While casting a horoscope, should we use Standard Time or Local Mean Time? Will the horoscope created using either of them would be significantly different?An esteemed reader has asked this question through email. Often, there is a confusion, especially among the beginner students in Astrology who are learning necessary calculations required in casting horoscopes.
Change Standard Time to Local Time
While casting horoscopes, we use Local Mean Time (LMT) only. We have to convert the time given in the format of Indian Standard Time (IST) or Standard Time (ST) of a particular Country to the Local Mean Time of that place. While casting a horoscope, we first change the Standard time to LMT depending on the latitude and longitude of a location. Earlier manually casting horoscopes was a tedious process when computers were not there. However, now, it is effortless as the software does all the calculations in seconds.
Importance of Lagna or Lagnam
Let us understand this further. Our Vedic Astrology system called the Nirayan Jyotish (फ़लित ज्योतिष) is fundamentally different from the Western Astrology system called as the Sayan Jyotish. In Nirayan Jyotish, while casting horoscopes, we place extremely high importance to the concept of Lagna or lagnam (the point on the easternmost horizon which rises each day at the time of sunrise). To calculate Lagna, we need to find the Sidereal Time of that place where the person is born. (Sidereal time is the time taken by Earth to make one full rotation around its axis such that a particular fixed star is found at the same place each subsequent day). Sidereal time is crucial to compute lagna.
During earlier days, when there was no concept of a time zone, people used to note the time of a local place through the use of shadows in the day time and position of stars in the night time. However, in the current times, there is hardly anyone who sets the watch as per the local time. We all live in the standard time Zones. Almost all the clocks/watches across the world are set according to the respective standard time zones which in India is 5 hrs. 30 mins east of Greenwich Mean Time (GMT).
For example, while casting horoscope from a given Indian Standard Time (IST), we take the vernal equinox adjustments as 4 minutes for each one degree, calculate the LMT of that place and further calculate the sidereal time. Using the sidereal time, we finally find lagna and longitude cusps of other planets. In case someone records a birth as per the Local Mean Time, we should take that local time and proceed to find lagna straight away.
It’s LMT only
In a nutshell, while casting horoscopes, we start from IST but convert this to LMT of that place. Then we find sidereal time to calculate lagna and house cusps. In other words, LMT is already taken care of while casting horoscopes. It is incorrect to say that we can construct horoscopes without converting the Standard Time to the Local Mean Time. We should never read such charts as they would be fundamentally faulty and the readings would be highly inaccurate.