The revolutions of our sun (and its solar system) around its dual periodically bring it closest and farthest from the Vishnu-nabhi. Due to its motion around the dual, the equinoxes have a retrogade motion with respect to the constellations -- moving out of a constellation pair in every 2000 years. So the Autumnal Equinox that is currently falling in Virgo (and its opposite Vernal Equinox is in Pisces) will lead to Leo-Aquarious constellations for the equinoxes next, followed by Cancer-Capricorn, and so on.
The Sun is closest to the Vishnu-nabhi when the Autumnal Equinox comes to the first point of Aries. An event that last occured in 11,501 BC.
One (ancient) explanation is that the Sun is a part of a binary star
system and revolves around its dual with a period of 24,000 years.
However, this explanation is not validated by modern astronomy. The nearest
known star Proxima Centauri is 4.2 light years away. (Alpha Centauri A, B
are at 4.3 light years-- Alpha and Proxima centauri are part of a three-star
Though we have not detected any luminous body nearer than Alpha/Proxima
Centauri circling the Sun, it does not rule out another celestial body
(planet, dead star etc -- not necessarily a star) that can act as a dual
of the Sun.
Note that the Sun itself orbits around the galactic center at a radius of
about 8,500 parsecs and takes over 200 million years to make one full orbit.
While the dual star explanation may not be validated, there is, however,
Milankovitch Cycle of 25,800 years that is related to the precession
of earth's axis. This precession leads to change in the backdrop of
constellations against which Sun's motion is played out (and thus the
equinoxes as mentioned here). Then the natural question is: what is so
special about Autumnal Equinox falling on Aries?
chatvaaryahu sahsrani varshanaantu kritam yugamFour thousands of years, they say, is the Krita Yuga (Satya Yuga). Its morning twilight has just as many hundredes, and its period of evening dusk is of the same length (i.e., 400+4000+400=4800). In the other three ages, with their morning and evening twilights, the thousands and the hundreds decrease by one (i.e., 300+3000+300=3600; etc.). That fourfold cycle comprising of 12,000 years is called an Age of the Gods. The sum of a thousand divine ages constitutes one day of Brahma; and of the same length is its night.
tasya tavchhati sandhya sandhyanshrshch tathavidhah
itreshu sasandhyeshu sasandhyaansheshu ch trishu
ekapayen vartante sahsrani shatani ch
yadetat parisankhyaatmadadev chaturyugam
aetad dwadash-sahstram devaanaam yugmuchyate
daivikanaam yuggnaantu sahastram prisankhyayaa
braahmaikamahgyeyam taatati raatriraiv ch
When the Sun is closest to the Vishnu-nabhi, the influence of Brahma on Dharma is greatest, and Dharma (or mental virtue) is most advanced (and mankind is thought to understand the subtlest details of material and spiritual worlds). On the other hand, when the Sun is farthest from Vishnu-nabhi, Dharma is diminished to the point that mankind can not grasp anything beyond the gross material creation. The interval of 12,000 years from the farthest point to the nearest point marks the period of growth in Dharma (with subsequent effects on intellectual, material and spiritual growths) for the mankind.
From 11,501 BC the Sun began to move away from its position nearest to the galactic center. In the following 12,000 years the mankind gradually lost its understanding of the spiritual and physical worlds. The period around AD 500 was the darkest period in human intellect. After this period, the Sun began to advance towards the galactic center. The next 1200 years (100+1000+100) defined the first period of Kali Yuga -- marked by human understanding at the most gross level. The last 100 years (from AD 1599) marked the transition out of Kali Yuga to Dwapar (which began in AD 1699) brought about beginning of realization of Sukshmabhuta or the fine constituents of the physical world.
The transition into Dwapar (from 1699 to 1899) brought about understanding of electricity, magnetism and their role in material creation. We are currently into the first 100 years of 2000 years of Dwapar, followed by Treta Yuga (that begins in AD 4099). Transition to Treta will be marked by an understanding of the source of all electro-magnetic forces and their influence on creation (referred to as "divine magnetism"). This is followed by Satya Yuga (in AD 7699) when all matters of creation and spirit are within the grasp of human intellect. And the cycle repeats bringing us once again 24,000 year from BC 11,501.