Arunachala is the form of Shiva is worshipped in the famous Arunachaleswarar Temple at Tiruvannamalai in Tamil Nadu. It is worthy to note that Arunachala is that form of Shiva in which the Mother Shakti shares half His body. ‘Aruna’ itself stands for the Mother Goddess Shakti and ‘Achala’ for Mahadev Shiva.
Aruna is a name of Mother Goddess Shakti, because of her deep red color. When she appears is deep red color she is the Divine Mother of Power and is referred to as Kameshwari. The red color typifies creativity and love in Hindu tradition.
It is of note that when attributeless white incandescence resolves itself into seven primary colors it is red (infra-red) that comes out as the first evolute.
The attributeless Shiva is white incandescene and His Shakti (Maya) who is the origin of the world of attributes is appropriately the red Aruna.
Achala means non-moving. It symbolizes the static Shiva whose Shakti is the ever dynamic Dance of Dualities.
On one occasion Shiva rejected Mother Goddess Shakti who, for fun, had closed His eyes with Her palm. Fired by penitence, Mother Goddess did penance and regained Him as her consort in Kanchi, and on reaching Arunachala, She became His inseparable Consort sharing half of His very person.
O Mahadev! Was it not because I happened to be outside of yourself that you could reject me? Therefore, make me half of yourself — prayed the Divine Mother Goddess Shakti to Mahadev and thus came to share His body.
Source – excerpts from Maharshi, The Poet – IV an article published in the Mountain Path Magazine January 1988 Issue page 20.