Astrology is the study of all the living beings on earth, in relation to the heavenly bodies, the constellations, Rasi and the planets of the Solar system. Man relates his activities on the Earth to the movements of the heavenly bodies from time immemorial.
The moon transits throuh a specific nakshatra each day. When the Moon passes through different 27 lunar mansions, specific activities can bear more productive fruit. While thinking about auspicious times, naturally the movements of Moon comes to fore. Inauspiciousness or auspiciousness depends upon the decreasing or increasing phases of the Moon. Amavasi (30 Tithiya) and Tithis 1,2,3,4,5 in the bright half (Sukhla Paksha) are considered inauspicious, due to the weakening of the Moon. The ensuing Tithis of 6, 7, 8, 9, 10 witness increase in the strength of the Moon to a certain degree and so these are considered medium in auspiciousness. The next ensuing Tithis of 11, 12, 13, 14, 15 (Full Moon) show the Moon with all its Nitya Devis (Kalas) and so these Tithis are auspicious.
During the dark half (Krishna Paksha), in the Tithis of 1, 2, ,3, 4 and 5, Moon starts its weakening course when the Nitya Devis start to leave the Moon and goes to the Sun one by one, but still remains with a number of Nitya Devis; so these Tithis are fully auspicious. In the ensuing Tithis of 6, 7, 8, 9 and 10, Moons strength gradually decreases and so these are medium in auspicious. The last Tithis (11, 12, 13, 14, 15 New Moon) show Moon very weak and so these are inauspicious. After Amavasai (New Moon) the Nitya Devis return one by one on each Tithi and the moon starts shining again till its full on the Pournami (Full Moon) when the last Nitya Devi returns to it.