During the Vedic age itself, which is like 5500 years ago, we knew how to calculate time using light. We call it Siddhanta Jyotishya. Our Rg veda, Yajur veda, Samaveda, Atharvana Veda all have identified and documented a way to calculate time. But to explain the time system of ancient India, as a rabbit was once told by His Majesty in the Alice in the wonderland, let me begin at the beginning, go till the end and then stop. :)
Let me take you back to the Vedic era. In Bharatavarsha, we knew that at each place, the light, or Sun, moves at a pace of 6 amshas per vighati. With 60 vighati's making a Ghati and 60 ghati's making a tithi - which would be the exact time that Moon and Sun would differ by 12 degrees. As per the calculations we can understand today, if we reverse derive, 6 amshas/longitudinal degrees is traversed in 1 vighati or 24 minutes. That is 6 degree apparent movement of Sun takes 24 minutes or 1 degree movement takes 4 minutes.
That means if you are looking at the longitudes, 2 places on the same longitude (say New Delhi and Kanyakumari) will have exact same time, but two places 1 degree/amsha apart from each other longitudinally, will have a time difference of 4 minutes. Bharatavarsha is wide - Latitudinal extent: 8deg4min N to 37deg6min N and Longitudinal extent: 68deg7min E to 97deg25min E. The farthest point of India's west differs by 29deg 19mins from the farthest point in the India's east longitudinally. That is about 117.27 minutes of the clock which is nearly 2 hrs. So longitudinally farthest east coast will be 2 hours ahead of farthest west coast of India. This is true even today. This time is called local meridian time of a particular place. Our ancestors identified 12 places in India at different longitudes each apart by 10 minutes in meridian time to cover 2 hours or 120minutes from east to west all longitudes and named them as jyotirlingas or the light-indicators. These 12 places (dwadasha jyotirlinga) give the nearest time to the local meridian time from where every panchanga preparing guy would know how many ghatis he has to add/subtract to arrive at the exact local meridian time mathematically regardless of any tools like hourglass or so.
As an example, when a kid is born, the jataka is cast based on the the local meridian time of the place he is born. The calculation of this local meridian time is dependant on the longitude of the place he is born. The local meridian time is calculated as per the ghati-vighati after the sunrise of the day. So it is dependant on the exact moment of sunrise which is taken from the panchanga and adding/subtracting the local meridian correction to it. You can see all such location specific correction amount also listed out for major places in the panchangas of these days. So to know the exact birth time, we depend on the place (there by the correction factor) and the sunrise time listed in the panchanga. The sun rise time is listed after applying the exact calculation of difference between the nearest jyotirlinga longitude and what is known as a Indian prime meridian - called the Lanka Rekha. Lanka Rekha is a standard longitude which happens to be the average of the 12 jyotirling longitudes. This longitude passes via Lanka-Kanyakumari-Ujjaini-Hastinapura (the now new Delhi). So, as per the mathematical rules of writing a panchanga, the exact time of sunrise is normalized to the prime meridian. The sunrise time listed in the panchanga will be as if it is the exact moment of being able to see the disc of Sun appear on the horizon at the Lanka Rekha, say at Kanyakumari. There are standard deviations that can be arrived for each of the jyotirlinga places to get a more accurate local timing of sunrise and a local meridian correction that needs to be applied to this nearest-jyotirlinga sunrise time to get the exact time of sunrise at the location in question. Every year, there used to be a panchanga shravana to re-calibrate the 12 jyotirlinga timings and the prime meridian timings at the exact time the Sun would rise as well as cross the prime meridian. That also took into account the tithi - which was the difference between the Sun and the Moon - thus ensuring cross check of the time. That process of re-calibration was called the panchanga sravana which would sync up the lunisolar bhagas and amshakas to ensure the panchanga is sync at each location every year, year after year. Once this local sunrise time is known, the kundali/chakras (charts) and amsha-kundalis (divisional charts) are cast as per the number of ghati's and vighati's post the sunrise.
This way the entire India had a standard process that every astrologer followed to ensure the jatakarma (first of the 16 initiations - shodasha samskara) is done accurately that is valid at the standard prime meridian clock, but calculated at the local meridian accuracy.
If you look at the time calculations we followed during that time, we started a dina or vaara at Sunrise that is calculated thus. If at Sunrise, the tithi is ekadashi (ekadashi is when 120 deg or 240 deg diff between Sun and Moon), we called that entire day to be ekadashi regardless of the exact time of tithi pravesh (exact time the diff between sun and moon exceeding the subsequent 12 degrees). Then at that Sunrise, the constellation that the sun was in for that month, started the lagna - which is the 2 hrs duration. So since as of today, we are in Mithuna maasa (Sun in Mithuna rashi), at sun rise, lagna will be mithuna lagna and after 2 hrs of sunrise, karka lagna and then after 2 more hrs simha lagna, etc. This 2 hr duration was what was used to invite people to events like upanayana, vivaha, etc. If today we say we are inviting you to a vivaha taking place in the subha kanya lagna, knowing that today is mithuna rashi, one would know that the marriage is between noon 12 to 2pm of today's clock. That was the exact reason why 'during so-and-so shubha lagna' is written in the invitations, so that people could plan to reach the event place by that time. We do write the same these days in the traditional invitations, not knowing the significance of it.
