As described in an article on Abhijit muhūrta, there is a difference between muhūrta as a time measurement unit and muhūrta as a daily astrological phenomenon. In this article, it is investigated whether the duration of Brahma muhūrta is fixed (by taking a period of 48 minutes, starting 1 hour and 36 minutes before sunrise) or it, too, varies depending on sunrise and sunset.
According to the references below, there are 30 muhūrtas in a day, which are divided into 15 daytime muhūrtas and 15 nighttime muhūrtas. The duration of daytime muhūrtas is calculated as 1/15th of the daytime and, similarly, the duration of nighttime muhūrtas is calculated as 1/15th of the nighttime. This method of calculation is described in the following works:
- “Hindu Electional Astrology”, section 12.1 by V. K. Shridhar (2002)
- “Muhurta Traditional and Modern”, chapter 2 by K. K. Joshi (2003)
- “Muhurtha (Electional Astrology)”, chapter 5 by B. V. Raman (1993)
If we consider Brahma muhūrta as one of those 30 muhūrtas, then its duration will vary during the year depending on sunrise and sunset. So the question is whether Brahma muhūrta is one of those 30 muhūrtas or it is an independent concept.
In a Wikipedia article on muhūrta, the names of the 30 muhūrtas are given and Brahma muhūrta is the 29th muhūrta on the list (thus, the 14th muhūrta of nighttime). Unfortunately, that article does not seem to cite its sources, so it cannot be considered authoritative.
“Hindu Electional Astrology” and “Muhurtha (Electional Astrology)” give slightly different names than the Wikipedia article above and, in particular, a different name for the 29th muhūrta. The names are mostly identical, so we can infer that all of these sources are talking about the same concepts, but maybe using different terminology. In any case, Abhijit muhūrta, which is the 8th muhūrta of the daytime as per related research, is not called by that name in any of these sources either.
An article on muhūrta by Pt. Sanjay Rath seems to be a bit confusing in this regard. On the one hand, it supports the idea that Brahma muhūrta is the 29th muhūrta of the day by explicitly associating the 29th muhūrta with Lord Brahma, continuing the same calculation logic as for Abhijit muhūrta. On the other hand, it also explicitly mentions the 96 minute interval, which seems contradictory. We have decided to solve this contradiction for now by considering the mention of 96 minutes as a simplification, which is common, and considering the conceptual description of Brahma muhūrta as more important.
Based on the research above, Chakra Darshana implements the varying approach, because there seems to be more evidence to considering Brahma muhūrta as the 29th muhūrta of the day (14th muhūrta of nighttime) than treating it as an independent concept. Still, this is a topic for further research.