Adityaya cha Somaya Magalaya Budhaya Cha
Guru Sukra Sanibhyascha Rahave Ketave Namaha
MahaVishnu created the Navagrahas as the first step in the creation of the Universe to keep track of the deeds (good and bad) of his creation. After the Trinity (Brahma, Vishnu and Shiva), the Navagrahas have the maximum importance in Hindu Rituals.
The Navagrahas are Surya (Sun), Chandra (moon), Mangala (mars), Budha (mercury), Guru (jupiter), Sukra (venus), Shani (Saturn), Rahu and Ketu. Rahu and Ketu are described as invisible or shadowy planets because they cannot be observed like the other grahas, but in fact, they are mathematical points of calculation. Imagine the path of the Sun across the Sky as a great wheel. This path is called the Solar Eclipse. Now imagine a similar path of the moon as it makes its monthly journey across the sky. Where the moon's path crosses the sun's path at the top of the circle is called the point of Rahu and 180 degrees to the south where these two lines again intersect is called the point of Ketu.
These Navagrahas show their effects on humans since birth. A horoscope shows the locations of the Navagrahas at the time of ones birth. An interpretation of these locations shows the effects of the Navagraha on the life of a person.
The life style of a person can be influenced by the movement of these Navagrahas with respect to the Sun (called Lagna in horoscope). and with respect to the moon (called Rashi in horospcope). Some time, in our horoscope, these Navagrahas are in the location of the enemy causing severe pain in one's life.
Similarly, one faces lots of difficulties during the seven and half years period (known as Shadhe-saati) when Shani (Saturn) stays in the preceding sun sign, followed by the person's sun sign, and lastly in subsequent Sun sign. Each graha stays in each rashi for 2 1/2 years. Also, the wrong locations of Mangla and Sukra can cause unnecessary delays in marriage.
People who are facing lots of difficulties, can help themselves by praying to the Navagrahas. However performing Yagnas or Homams of the entire Navagrahas may not help. Rather one should first find the exact graha that may be causing the troubles. They can then perform Yagna for that particular graha to lower their effect on the person.