Astronomy

The layout of Harappan towns and cities provides evidence that the Harappans had a good knowledge of astronomy. The orientation of the main streets in cities and towns followed the cardinal directions. This may not be sufficient evidence of astronomy since a simple method of establishing an east-west line known from a later Indian text may have been employed in Harappan times. (A circle was drawn around a gnomon post, with its radius equal to the height of the gnomon. Markers were placed at the points where the gnomon's shadow touched the circle when exiting and reentering it; a line joining them would run exactly east-west.) However, Holger Wanzke's study (1987) of the orientation of Mohenjo-Daro's streets demonstrated that they deviated from the north-south line by 1-2 degrees. A slight divergence was also observed at other Harappan sites. Wanzke therefore proposed that the Harappans were establishing the cardinal directions by sighting on the stars, in the case of Mohenjo-daro using the profile of the Kirthar Mountains to the city's west as the horizon against which to record their movements. A star whose setting in the west would have been clearly visible to the inhabitants of Mohenjo-daro was Aldebaran (Rohini); its setting point, slightly north of cardinal west, exactly matched the orientation of the city's streets.

A star calendar based on an intimate knowledge of the movements of the heavens is recorded in later Indian literature. The relative position of the aster-isms that compose this nakshatra calendar most closely match the arrangement of the heavens that was visible around the twenty-fourth century BCE, during the Harappan period, demonstrating that the calendar was devised by the Harappans. At this date, the North Star was not Polaris but Thubron (Alpha Draconis). The nakshatra calendar was composed originally of twenty-four as-terisms, later increased to twenty-seven and then twenty-eight, selected from the fixed stars and constellations that appeared in the night sky during the course of one year. The sidereal year (the time taken for the stars to return to

A series of graduated cubical weights from Harappa. The smallest weighs 0.865 grams. At this end of the weight scale, each unit is double the weight of the one before. (J. M. Kenoyer, Courtesy Department of Archaeology and Museums, Government of Pakistan)

Their starting point) and the solar year (the time taken for the earth to complete a revolution around the sun) differ in length by only 20 minutes, a discrepancy that had a minor cumulative effect on the calendar. Observation of the fixed stars therefore enabled the Harappans or their predecessors to establish the duration of a solar year and the timing of particular points within it, vital information for agriculture. During the Mature Harappan period, the beginning of the year was fixed by the heliacal rising of the Pleiades at the spring equinox: These events coincided exactly in 2240 BCE, and the Pleiades remained the constellation that rose closest to the equinox throughout the period 2720-1760 BCE. There is some evidence that originally the star used to mark the start of the year was Aldebaran, which rose heliacally at the spring equinox in 3054 BCE; as the position of the earth with respect to the fixed stars gradually changed, Aldebaran became unsatisfactory as the marker of the vernal equinox and was therefore replaced by the Harappans with the Pleiades sometime during the midthird millennium. Ashfaque (1989) suggests that the myth of the incestuous seduction of Rohini (Aldebaran) by her father Prajapati (Orion) reflects the observed rapprochement between these asterisms over time and the rejection of Aldebaran in favor of the Pleiades as the marker of the vernal equinox.

The Indian calendar, according to later texts, also made use of both the solar year, calculated at 366 days (the true length being around 365.25 days), and the lunar year of 354 days (composed of twelve cycles of the moon, each of around 29.5 days). The solar year was divided into twelve months each of thirty days, totalling 360 days, and the lunar year into twelve months alternately of twenty-nine and of thirty days. To bring these into line, both with the true length of the solar year and with each other, the calendar was reckoned over a five-year period, during which one intercalary solar month and two intercalary lunar months of thirty days were added.

In addition to the asterisms of the nakshatra calendar, Indian astronomers through the ages have shown particular interest in the planets, whose deities are considered to have a powerful and often malign influence. These numbered nine: the five planets visible with the naked eye (Mercury through Saturn), with the addition of the sun, the moon, the eclipse demon Rahu, and Ketu representing a comet or meteor. With the sun as the center, the planets also represent the cardinal directions and those between them (such as northwest). Worship of the planets is common in south India, particularly in conjunction with the worship of Shiva. Though the evidence is not conclusive, it is likely that all these astronomical aspects of Indian religion date back to Harappan times. Allchin and Agueros (1998) discuss the possible depiction of a comet on two seals, along with a man with a bow and in one case a stylized tree. Its identification as a comet is convincing, reflecting the Harappan interest in the heavens; it may, as Allchin and Agueros suggest, have been the comet Hale-Bopp, which was visible around 2000 BCE.