Urvasi & Pururavas – How the Ancients Used Universal language of love to explain Lunar Nodal Precession!

 

The love story of Urvashi & Pururavas explains a phenomenon called Lunar Nodal Precession. This is a teeter-totter effect that relates to how high or low the moon appears in our sky. The moon’s orbit is tilted at 5.14 ° to the ecliptic plane. The moon wobbles as if it is on a teeter-totter by going above the ecliptic and below the ecliptic. This movement is called lunar nodal precession. It takes 19 years (18.61) to complete one lunar nodal precessional cycle.  

The nodal precession of the moon is subdivided into two categories.

a.       Major Lunar Standstill

b.       Minor Lunar Standstill

Major Lunar Standstill: The Moon's orbital angle of tilt adds to the Earth's axial tilt to reach a maximum declination of ~28.5° in terms of ecliptic latitude; the Moon reaches its maximum of 5.14° north of the ecliptic.

Minor Lunar Standstill:

The Moon's orbital tilt subtracts from the Earth's axial tilt to reach a maximum declination of ~18.2° in terms of ecliptic latitude; the Moon reaches its maximum of 5.14° south of the ecliptic.

Occultation:

An occultation is a phenomenon in astronomy when one celestial body passes in front of another and hides the distant object from an observer’s view.

Even though the moon travels across the zodiac, the moon does not exactly pass in front of a star. It either passes above or below a star because of its tilt. For the moon to occult a star, that star must be within 5 ° of the ecliptic. Of the 27 stars in the zodiac, only Aldebaran (Rohini) is within this range, as it is approximately 5.5 ° south of the ecliptic.

We compared the moon’s tilt moving above or below the ecliptic to a teeter-totter. This is not the ordinary teeter-totter we see in playgrounds, but this celestial teeter-totter is also slowly rotating circularly.

Now, imagine that the ecliptic plane is the floor. The moon on the teeter-totter goes towards the ceiling from the floor, or it can go below towards the floor. The teeter-totter; the moon’s orbital plane is going up and down while also rotating.  The teeter-totter is never flat along the ecliptic; it is always tilted at an angle of 5.14 °.

Imagine yourself standing in this room on a wedge that is inclined to the floor at an angle of 23.4 °. The slope of the wedge is the celestial equator. This is a unique wedge with half of it above the floor and half below the floor, such that its highest point on the wall is +23.4° and its lowest is -23.4 ° below the floor. The lunar teeter-totter is suspended above the wedge you are standing on in such a way that the fulcrum of the teeter-totter aligns with the centre of the wedge.  The spinning teeter-totter’s high and low ends do not point at the same wall. They move in a circle while the teeter-totter is going up and down. The lunar spinning teeter-totter takes 18.61 years to complete one rotation.

 

During a minor lunar standstill, when the teeter-totter board is below the floor of the ecliptic, the moon's ascending node is located near the autumnal equinox, making it feasible for the low end of the teeter-totter to point at the star Aldebaran (Rohini). Even though the moon is at 5.14 °, it does not perfectly align with Aldebaran, located at 5.5 ° south of the ecliptic, but because of parallax error, to an observer on Earth, the moon appears to occult Aldebaran.

Every 18.6 years, when the moon is in a minor lunar standstill, for a period of 3 to 4 years, the moon's ascending node is located near the autumnal equinox, and the moon frequently occults Aldebaran (Rohini). During this period, the moon occults Aldebaran once every sidereal month (roughly every 27.3 days) for a period of about 49 consecutive months. This is the reason why Rohini is the favourite wife of the Moon out of the 27 stars, because the Moon does not frequently pass in front of the other 26 stars.  Also, Rohini is at the perfect location, around 5.5 degrees South of the ecliptic for the moon to occult her.

These 3 to 4 years, when the moon frequently passes in front of Aldebaran (Rohini), are the 4 autumns spent by Urvashi with Pururavas. As seen earlier, occultations of Aldebaran happen specifically when the moon's ascending node is located near the autumnal equinox (the point where the Sun crosses the equator in September). This is the four autumns. The moment the moon’s tilt changes, Rohini no longer appears in front of him.

As it is the moon who is on the spinning teeter-totter, it is he who must travel across the vast celestial field to be with Aldebaran (Rohini) again. It is Pururavas, the moon, who must seek Urvasi (Rohini) in the celestial plane by coming to her plane.