Vedic astrology is a myth and a misnomer. Vedas do not deal with anything relating to astrology. It do not anywhere state about any relationship between stars & planets and human life & destiny. Astrology came to India from the middle-east. Vedic Sages rejected it on the ground that it is not useful to to human life and society. The Vedas and the Upanisadas condemned such faith stating that it takes a person to the darkest regions of ignorance. But some powerful kings and powerful people got interested and urged their priests and scholars to study the subject. Thus astrology had its beginning in India. The priests & some scholars studied & developed the subject and also used it as a means to enhance their income and developed it as a profession. People, being mostly weak & suffering, accepted it as a means to remedy their mundane problems with the help of advice from the astrologers. Even kings and the powerful people also became interested in the subject and started relying on it. There are mentions of such faith in the stories in the Puranas. But the first text on astrology was written sometime after the introduction of Sanskrit as a language. It is true that many Hindus believe that astrology is a part of and is approved by religion. But it is because of a misconception, misunderstanding and mis-propaganda. It may be mentioned that as the Vedas & the Upanisadas, the very basis of Hinduism, condemned such faith, no other scripture can authorise or ratify the subject as a part of Hindu faith.
Predictions are made on the basis of position of stars and planets in different zodiac signs. These zodiac signs on which the accuracy of prediction depends, are imaginary figures. A zodiac sign is a creation of human imagination by drawing imaginary lines between a few stars which appear to be lying close to each other on a plane surface in the sky. But those stars are actually hundreds of light years ( a light year means the distance covered by moving at a speed of 1,86,000 miles per second for so many years or one light year distance = 584260 crore miles) away from each other both horizontally and vertically and have no relation, as such, between those stars. The zodiac sign, being an imaginary body does not have any homogenous existence and is not a body formed by nature. Therefore the so-called effects of a planet being in such an imaginary house is also imaginary and not real.
Then again the idea of enmity, friendship between planets, exaltation, debilitation, aspects of planets, ownership of zodiac signs by planets etc. etc do not have any logical or scientific basis. Considering the above facts it may be concluded that the ideas of various yogas are also all imaginary and do not have any logical or scientific basis. Correctness or mistake in predictions are all matters of pure guesswork. Besides this, even Sun & Moon are considered as planets and Rahu & Ketu are also planets in astrology though according to science Sun is a star, Moon is a satellite of earth and Rahu & Ketu are imaginary points only.
Then comes the questions about planets and their powers. You are aware that places, roads, buildings etc are named after some reputed persons, such as Gandhi Bhavan, Patel Nagar etc. So, in earlier days when people studying the sky, discovered a planet, they used to recognise it in the name of some reputed person of that or earlier period.. Thus when a planet was discovered, they used to name it after a renowned person like Dev Guru Brihaspati (Jupiter) or Shukracharyya (Venus), Shanaiischara (Saturn )etc. When a building is named after, say a poet or a singer, the building does not acquire the qualities of the poet or the singer and cannot write poems or start singing.. A road named after a politician statesman, does not become a statesman. The building or the road remains a building or a road only. Thus when a planet was named after a renowned person the planet does not acquire the qualities of that person but remains what it is, that is an inanimate object in the sky, revolving round the Sun. According to Hindu Puranas, Budha (Mercury) is the son of Chandra (Moon). But, as we know, Budha (Mercury) is a planet of the SUN and Chandra (Moon) is a satellite of earth. Budha (Mercury) was not born out of Chandra (Moon) and has no relation, as such, between them. It is thus clear that the planets in the sky are not the same as the characters in the Hindu Puranas. The objects in the sky are only named after those Purana Characters. Therefore two or more planets occupying particular places in the sky is a natural phenomenon only and have no relation to a person's life and destiny.
Next we come to Dasha Vichar. ..Dasha and Mahadasha is determined according to position of Moon vis-a-vis a star at the time of your birth and subsequent Mahadasha & antardasha are determined as per chart in astrology texts. The main issue is when we see a star in the sky we actually see the light emitted by it. A star which is , say, 100 light years away from us, we see the light emitted by it 100 years ago and assume that the star is at the place in the sky where we see it. But in fact, the star may have moved away from that position long long back and is totally somewhere else than the place we see it in the sky now. But while calculating the dasha we calculate on the basis of the position where we see it and not on the basis of its actual existence. This is the reason that in many cases readings about dasha vichar goes wrong.The entire concept is imaginary and it is now scientifically established that the basis of calculation of Dasha is also wrong.
Lastly it must be mentioned that the Vedas and the Upanisadas condemned such faith stating that it takes a person to the darkest regions of ignorance. It is for this reason that the great religious teachers, real Sadhus and saints never preached or practiced on the basis of such faith. If you wish to have detailed study on the subject please log on to : roymukherjeearun.blogspot.com and go through the articles therein starting with the one titled "Vedic Astrology" through other articles and particularly on "How accurate is my horoscope?" and on "Dasha Vichar