The Bhagavadgita describes that there are two eternal paths by which the dead transit to the next world after the soul leaves the body. The two paths are the white or the bright path (Shukla gati) and the black or the dark path (Krishna gati).
Lord Krishna explains this in the eighth chapter of the scripture.
By the Shukla gati, one will never return to the earth to take birth, and by the second Krishna gati, one keeps coming back again and again. The two paths are also known as the Northern path (uttarayana) and the Southern path (dakshinayana).
They are the obvious reference to the journey of the departed souls to the two higher worlds. These world are believed to exist in the sphere of the sun and the moon respectively. The world which exists in the sphere of the moon is the ancestral world and that which exists in the sphere of the sun is the immortal world of Brahman.
Agni and Sun worship
Vedic people were primarily sun worshippers. In Vedic tradition, the sun symbolizes Brahman as well as Agni, the fire god.
They worshipped sun as the highest God (Brahman) and the moon (Chandra or Soma) as one of the heavenly beings.
Although they worshipped numerous gods and goddesses and made them offerings, the ultimate recipient of those offerings was the sun god only. Agni is his dynamic aspect upon earth as well as in heaven. He radiates warmth and protects worshippers, driving away darkness and evil from their homes, hearts and minds.
Agni can also be extremely destructive and unleash great fury. Hence, Vedic people kept sacrificial fires continuously burning in their homes to keep darkness and evil forces away. If by any chance, the fires died down, they had to rekindle them ritually after performing penitentiary rites. In other words, the sacrificial fires were no ordinary fires which were used for domestic purposes. They were the sacred fires such as those which burned in the sun, the moon and the heavens, and into which the devas descended every day when the daily sacrifices were offered to them.
The first recipient of all offerings in a Vedic sacrifice is Agni (Sun). He distributes them among gods (devas) who are light beings and partake the nature of Sun (Brahman) just as the rays that emanate from him. Through them the offerings ultimately reach Brahman (Sun) only. This idea is reemphasized in the Bhagavadgita when Lord Krishna says that all paths eventually lead to him only. However, those who worship gods and others go to them before they reach him, while those who worship him reach him directly. It is why the first daily offerings or the morning prayers in Vedic tradition are customarily made to the sun before they are offered to others.
The sun and the moon symbolism
The sun completely drives away darkness upon earth and ushers in bright light. He is the great nourisher who keeps all life thriving and ensures the regularity of day and night, seasons and other celestial phenomena. If the sun symbolizes life, the darkness of night represents Death itself, who is described in the Vedas as the great devourer with insatiable appetite for the things of the world.
As long as darkness exists in our hearts and minds, death exists in our being and slowly devours us. Only the sun can keep the darkness of Death forever out of our existence. The moon also does it, but only partially. It cannot completely drive away the darkness of night as the Sun does. However, the moon is still important. He can shine light upon earth and provide a temporary and partial relief from darkness. He can help people find their way in darkness from one place to another or safeguard themselves from hidden dangers. It is not a complete solution to the problems of darkness and suffering (death and aging). However, it is much better than being in total darkness or in the sunless hells where no light can penetrate.
The Bhagavad Gita specifies two conditions for yogis to reach the immortal world of Brahman. They must die at the right time and in right conditions. They must be knowers of Brahman.
The right conditions are fire (when the sacrificial fires are burning in the home or when the body is cremated), daytime, the waxing period of the moon or the bright fortnight, and the six months in a year when the Sun is traveling in the northern hemisphere. They are auspicious signs. Those who die in such circumstances are most likely to travel to the immortal world, never to return again.
The dark path which leads to transmigration is also described. It refers to the fate of those who have not yet attained perfection or resolved their karma, and who are still caught in the cycle of births and deaths (samsara). The conditions which lead to rebirth are said to be smoke (mist and fog), night (darkness), the waning period of the moon or the dark fortnight, and the six months when the sun is traveling in the southern hemisphere.
The scripture describes many yogas, but puts the onus of attaining liberation upon two fundamental factors namely self-effort and divine intervention. The latter also depends upon the first only. God intervenes in your liberation only when you do your part in resolving your karma through various transformational yogas, and surrender to him completely and unconditionally in pure devotion.
Thus, whether you practice karma sanyasa yoga or jnanayoga or atmasamyama yoga or buddhi yoga or bhakti yoga, self-effort and selfless effort come first. To attain liberation, one must engage in actions without desires, cultivating detachment, devotion, purity etc. The scripture also speaks about the primordial propensities of Nature (gunas) namely sattva, rajas and tamas. It explains the specific qualities and behaviors they generate, and the need to cultivate the predominance of sattva, which lead to the preponderance of divine qualities and the righteous conduct, which are essential to attain liberation.