The duration of one day in the life of Brahma is calculated in solar years as being 4,300,000 x 1,000 years, or 4,300,000,000 years. Brahma's one day consists of one thousand cycles of the four yugas: Satya, Treta, Dvapara and Kali. The four yugas constitute one millennium.
In one day of Brahma there are fourteen manvantaras. Each Manu lives for seventy-two yugas, each consisting of some 4,320,000 years. The life-span of one Manu is called a manvantara.
- Fourteen Manus (1000 maha-yugas) pass during Brahma's day
- There are 420 Manus in one month of Brahma
- There are 5,040 Manus in one year of Brahma
- Brahma lives for one hundred years of his age
- There are 504,000 Manus in the duration of Brahma's life
This is the calculation for one universe, and there are innumerable universes. All these Manus come and go simply by the breathing process of Maha-Visnu. Since there are innumerable universes, no one can imagine the totality of the manvantara incarnations. Because all the universes are produced simultaneously by the exhalation of the Maha-Visnu, no one can begin to calculate how many Manus are manifest at one time. Each Manu, however, is called by a different name.
After the end of Brahma's night, the cycle of creation begins again in the daytime of Brahma, and they continue to exist through the life durations of fourteen consecutive Manus. At the end of each Manu there is devastation of the earthly planets, and the vast water is fearful even to Brahma. And, at the end of the life of each Manu there are shorter dissolutions.
The manvantara incarnation chastises all the miscreant rulers of different planets and disseminates the transcendental glories of the Lord. Manu creates scriptural regulations for the salvation of the conditioned souls, who come to the material world for material enjoyment. The Lord is so kind that any soul who wants to enjoy in this material world is given full facility for enjoyment, and at the same time he is shown the path of salvation. The Manus compiled the Manu-samhita. The word samhita means Vedic knowledge, and manu indicates that this knowledge is given by Manu.
From the beginning of Brahma's day of 4,320,000,000 years, six Manus appear and disappear before Lord Krsna appears. Thus 1,975,320,000 years of the day of Brahma elapse before the appearance of Lord Krsna. This is an astronomical calculation according to solar years.
Srila Bhaktisiddhanta Sarasvati Thakura, in his Anubhasya, gives a list of the Manus and their fathers' names. The calculations are given in the authentic astronomy book known as the Surya-siddhanta. This book was compiled by the great professor of astronomy and mathematics Bimal Prasad Datta, later known as Bhaktisiddhanta Sarasvati Gosvami, who was our merciful spiritual master. He was honored with the title Siddhanta Sarasvati for writing a commentary on the Surya-siddhanta, and the title Gosvami Maharaja was added when he accepted sannyasa, the renounced order of life.
[Note: It appears that the number of yugas is sometimes put at 71, and sometimes at 72. Because the number of yugas is divisible by 4, we have consistently used the number 72. For a more exacting description, please see Srila Prabhupada's original text and Purports.]
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.