(This is a research post and written in a divine mood so read it carefully )
It is mentioned in Hindu mythology that Brahma created this Universe and it took 27 years in creation. In this 27 years He also created 14 Lokas, 27 Nakshatras and 12 Signs.
Then Brahma entrusted the work to Lord Vishnu to activate the Universe . Vishnu created Sun from his own divine light to give life to the Universe . So Sun is the incarnation of Vishnu and Moon is the incarnation of Goddess Mahalaxmi, the Energy.
Sun expanded himself and created Mars Mercury Jupiter Venus and Saturn. These 14 Lokas also were distributed amongst 7 planets, 2 to each. Then Sun gave life, the PRAN to the Universe.
Mahalaxmi created the Maya to run the Universe. This Maya was entrusted to Rahu later on and liberation from this Maya was entrusted to Ketu. Rahu became in charge of 7 Lower Lokas whereas Ketu in charge of 7 Upper Lokas.
Day was in the control of Sun but darkness or night was entrusted to Rahu Ketu. Rahu took the charge of first half zone from Sun set to midnight and his nature was to create desires, ignorance , craze, lust , debauchery and intoxication during this time. Rahu takes one very far from his own soul.
Ketu took the charge of second zone starting from midnight to sunrise. Ketu’s nature is to bring one back to his soul, his own consciousness and make him introvert so Ketu inspires one to meditation in this time. Sunset awakens Rahu, the darkness. Sunrise awakens Ketu, the awareness.
The moment this Universe was live and active, Moon started his journey from Kritika Nakshatra and this way started dasha system from Sun. In Sarvatobhadra Chakra also Kritika is taken first.
The Natural Zodiac starts from Aries and Ashwini Nakshatra but first 27 years ( Ketu 7 plus Venus 20 ) were passed in creation so first dasha in universe was of Sun.
This way from the day of activation of the Universe, Sun dasha was in operation and souls started to enjoy Sun dasha first. Since the longevity of the different bodies on Earth was distributed different so all are not going through same dasha but this dasha is continuous journey of the soul from the day one of his Existance and continues till Liberation .
With the death of a person dasha is not over but it continues even after death because # Dasha is attached to soul and not the body #.
Whatever Karmas a person do on this earth in a particular dasha antardasha, the result he will get in the same dasha antardasha in his future births.
If I am suffering in Rahu Mercury dasha in this birth means I had done bad karma in Rahu Mercury dasha in my previous births. May be previous or pre previous.
Yes the birth chart is closed on the day of death so result of dasha after death will not be as per his birth chart but only as per his karmas done during that dasha on earth.
When a child is born in a particular Nakshatra we say that the child has enjoyed certain years of this dasha before birth in astral world including 9 months in fetus and now rest period he will enjoy after his birth.
This is how dasha system is working continuous for a soul irrelevant to his birth and death Chanel .
Where the soul goes after death is a complex question but from the dasha at the time of death we can get some clue or hints for his next journey but I am doing research on it.