Kalachakra, “the wheel of time,” names the Sanatan Dharma understanding that history does not move in a straight line from primitive to advanced, but cycles endlessly through four ages, or Yugas, each shorter and more morally degraded than the one before: Satya Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga.
Satya Yuga, the age of truth, is the longest and purest — dharma stands on all four legs, and humans live in near-total harmony with truth, requiring almost no external law. Each subsequent Yuga strips away one leg of dharma: Treta Yuga introduces the first cracks in virtue, Dvapara Yuga sees truth and falsehood roughly balanced, and Kali Yuga, the current age, is defined by dharma standing on a single leg — conflict, ignorance, and spiritual forgetting are the norm rather than the exception.
This is not meant as pessimism but as orientation: knowing which Yuga one lives in shapes what kind of effort is appropriate. Tradition holds that at the darkest point of Kali Yuga, Vishnu descends once more as Kalki to end the cycle and restore Satya Yuga, beginning the great wheel again — a Mahayuga of four ages repeating without end across cosmic time.