Ketu: effects and importance in Indian astrology

Ketu is an imaginary planet or a node that is positioned where the earth's trajectory around the Sun and the Moon's trajectory around the Earth meet. It is believed to represent the tail of demon. The South node or Ketu is regarded as a point of difficult Karma from a past life in which the individual furthered his own selfish interest at the expense of others. The South node, on the positive side, can develop strong concentration and mastery of obscure or arcane subjects. Ketu gives sudden and unexpected results for good or bad. Both Rahu and Ketu are the parts of the same danava (demon) but are 180 degree apart. When Ketu, Sun and Moon lie in the same zodiacal longitude lunar eclipse takes place. Those ruled by Ketu, are secret adversaries and relates to people in pharmaceuticals industries, doctors and astrologers. Both imaginary nodes are the mysterious forces and show karmic and spiritual influences in the birth chart.
Ketu also does not rule any zodiac sign since it is not a real planet. But it identifies as its own house (in spite of the lordship of Jupiter - "The guru of Gods"). Ketu is exalted in Scorpio, and debilitated in Taurus. It is strong in Jupiter’s sign (Sagittarius and Pisces) and that of Mercury (Virgo and Gemini). Ketu goes along beautifully with Mercury, Venus, Saturn and Rahu. Ketu is neutral to Jupiter while Mars, Sun and Moon are its archenemies. Ketu is known as "Moksha- Karaka" or the planet influencing spirituality. The person influenced by the planet is usually endowed with psychic powers, healing abilities, excellence in herb logy, tantrik, and ghost related ailments. Ketu influences pharmaceutical industry, medicine and astrology to a large extent. Ketu, if favorable, brings a lot of luxury, wisdom and intuition to the person. If unfavorable, it causes weak eyesight, poor concentration, boundless worries and anxiety.
Ketu is a killer, and strips the house where it tenets the life in a way, but can promote it in another way. The way it does both is through its strong empty feeling. By promoting emptiness in that house, the person often struggles to refill it. This can result in accumulation and abundance for that house, hence ketu in the 11th house in a horoscope increases riches, by creating a feeling of constant poverty or drainage of the 11th house.
Ketu in the 7th house in a horoscope causes a feeling of loss of connections with others, a natural result of Rahu in the first, which makes one often rejected and abandoned for being outcaste. This happens subtly throughout life for those bearing the mark of Rahu in the First.
Ketu always causes Rahu straight across. Thus, the nodes affect an axis always. On one side there is concern and expansion (Rahu) and on the other side there is detachment and loss (Ketu).There are head and tail of karmic reality-too much abundance and involvement for the mouth of Rahu caused him to lose his body but he became eternal in a sense.Ketu is the highest planet in Jyotish spiritually, because it rules the ultimate truth for human. Jupiter can teach us all about religion but ketu really drives it home.Ketu and Saturn help us to realize the teachings of Jupiter and they alone teach us that we must be humble.Ketu represents the opposite node of Rahu.It the tail of the serpent. Venus and Rahu are its friends whereas moon and mars are its enemies. Forty two years is the age of ketu. Ketu is also considered to be auspicious for the birth of a son and as long as that bed is in the house.