The Tzolkin is a 260-day calendar based around the period of human gestation. It is composed of 20 day-signs, each of which has 13 variations, and was (and still is) used to determine character traits and time harmonics, in a similar way to Western astrology. The Maya also used a 365-day calendar called the Haab, and a Venus calendar, plus others. They measured long time periods by means of a Long Count, in which one 360-day year (a "Tun"), consists of 18 x 20-day "months" ("Uinals"). Twenty of these Tuns is a Katun; 20 Katuns is a Baktun (nearly 400 years); and 13 Baktuns adds up to a "Great Cycle" of 1,872,000 days, (5200 Tuns, or about 5125 years). Jose Arguelles has pointed out that the Tzolkin is a harmonic of the Great Cycle, and can be used to map history, as if it is measuring not individual gestation but species gestation, since 5 Great Cycles add to exactly 26,000 Tuns; the "Grand Year" or precession of the equinoxes - a higher harmonic of 260.