So, this strong was (is) the time calculation system in India. Then over the centuries, Greek came along. This was not recent, it was nearly 3500-4000 yrs ago. We identified them as Yavanas or Yavana Rishis. They originated from west near the today's UK. They came via land over the Gandhara desha (today's Kandhahar - Afghanistan). They maintained in touch with their origin and after some centuries of learnings, went back with the learnings they got here. They however still wanted to be in 'sync' with us. So they maintained the same calculations as we did but traveled so far away back that the longitudinal time difference became 330 minutes apart = longitudinally 82.5 degrees away. Because of this, they saw that they expected the sun rise at 0 ghati and 0 vighati as per the mathematical calculations they did as per Indian calculations, but the sun did not rise at that time!! There was a delay of 13.75 vighatis for sun rise. Now they had the choice of calibrating the entire time calculating mathematics by 13.75 vighatis or 330 minutes or continue to call the same 0 ghatis and 0 vighatis as a start of the day. That is when an educated choice of keeping the same 0 ghati 0 vighati as the start of the day made more sense. So they started their day at almost 330 minutes earlier than the sunrise, the exact time when a person sitting in Kanyakumari would see the sun rise. That is the origin of a day at 00.00.00 hrs at GMT which is nearly 5hrs 30 mins behind us. why 5 hrs 30 mins, because that is the earliest time a sun can rise (sun rise time varies based on seasons from as early as 530am to 7am). We calculate the start of the day as the sun rise time at prime meridian, the lanka rekha, and it varies based on seasons. But the Greeks kept the earliest sun rise time that is 5:30 hrs behind India.
If you search for the origin of GMT you may not find anything that is older than AD 1800s or so. Deeper investigation only ends up in the answer of popularity among the locals of UK and only French people not using the GMT meridian. But nowhere you can find why GMT was based out of Greenwich. But Greenwich is exactly 82.5 degrees away from our Lanka Rekha and we are told India is exactly 5 hrs 30 mins ahead of GMT. Fortunately IST is defined at New Delhi which happens to be on the Lanka Rekha and hence it avoids further confusion.
Now think of 3 immortal beings - one - a rishi from vedic age sitting in Ujjaini, two - a Greek descendant who follows GMT sitting in Greenwich, UK and three - a IT software guy in India sitting in New Delhi.
Lets use time reference as the IT guy of India so that you appreciate the fact easily. At 12AM on his ultra slim Timex watch today, he will say 17th June 2012 has begun and it is Sunday in India. But the Greek guy at that very moment, being 5 hrs 30 mins behind, says it is still Saturday the 16th of June. The vedic rishi sitting in India also agrees with the Greek guy and says it is still Saturday the 16th of June, because he is yet to see the sun rise. After exactly 5 hrs and 30 mins, The vedic rishi sees a beautiful sun rising in the east horizon and pronounces the start of Sunday and bows down to the Suryanarayana in the east. The Greek guy sitting at UK, at the same time says, Sunday has begun for him too.
The only guy out of sync is the guy with the modern watch. Fortunately for us, the percentage of such guys is so much that a solid pseudo sync is in place. Everyone calling a new day a day 5 hrs 30 mins earlier itself at India. Everyone around the world follows GMT plus/minus their longitudes so that the day begins at 00.00.00 hrs and GMT in itself is following the earliest sun rise time of India.
Unfortunately, sitting in India itself, we follow GMT plus 5 hrs 30 mins as IST when we already had a solid time calculating methodology. Who is wrong? Nobody. Just that a vedic rishi of India and a person using GMT in UK are in sync with the time as per the original definition and the timex guy has done 3 wrongs (Trishanku? - two wrongs had made it right for the Greenwich guy), thus going out of sync.
The impact you might have seen in some people in India getting confused what day they were born. They would have born post midnight say at 2AM and think the day is say Monday, while their traditional horoscope mentions the day as Sunday! If you think, at 2AM when the person is born, the vedic rishi will say it is a Sunday still as the sun is yet to rise. The Greenwich Greek would say it is still Sunday because his clock is 5 hrs 30 mins behind and showing 8:30PM Sunday. Both of them agree that it was Sunday when he was born, but the timex guy of India in our brain is thinking it is already Monday as the timex shows 2AM. So, ideally what is mentioned in the jataka during such cases is correct regardless of what timex shows.
