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New millennium astrological chart
Astrological compatibility (synastry) is the branch of astrology that studies relationships by comparing natal horoscopes. A natal horoscope is a chart, map, or imaginary snapshot of the planets in the Solar System and their positions in the zodiac at the exact time of a person's birth. The signs of the Zodiac represent 12 different types of spiritual energy and purpose. The planets in the Solar System are associated with at least one, but most often two signs of the zodiac.
Relationships between planets, signs (types of energy), and houses (sectors of the chart) are described in words and/or often rated in numbers to show to what degree, how, and in what ways one person is compatible or incompatible with another. The assessment is based on the patterns and distributions of energy in the respective and combined charts.
Astrology is generally considered a pseudoscience by the scientific community and what little statistical evidence that does exist has failed to find a correlation between astrological compatibility and real world consequences.
Compatibility in Indian astrology 
The Hindu/Indian system of examining compatibility based on horoscopes of the aspirant couple is unique. The fundamental concept of matching horoscopes emanates from constellations occupied by the Moon (moon sign) at the time of births of bride and bridegroom. Individuals inherit qualities of the birth constellation. Various constellations represent different nature in respect of their casts, animals presented by them, sex, Gana (God/man/demon), humour, birds ruled, primordial elements, Gotra, directions ruled, consonants and vowels, mutual harmony and repulsion with certain stars, beneficence to the other stars by virtue of mutual distance etc.
In addition, a host of other factors such as longevity of the individuals, character (sexual), widowhood, poverty, progeny, body status, radical strengths and indications, planetary nature and afflictions, currency of major and minor periods, Marakaas (death inflicting planets), placement of Mars in their nativity (Mangal Dosha), time of query, omen (Shagun), antidotes, propitiations, auspicious time for marriage etc. should be examined to arrive at a suitable union of the couple.
Based on the birth constellations, the following aspects are examined Varna, Vashya, Tara, Yoni, Gana, Graha Maitri, Bhakoota, Nadi, Mahendra, Vedha, Rajju, Stree Deergha, Linga, Gotra, Pakshi, Chandra Yoga, Ayya Varga and Yujja for checking compatibility.
In another system, only eight of these factors are given importance and are assigned numeric values - Varna (1 point), Vashya (2 Points), Tara (3 points), Yoni (4 points), Graha Maitri (5 points), Gana (6 points), Bhakoota (7 points) and Nadi (8 points). The total of these factors adds up to 36 points and a horoscope is considered to be matched only if the compatibility score is more than 18.
Significance of various aspects in compatibility are as follows:
- Varna: Grade of spiritual development, obedience
- Vashya: Mutual control – friendship.
- Tara: Mutual beneficence-luck-auspiciousness
- Yoni: Sexual aspects, affinity, urge, compatibility.
- Graha Maitri: Psychological disposition, friendship.
- Gana: Nature: Dev – Manush – Raakshas
- Bhakoota: Children, growth of family.
- Nadi: Temperaments, hereditary (astrological), affliction and death to other
- Mahendra: Promotes attachments, increase in longevity.
- Stree – Dergha: Service to husband.
- Rajju: Felicity and duration of married life.
- Vedha – Varga: Mutual repellence and enmity.
- Varga: Mutual friendship/enmity judged from their names
- Yujja: Type of mutual love – from one side – both sides.
Based on the above aspects, mathematical computation is performed to find a match. In certain cases need based matching is done and the astrologer has to use his skills in examining compatibility. It is felt essential to match the horoscopes before further marriage negotiations are done. The above system is in practice for the last two millennia.
A statistical study was done on the predictive power of Indian astrology. The study found no statistically significant effects in the predictive quality among 51 astrologers.
The study was actually specific to the predictive power of Indian astrology in regards to intellectual and academic ability not physical/psychological/emotional compatibility. The birth details of 200 students, 100 high academic achieving students and 100 mentally disabled students, were collected. Natal charts were created for each student for the purposes of this study. The charts were then mixed and randomized. Sets of 40 randomly selected charts were sent to each of the 51 participating astrologers. They were asked to determine which category each of their 40 charts belonged in, high academic achievement or mental disability. Only 27 of the 51 participating astrologers sent results back in. The data was then calculated and analyzed to reveal that the "success rate [was] marginally less than that what would be achieved by tossing a coin" (Narlikar, 641).
See also 
For astrological compatibility in Chinese astrology, see:
- Theodora Lau. (1997). "Chinese Horoscopes Guide to Relationships". Publisher: Doubleday Publishing. ISBN 0-385-48640-5
- Suzanne White. (1998). Chinese Astrology: Plain and Simple. Publisher: Tuttle Publishing. ISBN 0-8048-3188-2
- Suzanne White. (1998) New Astrology: A Unique Synthesis of the World's Two Great Astrological Systems: The Chinese and Western
- Theodora Lau. (1999). Handbook of Chinese Horoscopes. Publisher: HarperCollins Publishers. ISBN 0-06-273731-7
For astrological compatibility in western astrology, see:
- The Diagram Group. (2004). "The Book of the Zodiac". Publisher: Sterling Publishing Co., Inc. ISBN 1-4027-1306-1
- Ronald Davison. (1983). "Synastry: Understanding Human Relations Through Astrology". Publisher: Aurora Press, Inc. ISBN 0-943358-05-1
- E. W. Neville. (1997). "Planets in Synastry: Astrological Patterns of Relationships". Publisher: Whitford Press, Inc. ISBN 0-924608-01-3
- Christine Rakela. (2004). "Love Relationship Formula: Predicting Romantic Success with Astrology ". Publisher: Llewellyn Publications, Inc. ISBN 0-7387-0424-5
For astrological compatibility in Indian Astrology
- Gayatri Devi Vasudev (2002). "The Art of Matching Charts". Publisher: Motilal Banarasidass. ISBN 81-208-1769-9
- H.Defouw - R.E.Svoboda(2000). "Light on Relationships: The Synastry of Indian Astrology". Publisher:
Weiser Books. ISBN 1-57863-148-3
- V K Shridhar (2002). " HINDU ELECTIONAL ASTROLOGY". Publisher: Suvaas Publishers, 25, Friends Colony, Yamuna Nagar, 135 001 Haryana India. ISBN :81-901413-0-9.