John Stringer and Dinah Starkey explore religious festivals and the science of illumination
All religions use symbols in an attempt to illustrate spiritual truths, and the equation of light with goodness is a universal metaphor.
From the Zoroastrian wheel of fire to the Advent candle, light is seen as a purifying force and a weapon against the powers of darkness. It is this symbolism which gives the festivals of light their power.
Diwali is probably the most widely celebrated of all the Hindu festivals. It falls in October or November but the precise date varies according to the year and the community. The word "Diwali" means a garland of light and - in the old days - Hindu households were decorated with rows of little clay bowls filled with oil or clarified butter that were lit as evening approached. Nowadays, the lights are often electric, but the custom continues to thrive.
Like Christmas, Diwali is a complex mix of different traditions. In some communities the lights are lit to welcome the Lord Rama and commemorate his return from banishment after his battle with the demon king Ravana. In other parts of India, they guide the path of Lakshmi, the goddess of Wealth, when she visits her worshippers at the start of the new year. Traders close their accounts on the day before Diwali and bring their ledgers to the temple for a blessing from her. It's a time for the exchange of greetings cards and presents, and for feasting at candlelit tables.
Diwali is an important festival for Sikhs, too, but its significance is quite different from the Hindu festival. It marks three key events in the history of the religion: the foundation of the city of Amritsar (1577), the release from imprisonment of Guru Hargobind and the martyrdom of Bhai Mani Singh, the custodian of the Golden Temple.
Houses are cleaned from top to bottom ready for the time when the cold nights begin and beds are brought indoors. Candles and fireworks illuminate the moonless Diwali night and friends exchange presents and visit the temple.
Hanukkah falls in late November or December and it commemorates a time when the Jews reclaimed the Temple from Antiochus, king of Syria in 165BC. When he conquered Jerusalem he rededicated the Temple to Zeus and ordered its priests to extinguish the flame, which burns perpetually on the High Altar.
The Jews, led by Judas Maccabaeus, rose up against him. They took the temple and relit the lamp, but the sacred oil had run so low that there was only enough to keep it burning for one day. A miracle kept the lamp burning brightly for eight days until fresh supplies arrived and the festival of Hanukkah marks this event. At its heart is the kindling of light in darkness. The Hanukkah lamp is lit soon after sunset: even the youngest child can witness the event and remember the miracle.
Buddhism Every Buddhist country has a festival of light but it takes a different form in each one. The Thai festival, in November, celebrates the selflessness of the Buddha who, in a previous incarnation, gave up his wife and children. There are fireworks and in the late evening thousands of candles are sent floating away down rivers and streams as an offering to the river spirits.