Everyone is a moon, and has a dark side which he never shows to anybody — Mark Twain
What does Occultation mean?
An occultation, in astronomy, is a phenomenon when one celestial object passes through another blocking the light emitted from the former. A Lunar Occultation, in the same sense, occurs when the moon passes through another celestial object, say a star, for a brief period of time, blocking the light emitted by the star.
How did the ancient Hindus explain this phenomenon?
Chandradev (The Moon) was born to Maharishi Atri and Anasuya. He was married to the 27 daughters of Daksha, the son of Lord Brahma. Chandradev made a promise to Daksha that all his 27 daughters will be treated equally with respect. But, failing to keep up with his promise, Chandradev favored his fourth wife, Rohini, more than the others.
The other 26 wives, not happy with his actions, expressed their concerns to their father, Daksha. Furious upon the complaint received from his daughters, Daksha imprecated a curse on Chandradev that he would slowly perish and die. Out of fear, Chandradev landed up at Prabhas Patan, in Gujarat, and set up a Shivalinga, popularly known as Somnath Jyotirlinga, in hopes that the curse can be reversed with the help of Lord Shiva. But as no curse can be completely reversed, Lord Shiva was helpless. But he did agree that he will help Chandradev.
Instead of having him die, Lord Shiva arranged for a less fatal punishment. He said, starting from that particular day, the moon will go through different stages of increase and decrease in brightness (waxing moon and waning moon) while passing through all the 27 stars. The increase in brightness will last for approximately 14 days (Shukla Paksha), and the decrease in brightness will last for another 15 days (Krishna Paksha). This way, Chandradev will devote his time equally to all 27 of his wives. Lord Shiva then told Chandradev that he will forever hold the crescent moon on his head (Ardhachandra) as a constant reminder to Chandrdev of Daksha’s curse.
The ancient Hindus embedded complex yet simple astronomical information in this story. They noticed that the moon rose in the east at a different time with a different alignment of stars every day. So, they divided the spherical earth, 360 degrees, into 27 different sections. Given the cornucopia of stars visible in the night sky, they picked 27 of the brightest stars and named each of them Ashvini, Bharani, Krittika, Rohini, Mrigashirsha, Ardra, Punarvasu, Pushya, Ashlesha, Magha, Purva Phalguni, Uttar Phalguni, Hasta, Chitra, Swati, Vishakha, Anuradha, Jyeshtha, Mula, Purva Ashadha, Uttar Ashadha, Sravana, Dhanishta, Shatabhisha, Purva Bhadrapada, Uttar Bhadrapada, and Revati. This is called the Lunar Mansion. This way, they were able to identify which phase the moon was currently in and determine the time of the month. These stars are called the principal stars.
What is an ecliptic?
An ecliptic is an imaginary line that indicates the path of the Sun around the Earth in one year. A Solar Eclipse happens when the moon momentarily passes through this ecliptic (between the Sun and the Earth). The moon passes through this ecliptic twice a month. But, an eclipse will occur only when this passage happens during a full moon for Lunar Eclipse and a new moon for Solar Eclipse.
But, why did Chandradev favor Rohini out of all the wives?
So, when the moon goes through its different phases, i.e. pass the 27 stars, it either comes close to the particular star, or it passes through it. This “passing through” is what Lunar Occultation is. So, the ancient Hindus observed that the moon occults with some principal stars, but not others. They also observed periodicity of approximately 19 years, also called the Metonic Cycle.
When a star is located between 4 — 6 degrees from the ecliptic, it experiences a cluster of occultations within a four-year period in the Metonic cycle. The star Rohini, or popularly known by its Arabic name, Aldebaran is located at 5 degrees from the ecliptic. This means that Rohini will experience a cluster of occultations in the 19-year cycle, making it the star (out of the 27 stars) that occults the most with the moon. This is why, the story, narrated by our ancestors, was framed to portray Chandradev favoring Rohini over the other wives.
This complex astronomical occurrence is hard to pass down through generations, and hence, our ancestors put it in the form of simple stories that will help retain such theories for thousands of years.