky, different calendar day.
How Parallel Earth does it
Our simulated India needs festival dates for every city, every year, so it computes them. It takes the Sun's and the Moon's positions from an open-source astronomy library, converts them to the sidereal zodiac Indian calendars use, works out each city's sunrise, sunset and moonrise, and checks the tithi at each festival's moment. It also names the lunar months by the sign the Sun is in, which is how it finds 2026's extra month, an adhik Jyeshtha.
Why we bother
In Parallel Earth the calendar is what moves people. On a Ramleela evening in Rohini the simulated residents head for the big ground; on Dussehra the offices close and people follow their weekend routine; on Diwali night the air turns smoky. All of it starts with getting the day right.
Frequently asked questions
Why is Diwali on a different date in some cities?
Diwali is kept on the day the Amavasya tithi holds at pradosh, just after sunset. Sunset comes at different times in different cities, so when the tithi changes near dusk, cities can land on different days.
What is a tithi?
A lunar day: the time it takes the Moon to move 12 degrees ahead of the Sun. A lunar month has 30 tithis, 15 waxing and 15 waning.