In Hindu Vigyan, Rahu Kaal is one of the 8 segments of the day and considered inauspicious period in Indian astrology due to its association with malefic, Rahu. The segments are calculated by taking the total time between sunrise and sunset, at a given place, and then dividing this time duration by 8. Astronomically in the Hindu Panchang Panchangas, the ever varying planetary configurations mean that the auspicious hours during each day will not always fall at the same moment of time. Rahu kaal (Raahukaala), Yamagandakaala (Yama Ghantam) and Vishaghati are the time periods which are especially considered inauspicious or unlucky (Ashubh).
The planets according to Indian astronomers are the Sun, Moon, Mars, Mercury, Venus, Jupiter, Saturn, Rahu and Ketu. Rahukaal (Raahukaala) and Gulikakaal (Gulikakaala) are calculated based on these planets. Even though Rahu and Ketu are not physical bodies, they are sensitive points on the intersection of the ecliptic (the apparent path of motion of the Sun) by the Moon's orbit. Rahu is the northern point and Ketu is the southern point. Ancient astronomers felt that the mighty Rahu and Ketu have the strength to obscure the sun, thus causing a solar eclipse. Hence it is considered inauspicious to begin any undertaking during this "Rahukaal". Every day this Rahukaal lasts for around 90 minutes, but the duration differs according to the length of the time between sunrise to sunset.
The timespan of Rahukaal varies each day and needs to be calculated as per the sunrise and sunset time.