Vedic Astrology considers 9 Planets which impacts human life. These are: Jupiter (Guru), Mercury (Budh), Venus (Shukra), Moon (Chandra), Sun (Surya), Saturn (Shani), Mars (Mangal), Rahu and Ketu. First 4 in this sequence are real benefics and the last 5 are malefics. Position of the planets in the Natal chart is one of the key pillars which holds the fortune of the native in this life time. Ascendant lord is the King of the chart, in needs to be analyzed first, it holds the key of the overall life of the Native. If Ascendant lord is weak, it shows that overall personality of the Native is weak. All the planets play different roles, in particular we need to see Jupiter (Most Benefic Planet), and Saturn (Planet of Karma). Without the blessings of Jupiter no auspicious event can happen, while Saturn shows the place where native has to burn most of his/her life Karma. Sun represents the soul and its wishes, Moon represents native's mind. Mercury represents knowledge or communication, Mars represents Men and masculine traits, Venus represents Women, luxuries and comfort. 

3. Temporal Strength (Kaal Bal - Time Strength) - This strength is based on the period of time when the planet is strong or weak at the time of birth or an event occurs.  Sun, Jupiter and Venus are strong during day time, whereas Mars, Saturn and Moon are strong during night time. Mercury is strong during sunrise and sunset. Similarly, we can also check the strength based on Moon Paksha, benefic planets Jupiter, Venus, Mercury and Moon are strong near to full moon (Purnima) while malefic planets Mars, Saturn and Sun are strong near to No Moon day (Amavasya) .

4. Motional Strength (Chesta Bal) - Strength of the planet arising because of the effort it is making during its movement. Planets are said to have Motional Strength when they are in retrograde motion as they do more effort then when they are moving steadily. Retrograde can be defined as the effort being made to move against the current. Sun and Moon are never retrograde. If any planet is Retrograde as well as Combust then it has highest Motional strength.

5. Natural Strength (Naisargika Bal) - The natural strength of the planets is called Naisargika Bal. Highest to lowest order based on the natural strength of the planet is Sun, Moon, Venus, Jupiter, Mercury, Mars and Saturn.

6. Aspectual Strength (Drig Bal) - Planets get Aspectual strength based on the planets aspecting them from other houses. Details of the aspecting capability of different planets is given in next section. If a benefic planets is aspecting another planet, it adds to the strength of the planet being aspected, whereas aspect by malefic planet reduces the strength. Also, if the aspecting planet is a friend of the planet being aspected then it adds to the strength, whereas if the aspected planet is enemy then it will reduce the strength.

Rahu (North Node) & Ketu (South Node) are always opposite to each other. Rahu and Ketu always move in the opposite direction to the rest of the planets.

Mythology: At the time of the churning of the ocean (Samudra Manthan), the Gods knew that they cannot carry out the immense task all by themselves and will need the help of the Demons to churn the mount Mandara. To do this, they invited the Demons to help churn the ocean and offered to share the Amrit (nectar) of immortality with them. Seeing this Lord Vishnu, disguised himself as Mohini, the most beautiful woman, and convinced the Demons, to follow what she said. She started distributing the nectar amongst the Devas first. However, a Demon named Swarbhanu, understood what is happening and disguised himself as a Deva, and went and sat with the Devs. However, Sun and Moon, recognized him and told Lord Vishnu of his plan. Seeing this, Lord Vishnu had cut his head with his Sudarshan Chakra, but he had tasted a drop of the immortality nectar and had become immortal. His head is called Rahu, which fell to the side of the demons and rest of his body came to be known as Ketu, which stayed at the devas side.

So, while analyzing Rahu’s placement in horoscope it is considered a head with malefic brain and no body, in a sense it means that because Rahu is having only brain it has desires. However because it doesn’t have a body, it cannot consume and therefore is never satisfied. So it runs after materialistic things and is never satisfied. While Ketu is considered body without head or brain, and since it stayed with devatas it became spiritual. Thus, Ketu represents detachment and spirituality.