The period of Rahu or Rahu Kaal, as more popularly known in the Indian Vedic calendar is a specific period of Rahu which occurs on a daily basis and is considered to be inauspicious for undertaking any particular task which may yield good results or to start a new venture.
There are 8 segments or Muhurats in which a day is divided into and these are deliberated by taking the complete time in between a sunrise and sunset of a specific place and then divided by 8. These deliberations in the Hindu Panchang are what signify auspicious and inauspicious times of a day to perform several tasks. Rahu Kaal, contemplated as inauspicious is variable and changes every day as planetary motions vary every passing second. And so, calculation of Rahu Kaal for each day is important and depending on the calculations, particular tasks should be performed.
Planets in the Hindu Vedic astrology are Sun, Moon, Mars, Mercury, Venus, Jupiter, Saturn, Rahu and Ketu. Although, Rahu and Ketu are not physical bodies, they are considered to be one of the 8 segments of the day in the form of sensitive points of the lunar nodes on the convergence of the ecliptic i.e., the path of the Sun. Both of these are considered inauspicious in Indian astrology and Rahu Kaal is denoted as the most unpropitious duration of the day.
Ancient astrology suggests that Rahu is a northern node and Ketu is a southern node, and these two together have the strength of concealing the sun and taking away all the light cast upon the universe. Due to this, astronomers labelled these two bodies as inauspicious or of adverse nature. Till date in the Hindu Panchang Rahu Kaal is given a special calculation and natives are suggested to avoid performing any tasks while Rahu Kaal lasts. Rahu kaal lasts for a total approximation of 90 minutes, each day, depending on the time of sunrise and sunset.
The calculation of Rahukaal is done entirely on the basis of sunrise and sunset and thus it varies each day. The above mentioned time slots are true for a day with 12 hours of sunlight, when the sun rises at 6.00 am. The time and duration will all change for different situations.