The familiar symbol, known as the yin yang symbol but properly called the taijitu is about a thousand years old from China. Similar designs are found in Iron-Age Celtic culture in 3rd century BCE.
It shows a dynamic harmony, within the circle of the polarities of black and white moving together.
Tantra as a spiritual practice is always ultimately about Oneness and the skilful use of polarities like Shiva and Shakti - or consciousness and energy. It is tempting to put the two together and think in terms of a harmonious balance of Shiva and Shakti. This can be misleading. Putting together two different types of qualities can cause more problems than it may solve. The familiar yin yang symbol of a black and white sector whirling is better than mixing black and white together and getting a static, uniform grey!
Within each, there is a dot of the opposite which points beyond separation to the reality that int he heart of each is the other. In some forms of this symbol there is an empty circle in the very centre of the symbol to show that this duality is based in emptiness.
Enjoy the dance. Today there is an equal balance of Yin and Yang and the turning of the year towards light and Spring. This can inspire us to believe that, along with the Earth, we too might change, release the past, and give birth to new passions, ideas and projects. Soon will be Easter and then May Day as Life comes riotously into abundance.
On a planetary level from the perspective of space there isn't day and night just a beautiful blue-green planet spinning in space.