In Vedic Astrology, Cancer is considered as the fourth sign among the twelve zodiac signs which are considered for making astrological calculations on the basis of Vedic Astrology. The last pada of Nakshatra Punarvasu, all four padas of Nakshatra Pushya and all four padas of Nakshatra Ashlesha are ruled by this zodiac sign called cancer. Cancer is a very important sign in a horoscope according to Vedic Astrology as it is involved in the formation of a couple of defects mentioned in Indian Astrology. The placement of the Moon in the fourth pada of Ashlesha Nakshatra forms a Gandmool Dosh in the horoscope and as the fourth pada of Ashlesha Nakshatra is ruled by Cancer sign, the study of this sign in a horoscope becomes important in Vedic Astrology. Apart from this, any serious afflictions to this sign by one or more than one negative planets in a horoscope form Matru Dosh which can pose many kinds of problems for the native depending upon the other important aspects of the horoscope. As Cancer is ruled by Moon, so any affliction to this sign makes the Moon afflicted and weak and thus a Matru Dosh is formed. Therefore the study of this sign in a horoscope becomes very important and it should be analyzed carefully in a horoscope.
Cancer is a watery sign and it is ruled by the Moon which is considered as the coolest of all the nine planets in Vedic Astrology and which is also considered as the fastest moving planet in Vedic Astrology. Therefore most of the planets placed in Cancer tend to show their positive or negative results in a less aggressive way compared to some other signs. Jupiter which is the planet that rules knowledge, spiritualism and prosperity, becomes exalted in this sign which means that out of all the twelve zodiac signs, Jupiter is considered strongest when placed in Cancer. A positive Jupiter well placed in Cancer can bless the native with great knowledge, spirituality and prosperity among many other things depending upon the other important aspects of the horoscope.
And whereas Jupiter is exalted in Cancer, Mars is considered debilitated when placed in Cancer which means that out of all the twelve zodiac signs, Mars is considered as the weakest when placed in Cancer. This is probably due to the reason that Mars is the signifier of energy and fire and the cool and calm nature of Cancer sign and its Lord Moon does not suit Mars and so it becomes the weakest when placed in Cancer. And for this reason, the natives who have Mars placed in Cancer in their horoscope are likely to suffer from the bad effects produced due to the weakness of Mars by virtue of its placement in Cancer. These bad effects can be different for different people depending upon the other important aspects of their horoscopes.
Cancer rules the Chest or Breast region and the organs in this region like lungs and heart etc. A strong Cancer sign and a strong and clean Moon in a horoscope blesses the native with good health and a very good working of the organs ruled by this sign whereas the weakness of this sign or serious afflictions to this sign by one or more negative planets in a horoscope can make the native suffer from ailments and diseases in the chest, breast, lungs, heart or other parts and organs ruled by Cancer. Serious afflictions to this sign can also make the native suffer from mental diseases and sleeping disorders as the Lord of this sign is directly rules the emotions of all the human beings. Therefore this sign should be studied carefully while analyzing a horoscope.