This demon, Rahu, doesnt just represent desire, but desire for things that aren't his to have. He seeks to become immortal like the Devas. While the story runs around and through all kinds of scenes and characters as good and evil fight over the elixir of immortality, called Amrit, it is during the distribution of the Amrit, when Rahu realizes the Devas aren't going to share.
Once Rahu knows the Devas aren't going to be sharing immortality with the Asuras, he sneaks in line between the Sun and Moon, seemingly undetected. Just as he's given a given a drop of the immortal elixir, which lands on his demon tongue, the Sun and Moon, who've noticed his presence between them, point him out to Vishnu. In immediate response, Vishnu hurls his diamond encrusted discuss and slices Rahu and his now immortal head from his severed immortal body. The head becomes Rahu and his body becomes Ketu.
The two parts of his body are thrust into the sky, where they are forever chasing and seeking revenge on the Sun and Moon. How do they enact their revenge? By swallowing the Sun and Moon. As they are disjoint bodies, they can swallow but not hold either the Sun and Moon.
In Vedic astrology, Ketu represents desire for wholeness and reunification of our heart and feelings with our ego. Rahu only knows intense and overwhelming desire for those ego-driven things indicated in his chart. Rahu isn't interested in unifying with his body because his ego-desire is overwhelming amd (mostly) unregulated. Ketu is left to feel his way through time, as he holds the heart, but lacks the senses that drive Rahu's desire.
Scientifically, the two points represent the location where the path of the Moon crosses the ecliptic, which represents where eclipses happen.
One Possible Insight Into Meaning
It seemed to me a possibility that a meaning of the description of the story of Rahu and Ketu is the story of when...
Sperm meets Egg
Here's a description of the biological process:The tip of the head of the sperm cell contains enzymes which break through the zona pellucida and aid the penetration of the sperm into the egg. Once the head of the sperm is inside the egg, the tail of the sperm falls off, and the perimeter of the egg thickens to prevent another sperm from entering.It's possible that Rahu represents, in our more mundane world, the head of the sperm and Ketu may be the tail that falls away, while the Egg is the discuss of Vishnu which slices Rahu into component parts. If so, then the Amrit becomes the drop of nectar that brings life at the point of conception.
Read more: Fertilization - Sperm, Egg, Cell, and Female - JRank Articles http://science.jrank.org/pages/2693/Fertilization.html#ixzz41qds0rj4
Here's a link to the story of the Samudra Manthan. The story is so full of imagery and meaning related to life, it's possible you'll revisit it many times.
King Indra and Sage Durvasa
There's much more meaning in the story. Like how Indra, the king represents grown-up Rahu. His ego so large and possessions so great even the largest elephant isn't big enough to hold him. Grown up Ketu may be represented by the sage Durvasa, who is deeply spiritual, but wildly emotional.
Separately, they are unable to understand the fragility and meaning of the garland of life that they attempt to share to their demise. Together, their only hope seems to be to unite their voice, which is the only part of their disjoint body they still have in common. But that's another possible layer upon this richly woven story.
