The truth is that there were not 27 but 28 nakshatras in the ancient Indian Jyotisha system. The Nakshatra system that originated in India really long ago, divides the visible constellations in the sky into 27 segments or nakshatras with each constellation named after a prominent star in that constellation. The moon’s journey through each of these nakshatras takes about 27.3 days, when the moon also completes a revolution around the earth.
But the number of nakshatras was not always 27. There were initially 28 which included a star called Abhijit placed between the Uttara ashaada and the Sravana nakshatras. According to the Mahabharata, Yudhishtra, the eldest of the Pandavas, was born under this star. This star subsequently lost its individual identity and the number of nakshatras were reduced to 27.
But why did this happen? Why did the number of Nakshatras become 27?
Scholars believe that this may have happened when Indian astronomy that followed a lunar calendar system encountered the solar calendar system in the early centuries of the Christian era. The tropical or solar calendar system, as we have seen, based its time-keeping system on the sun’s movement across the sky . At some point in time, these two systems – lunar and solar – got integrated and we ended up with a luni-solar system.
However, this integration created certain mathematical problems. We know that the lunar system divided the constellations in the sky into 28 arcs whereas the solar system divided them into 12 segments called Rashis. An integration of the two systems meant that the 28 nakshatras had to be accommodated across the 12 Rashis. But 12 could not divide 28 evenly. However, 27 could be divided neatly by 12 without leaving a messy fraction.
So the number of nakshatras was brought down to 27 and each of the 27 nakshatras was divided into 4 quadrants called paadas. So, we had 108 paadas for the 27 nakshatras. Now, when the paadas of these nakshatras were divided across 12 rashis, we ended up with 9 paadas spanning 3 nakshatras fitting neatly into each of the 12 rashis.
For example, the Mesha rashi has all the four paadas of Ashwini and Bharani and the first paada of Krithika. The following Rishabha rashi has the remaining three quarters of the Krithika nakshatra, all the four paadas of Rohini and two paadas of the next nakshatra, Mrigashirisha. This fitment goes on till all the 108 paadas of the 27 nakshatras are neatly fitted into the 12 rashis.
Moreover, 27 nakshatras also meant that each of the nine grahas or planets could be assigned 3 nakshatras each, which would not be possible with 28.
Similarly, 360 degrees, which is the measure of the the sun and the moon’s orbit around the earth, can be divided by 27 evenly into 13 degrees and 20 minutes per nakshatra (60 minutes make a degree). Whereas, dividing 360 by 28 leaves a messy fraction.
So, essentially, it was to align these lunar and solar calendar systems that the number of nakshatras was reduced from 28 to 27. So, we find that understanding astronomy and astrology calls for a deep understanding of numbers and their nature too!