No one has yet posted a solution to the riddle of the eighteen million years age of physical humanity versus the one hundred and twenty million years age of our current humanity. There seems to be no obvious explanation of why the Tirukkanda Panchanga gave the eighteen million years age for the current Vaivasvata manvantara, quoted in full agreement in The Secret Doctrine (vol. 2, pp. 68-69), while according to all known sources the age of the Vaivasvata manvantara should be one hundred and twenty million years. The answer to this may have been given by H. P. Blavatsky in a fragmentary article published posthumously as “On Cosmic Cycles, Manvantaras, and Rounds” (Blavatsky Collected Writings, vol. 13, pp. 301-306), if anyone who is good at numbers can figure it out.
In this article, HPB writes (p. 302): “The astrological work states that: — 3. ‘The number of years that elapsed since the beginning of Vaivasvata Manvantara — Equals 18,618,725 years.’” This very same statement is made in The Secret Doctrine (vol. 2, pp. 68-69), where it is taken from the Tirukkanda Panchanga (the one for the kali yuga year 4986; see SD vol. 2, p. 51). So obviously “the astrological work” referred to in HPB’s fragmentary article is the Tirukkanda Panchanga. The calculations that she gives in this article may therefore pertain to how the eighteen million years figure is obtained. We should here recall that, for HPB, this figure is the age of humanity “on our planet D, in the present Round” (p. 302). In the Tirukkanda Panchanga, this figure is apparently given for the age of the Vaivasvata manvantara as a whole, an age that from other sources should be one hundred and twenty million years. I am hoping that someone who is good at numbers can figure this out.
In a small book that Daniel Caldwell called my attention to, Occult Chronology, by James Arther (Adyar: Theosophical Publishing House, 1943; http://blavatskyarchives.com/theosophypdfs/early_theosophical_publications_chrono.htm; http://hpb.narod.ru/Occult_Chronology.htm), the author refers to the Tirukkanda Panchanga and says (near the end of the book, no page number): “The Adyar Library is not in possession of a copy, and I have not yet been able to secure one. If anybody is the fortunate possessor of, or can lay his hand on, a copy, it would be greatly appreciated if he would make the Adyar Library the happy recipient of it.” This may have happened. I contacted the Adyar Library and learned from Prof. C. A. Shinde that although the Adyar library does not have the particular issue that HPB used, for kali yuga 4986 or 1884-1885 C.E., it does have several previous years of the Tirukkanda Panchanga, 1870-1881. These should all give the same epochs that the 1884-1885 issue gives. I have requested material from these. When it arrives, it may (or may not) be possible to determine how the Tirukkanda Panchanga arrived at its eighteen million years age of the Vaivasvata manvantara.
The ratio and calculations given by HPB in the fragmentary article referred to above are highly unusual, and do not fit in with other things she gave. Nonetheless, no one has yet been able to figure out how the eighteen million year figure was derived by the normal calculations, using the ratio 1:2:3:4. So perhaps further efforts using the ratio 1:2:3:4:5:6:7 will solve this riddle. Right now, the eighteen million years age of humanity is based only on an occult claim, apparently supported by reference to a once used calendar/almanac, the Tirukkanda Panchanga. But unless we can determine how the Tirukkanda Panchanga arrived at this figure, its evidence is little more than one more unverifiable claim. I have not found any other reference in any other Hindu text supporting this age for the Vaivasvata manvantara.
Nicholas, is this the SD passage suggesting the sevenfold ratio that you were thinking of? It is from vol. 1, p. 36:
“The Seven Eternities meant are the seven periods, or a period answering in its duration to the seven periods, of a Manvantara, and extending throughout a Maha-Kalpa or the “Great Age” — 100 years of Brahma — making a total of 311,040,000,000,000 of years; each year of Brahma being composed of 360 “days,” and of the same number of “nights” of Brahma (reckoning by the Chandrayana or lunar year); and a “Day of Brahma” consisting of 4,320,000,000 of mortal years. These “Eternities” belong to the most secret calculations, in which, in order to arrive at the true total, every figure must be 7x (7 to the power of x); x varying according to the nature of the cycle in the subjective or real world; and every figure or number relating to, or representing all the different cycles from the greatest to the smallest — in the objective or unreal world — must necessarily be multiples of seven. The key to this cannot be given, for herein lies the mystery of esoteric calculations, and for the purposes of ordinary calculation it has no sense.”
Perhaps if we recall that between the root-races are transition periods equal in length to that of each race, we may approach 18 million. But one has to count from the 4th sub-race (or was it 3rd?) of 3rd RR and stop at the 5th sub-race of 5th RR. Have fun!
There is an SD passage that suggests the sevenfold ratio, if I can find it.