Ujjain is an important place in Indian astronomical history. Its historical reference meridian and observatory should be understood separately from the modern IST standard meridian at 82.5° east.
Start with the observatory
The district administration's Vedh Shala account describes an observatory established under Sawai Jai Singh and instruments used for astronomical observations. These instruments make an abstract subject tangible: sunlight, shadow and angles can be used to investigate time and the sky.
Historical descriptions sometimes give differing construction dates or date different stages of a complex. A useful guide should acknowledge that distinction instead of combining conflicting dates into a precise but unsupported story.
What is a reference meridian?
A meridian is a line of longitude. A calculation system can select one as a reference for recording observations or tabulating positions. This is different from a modern national agreement to use one civil time on clocks across a country. See the prime-meridian guide for the broader distinction.
A worked longitude example
Earth turns through 360 degrees in approximately 24 hours relative to the mean Sun, so one degree corresponds to four minutes of mean solar time. Using Ujjain at roughly 75.79° east, its longitude differs from 82.5° east by about 6.71°. Multiplying by four gives about 26.8 minutes. This is a longitude-based comparison, not a replacement civil timezone.
A sundial also follows apparent solar time, which differs seasonally from mean solar time. Do not use the longitude calculation alone to predict what a sundial must read at a particular instant.
Religion and astronomy in one city
Mahakaal connects the city with religious meanings of time. Vedh Shala connects it with observational instruments. Readers can appreciate both while asking different source questions of each: which tradition explains a belief, and which instrument or document supports a historical claim?
Using this history today
Keep devices and appointments on IST. Use local solar time as a subject of observation and learning. The time-units guide offers another way to compare traditional quantities with modern hours and minutes.