The Sun, the moon and the Festival of Light

The festival of light - Diwali science

The Sun, the moon and Diwali, the Festival of Light!

A friend of mine told me she was looking forward to Diwali this month. The lights, the food and perhaps some noise-free fireworks in the garden were all mentioned. But I’ve got to confess that I didn’t catch much of the conversation because I was thinking “this month? I thought Diwali was in October, it was last year!” I asked her if I’d imagined it, and she assured me that I hadn’t. So why the different day? She explained that Diwali and in fact all festivals fall on different days every year because Hindus have a completely different calendar system – called a Panchanga.


Now you might be thinking that science and religion don’t mix. But this is the perfect example of where they can! The Panchanga calendar system is lunisolar. It’s a multi-dimensional calendar people!

What does that mean? OK here’s where it gets a bit complex so bear with me…

This calendar system dates back to BCE (Before Common Era) and combines information about lunar days, solar days, lunar months, solar months, the movements of the Sun and the Moon in relation to stellar constellations, and other astronomically defined time spans. Compared to the Western system that’s based on solar days and solar years, it’s massively intricate, but it allowed different regions in India to have their own calendar. A process that has now fallen out of favour, especially now that all countries use the solar Gregorian calendar system.

So how does a lunisolar calendar system work? Well, you have 12 lunar months – which are based on the time it takes the Moon to orbit the Earth in relation to the Sun, and 12 solar months – which are defined by and named after the zodiac signs the Sun traverses during different parts of the year, as seen from Earth.

The lunar month

Each lunar month is divided into 30 lunar days, but it doesn’t stop there! The 30 days are divided into 2 groups of 15 days, One ‘fortnight’ takes us through the waxing of the moon up to full moon,(the “bright” fortnight) and the other takes us through the waning of the moon (the “dark” fortnight). Festival dates are determined by the lunar months which is why they shift around a bit, and Diwali always falls on the same day that there’s a new moon, take a look here if you don’t believe it!

The solar month

Sun gives us lightSolar months aren’t that different to our Gregorian months because they’re broadly based on the same idea and they don’t shift around so you can see how they match up with the months and seasons that we all use.




12 lunar months only amount to something like 354.367 days, whereas 12 solar months give us 365 days as we know, that’s a difference of around 11 days. How do you synchronise the two? You just make the lunar year longer every 2 and a bit years! But you don’t just chuck in an extra few days at random – oh no. Hindu scholars look at the seasons and insert an extra month in sync with the seasons and the cycle of nature, to align the lunar and solar calendar and retain balance with our natural environment. Clever!

There’s loads more to talk about when it comes to lunisolar calendars, not least that many cultures including the Chinese and the Egyptians also have them. But I think we’ve answered the question of why Hindu festivals fall on different days so let’s leave it there for today! And a very Happy Diwali to anyone celebrating. Peace, love and light to you all!

Science of Fireworks

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If this has lit up your curiosity about light, then why not take a look at our Light Boxes

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