Think over it - is time, that which is shown in timex or that which we universally follow and are in sync??
Let me take you back to the Vedic era. In Bharatavarsha, we knew that at each place, the light, or Sun, moves at a pace of 6 amshas per vighati. With 60 vighati's making a Ghati and 60 ghati's making a tithi - which would be the exact time that Moon and Sun would differ by 12 degrees. As per the calculations we can understand today, if we reverse derive, 6 amshas/longitudinal degrees is traversed in 1 vighati or 24 minutes. That is 6 degree apparent movement of Sun takes 24 minutes or 1 degree movement takes 4 minutes.
That means if you are looking at the longitudes, 2 places on the same longitude (say New Delhi and Kanyakumari) will have exact same time, but two places 1 degree/amsha apart from each other longitudinally, will have a time difference of 4 minutes. Bharatavarsha is wide - Latitudinal extent: 8deg4min N to 37deg6min N and Longitudinal extent: 68deg7min E to 97deg25min E. The farthest point of India's west differs by 29deg 19mins from the farthest point in the India's east longitudinally. That is about 117.27 minutes of the clock which is nearly 2 hrs. So longitudinally farthest east coast will be 2 hours ahead of farthest west coast of India. This is true even today. This time is called local meridian time of a particular place. Our ancestors identified 12 places in India at different longitudes each apart by 10 minutes in meridian time to cover 2 hours or 120minutes from east to west all longitudes and named them as jyotirlingas or the light-indicators. These 12 places (dwadasha jyotirlinga) give the nearest time to the local meridian time from where every panchanga preparing guy would know how many ghatis he has to add/subtract to arrive at the exact local meridian time mathematically regardless of any tools like hourglass or so.
As an example, when a kid is born, the jataka is cast based on the the local meridian time of the place he is born. The calculation of this local meridian time is dependant on the longitude of the place he is born. The local meridian time is calculated as per the ghati-vighati after the sunrise of the day. So it is dependant on the exact moment of sunrise which is taken from the panchanga and adding/subtracting the local meridian correction to it. You can see all such location specific correction amount also listed out for major places in the panchangas of these days. So to know the exact birth time, we depend on the place (there by the correction factor) and the sunrise time listed in the panchanga. The sun rise time is listed after applying the exact calculation of difference between the nearest jyotirlinga longitude and what is known as a Indian prime meridian - called the Lanka Rekha. Lanka Rekha is a standard longitude which happens to be the average of the 12 jyotirling longitudes. This longitude passes via Lanka-Kanyakumari-Ujjaini-Hastinapura (the now new Delhi). So, as per the mathematical rules of writing a panchanga, the exact time of sunrise is normalized to the prime meridian. The sunrise time listed in the panchanga will be as if it is the exact moment of being able to see the disc of Sun appear on the horizon at the Lanka Rekha, say at Kanyakumari. There are standard deviations that can be arrived for each of the jyotirlinga places to get a more accurate local timing of sunrise and a local meridian correction that needs to be applied to this nearest-jyotirlinga sunrise time to get the exact time of sunrise at the location in question. Every year, there used to be a panchanga shravana to re-calibrate the 12 jyotirlinga timings and the prime meridian timings at the exact time the Sun would rise as well as cross the prime meridian. That also took into account the tithi - which was the difference between the Sun and the Moon - thus ensuring cross check of the time. That process of re-calibration was called the panchanga sravana which would sync up the lunisolar bhagas and amshakas to ensure the panchanga is sync at each location every year, year after year. Once this local sunrise time is known, the kundali/chakras (charts) and amsha-kundalis (divisional charts) are cast as per the number of ghati's and vighati's post the sunrise.
This way the entire India had a standard process that every astrologer followed to ensure the jatakarma (first of the 16 initiations - shodasha samskara) is done accurately that is valid at the standard prime meridian clock, but calculated at the local meridian accuracy.
If you look at the time calculations we followed during that time, we started a dina or vaara at Sunrise that is calculated thus. If at Sunrise, the tithi is ekadashi (ekadashi is when 120 deg or 240 deg diff between Sun and Moon), we called that entire day to be ekadashi regardless of the exact time of tithi pravesh (exact time the diff between sun and moon exceeding the subsequent 12 degrees). Then at that Sunrise, the constellation that the sun was in for that month, started the lagna - which is the 2 hrs duration. So since as of today, we are in Mithuna maasa (Sun in Mithuna rashi), at sun rise, lagna will be mithuna lagna and after 2 hrs of sunrise, karka lagna and then after 2 more hrs simha lagna, etc. This 2 hr duration was what was used to invite people to events like upanayana, vivaha, etc. If today we say we are inviting you to a vivaha taking place in the subha kanya lagna, knowing that today is mithuna rashi, one would know that the marriage is between noon 12 to 2pm of today's clock. That was the exact reason why 'during so-and-so shubha lagna' is written in the invitations, so that people could plan to reach the event place by that time. We do write the same these days in the traditional invitations, not knowing the significance of it.
