The Celtic Cross, or wheel cross, is one of the most widely recognized patterns of this Christian symbol. It comes to us through at least three different paths, or interpretations of its development. First, the circle with rays coming out from the center through its sides, top and bottom, had been a widespread symbol for the sun. In the Celtic world, this sun symbol had often been represented as an actual wheel with numerous spokes, deriving from the old belief that the sun was drawn by a chariot with wheels. This interpretation is adaptable to Christianity on the basis that Christ is the Light of the world. Moreover, with its longer bottom line, the Celtic cross is reminiscent of the Star of Bethlehem, with the light directing us to the birth of the Savior.
The second path is more traditionally Christian and can be traced in the development of the cross itself on monuments in Britain and elsewhere. One of the earliest Christian symbols (even before the cross) was the Chi-Rho -- a combination of the first two Greek letters in the name Christos, resembling a P over an X and often placed within a circle or wreath. Gradually, the X was turned to become a crossed vertical and horizontal line, with the vertical line merging with the vertical line of the P. The loop of the P eventually disappeared, leaving us with the simple cross within a circle. By extending the lines outside the circle, we have the traditional Celtic cross.
In a more basically Celtic tradition, however, the cross is indeed a "wheel cross." The wheel was a symbol of the Indo-European peoples who had come into the West with the domesticated horse and the chariot. So the wheel in its simplest symbolic representation of a circle (the rim) with internal vertical and horizontal lines (the spokes) came to be associated with the Europeans and especially with the Celtic peoples. The Christian cross then is a traditional wheel symbol with the arms extended to form the cross of Christ superimposed upon the circular wheel. This interpretation is highly symbolic of Christ's Lordship over the Celtic people, but it also represents a combination of Christianity with traditional Celtic spirituality.
One thing that the interpretation of the Celtic cross as a wheel cross does give us that is most characteristic of the Celtic way is the idea of connectedness. As the Celtic knots that often adorn it show a connectedness through the single unbroken thread, the wheel cross provides us with another unbroken symbol in a circle -- often used as a symbol itself for the unity of the people of God -- connected and embraced by the arms of the cross.