The reason for moths to be apparently attracted to light is not known. It is thought that moths may navigate by maintaining a constant angular relationship to a bright celestial light (such as the moon), but when encountering a bright artificial light they navigate by maintaining a constant angle to the light. This spiral flight eventually ends on the moth hitting the light source. There are many theories but none is confirmed. Hope this helps.

here is a good poem about it i was talking to a moth the other evening he was trying to break into an electric light bulb and fry himself on the wires why do you fellows pull this stunt i asked him because it is the conventional thing for moths or why if that had been an uncovered candle instead of an electric light bulb you would now be a small unsightly cinder have you no sense plenty of it he answered but at times we get tired of using it we get bored with the routine and crave beauty and excitement fire is beautiful and we know that if we get too close it will kill us but what does that matter it is better to be happy for a moment and be burned up with beauty than to live a long time and be bored all the while so we wad all our life up into one little roll and then we shoot the roll that is what life is for it is better to be a part of beauty for one instant and then cease to exist than to exist forever and never be a part of beauty our attitude toward life is come easy go easy we are like human beings used to be before they became too civilized to enjoy themselves and before i could argue him out of his philosophy he went and immolated himself on a patent cigar lighter i do not agree with him myself i would rather have half the happiness and twice the longevity but at the same time i wish there was something i wanted as badly as he wanted to fry himself archy -- Don Marquis

I think it's because they think the light is the sun (or moon) which they use for orientation.

A friend of mine said it has something to do with navigation. Check this out: Moths frequently appear to circle artificial lights. One hypothesis advanced to explain this behavior is that moths use a technique of celestial navigation called transverse orientation. By maintaining a constant angular relationship to a bright celestial light, such as the Moon, they can fly in a straight line. Celestial objects are so far away, that even after travelling great distances, the change in angle between the moth and the light source is negligible; further, the moon will always be in the upper part of the visual field or on the horizon. Human light sources have not existed long enough to affect the evolution of moth navigation systems. When a moth encounters a much closer artificial light and uses it for navigation, the angle changes noticeably after only a short distance, in addition to being often below the horizon. The moth instinctively attempts to correct by turning toward the light, causing airborne moths to come plummeting downwards, and - at close range - which results in a spiral flight path that gets closer and closer to the light source. Reference: http://en.wikipedia.org/wiki/Moth#Attraction_to_light

The used flames.

They fried in the fire.

There were these crazy things, made of the same stuff as crayons (i know, can you believe it?) called "candles"... and then there was fire, and you could light it without a stove! :O Insane, I know.

They use the moon for navigation, indoors light fittings replicate the moon. Thus they try to fly with the light in a constant position from their view, leading to the spiralling.

Huddled together in dark places.

Sat around tapping their feet.

They moth-fornicated by moonlight.

I'm gonna go on a limb here and say that they were still attracted to candle light

Actually, they use dim light sources (the stars and the moon) to navigate by flying in certain ankles regarding the sources. After the invention of electric lights and candles, they got a problem because their instinct misfires and can even kill them in case of a candle.

http://www.answerbag.com/q_view/67834 Question's already been asked.