So, this strong was (is) the time calculation system in India. Then over the centuries, Greek came along. This was not recent, it was nearly 3500-4000 yrs ago. We identified them as Yavanas or Yavana Rishis. They originated from west near the today's UK. They came via land over the Gandhara desha (today's Kandhahar - Afghanistan). They maintained in touch with their origin and after some centuries of learnings, went back with the learnings they got here. They however still wanted to be in 'sync' with us. So they maintained the same calculations as we did but traveled so far away back that the longitudinal time difference became 330 minutes apart = longitudinally 82.5 degrees away. Because of this, they saw that they expected the sun rise at 0 ghati and 0 vighati as per the mathematical calculations they did as per Indian calculations, but the sun did not rise at that time!! There was a delay of 13.75 vighatis for sun rise. Now they had the choice of calibrating the entire time calculating mathematics by 13.75 vighatis or 330 minutes or continue to call the same 0 ghatis and 0 vighatis as a start of the day. That is when an educated choice of keeping the same 0 ghati 0 vighati as the start of the day made more sense. So they started their day at almost 330 minutes earlier than the sunrise, the exact time when a person sitting in Kanyakumari would see the sun rise. That is the origin of a day at 00.00.00 hrs at GMT which is nearly 5hrs 30 mins behind us. why 5 hrs 30 mins, because that is the earliest time a sun can rise (sun rise time varies based on seasons from as early as 530am to 7am). We calculate the start of the day as the sun rise time at prime meridian, the lanka rekha, and it varies based on seasons. But the Greeks kept the earliest sun rise time that is 5:30 hrs behind India.
If you search for the origin of GMT you may not find anything that is older than AD 1800s or so. Deeper investigation only ends up in the answer of popularity among the locals of UK and only French people not using the GMT meridian. But nowhere you can find why GMT was based out of Greenwich. But Greenwich is exactly 82.5 degrees away from our Lanka Rekha and we are told India is exactly 5 hrs 30 mins ahead of GMT. Fortunately IST is defined at New Delhi which happens to be on the Lanka Rekha and hence it avoids further confusion.
Now think of 3 immortal beings - one - a rishi from vedic age sitting in Ujjaini, two - a Greek descendant who follows GMT sitting in Greenwich, UK and three - a IT software guy in India sitting in New Delhi.
Lets use time reference as the IT guy of India so that you appreciate the fact easily. At 12AM on his ultra slim Timex watch today, he will say 17th June 2012 has begun and it is Sunday in India. But the Greek guy at that very moment, being 5 hrs 30 mins behind, says it is still Saturday the 16th of June. The vedic rishi sitting in India also agrees with the Greek guy and says it is still Saturday the 16th of June, because he is yet to see the sun rise. After exactly 5 hrs and 30 mins, The vedic rishi sees a beautiful sun rising in the east horizon and pronounces the start of Sunday and bows down to the Suryanarayana in the east. The Greek guy sitting at UK, at the same time says, Sunday has begun for him too.
The only guy out of sync is the guy with the modern watch. Fortunately for us, the percentage of such guys is so much that a solid pseudo sync is in place. Everyone calling a new day a day 5 hrs 30 mins earlier itself at India. Everyone around the world follows GMT plus/minus their longitudes so that the day begins at 00.00.00 hrs and GMT in itself is following the earliest sun rise time of India.
Unfortunately, sitting in India itself, we follow GMT plus 5 hrs 30 mins as IST when we already had a solid time calculating methodology. Who is wrong? Nobody. Just that a vedic rishi of India and a person using GMT in UK are in sync with the time as per the original definition and the timex guy has done 3 wrongs (Trishanku? - two wrongs had made it right for the Greenwich guy), thus going out of sync.
The impact you might have seen in some people in India getting confused what day they were born. They would have born post midnight say at 2AM and think the day is say Monday, while their traditional horoscope mentions the day as Sunday! If you think, at 2AM when the person is born, the vedic rishi will say it is a Sunday still as the sun is yet to rise. The Greenwich Greek would say it is still Sunday because his clock is 5 hrs 30 mins behind and showing 8:30PM Sunday. Both of them agree that it was Sunday when he was born, but the timex guy of India in our brain is thinking it is already Monday as the timex shows 2AM. So, ideally what is mentioned in the jataka during such cases is correct regardless of what timex shows.
Think over it - is time, that which is shown in timex or that which we universally follow and are in sync??