Ujjain was the capital of Avanti Mahajanapada during the Vedic period. Even after Avanti was no longer an independent kingdom, it remained the capital of the western provinces during the Mauryan and Gupta times. Along with Varanasi it seems to have been a great centre of learning, especially in astronomy in ancient times.
One of the earliest available texts on Indian astronomy, the 'Surya Siddhanta', dated to the 4th century BC, explains with clarity that the earth is a sphere. It also describes the scientific methods for determining the Prime Meridian. From the text, it is seen that these methods were thousands of years old, even at the time of its composition and indicated that the "Prime Meridian" passed through the historic city of Ujjain.
Aryabhata in AD 499 said " when it is morning in Lanka, it is sunset at Siddhapura, noon at Yavakotya and midnight in Romakdesh "
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Ujjain is a zero point for both time and geographical calculations and in the ancient and medieval world, the " Greenwich" line was always drawn through Ujjain. Since 'time' in Sanskrit is called "kala" the main Shiva temple located exactly there, is called as "Mahakala temple". All Indian traditional calendars and all sacred time is calculated even now from this line (Ujjain time which is roughly 29 minutes behind the Indian Standard Time) including birth charts in astrology. It was only in 1884 that the present " Greenwich" was agreed to in the International Meridian Conference to determine a common zero meridian, probably due to the political and economic conditions of that time.
Additionally, it was here in Ujjain that the legendary king Vikramaditya drove away the Sakas and started a new era, the Vikram Samvat or the Ujjain calendar around 58-56 BC.
Ujjaini Symbol
The Ujjaini symbol is common to Indian numismatics. It consists of a cross connecting four circles/orbs. There are various types even for the orbs, they are either hollow or filled with pellets or swastika symbols. A single or double ring also appears inside the orbs. Amongst the various theories about this symbol, the most likely one is that the Ujjaini symbol represents the occurrence of a phenomena that the equator passes through Ujjain and the tropic of cancer cuts the equator just in the middle of Ujjain city, thus forming a cross at Ujjain.
There are a few who believe that it represents the junction of four roads to the city of Ujjain as it was an important trade centre or some who believe it represents " Damru" of Lord Shiva, whereas some say it represents four gates of Ujjain.
There are various ancient dynasties and kingdoms of ancient India that used "Ujjaini symbol" on their coinage. It is seen on coins of Satavahanas, Vidharba , coins from Eran, Bhadra Mitra, Kaushambi, Erikachha etc. This was a symbol accepted on coins by the traders of Ujjain.
Obverse: Three headed standing Shiva holding Danda and kamandalu, railed tree on right and chakra above fish tank on left
Reverse: Ujjaini symbol with swastika in each orb
Obverse: Shiv Parvati holding hands, Parvati's right hand raised to her head, Shiv's left akimbo, hair decorations on side of head, river at the bottom.
Reverse: Ujjaini symbol with a taurine in each orb.
Obverse: Lakshmi seated with feet parallel to each other, right rests on her belly and her left on her leg: Lakshmi is flanked by two elephants which are sprinkling water on her head.
Reverse: Ujjaini symbol.
Obverse: Elephant with raised trunk to right with chakra on top left, railed tree on right
Reverse: Ujjaini symbol with a taurine in each angle.
Obverse: Elephant facing right and Brahmi legends
Reverse: Ujjaini symbol.