I recently received a request through Yahoo!Answers on how to cast a Wiccan circle. This is the response I sent. Hopefully it makes sense.
Hello, _______:
I believe your original question was about how to cast a circle. The following may be a little long-winded, but please bear with me. :-)
The purpose of a circle of power in a Wiccan context, as I was taught, is threefold. First, it is a temple, a consecrated space marked off from the mundane world; it is commonly referred to as a space "outside the world, between the worlds, and in all the worlds". It is the place where Wiccans invite our Gods to join us, where we may meet with Them and honor and adore Them. Second, it is a container for the power that is raised within it, to keep that energy from dissipating before it is used for spellwork. Third, it is a protection against any negative outside influences -- not demons or any such thing, but rather any energies that might have a negative effect on the work we are doing in circle. A good analogy is the way that a surgical theatre in a hospital is sterilized prior to and during an operation.
The circle of power is actually a sphere of power. The circle is the point at which the sphere intersects the ground or the floor; it actually continues below the floor and upward into the air above the heads of those within the circle.
You need to be able to raise and hold energy in order to cast an effective circle. A good exercise for this is to sit or stand quietly, in a place where you will not be disturbed, and concentrate on your breathing. With each breath in, imagine that you're pulling up energy from the ground beneath your feet and storing it in a "battery" just behind your solar plexus. Do this for ten breaths. Then, continuing to concentrate on your breathing, imagine that with each breath out you are returning that energy to the earth, until the "battery" is empty. Practice this exercise until you can easily fill and empty your energy battery.
The next step is to practice directing that energy after you have raised it. Perform the exercise of filling your energy battery; then put your palms of your hands together in front of you, and slowly pull them apart, visualizing the energy in your battery flowing up your arms and into that space between your palms to form a growing sphere of energy. Hold your hands about six inches apart, concentrating on maintaining the sphere, for the span of several breaths; then bring your hands slowly together again, reducing the size of the sphere by drawing the energy back into yourself. When you are done, concentrate on draining energy back into the earth with each breath until the battery is empty again.
When you cast a circle, you will be raising and directing energy in a similar way, but instead of forming the sphere from the inside out, you will be creating it the way you would create a circle with a pencil on a piece of paper. Before casting a circle you will have gone through the steps of ritually purifying yourself and grounding and centering; then, going to the eastern edge of where you want the circle to be, you will trace an Invoking Earth Pentagram in the air in front of you, then point the tip of your athame at the ground and visualize the energy flowing up through you and out through the tip of the athame. Walking slowly clockwise (or deosil), you will trace the edge of the circle on the ground where your athame is pointing, until you come back to the east edge (or Quarter) and complete the circle, tracing another Invoking Earth Pentagram to seal the circle.
To uncast the circle, you repeat the process in reverse, using Devoking Earth Pentagrams and walking counter-clockwise (or widdershins). You have the option of either visualizing the energy of the circle flowing back through you and down into the earth, or simply dissipating.
Here is a picture of the Invoking and Devoking Earth Pentagram:

I hope that helps. :-) If you have any further questions, please feel free to ask.
Blessings,
Prairie Crow
Hello, _______:
I believe your original question was about how to cast a circle. The following may be a little long-winded, but please bear with me. :-)
The purpose of a circle of power in a Wiccan context, as I was taught, is threefold. First, it is a temple, a consecrated space marked off from the mundane world; it is commonly referred to as a space "outside the world, between the worlds, and in all the worlds". It is the place where Wiccans invite our Gods to join us, where we may meet with Them and honor and adore Them. Second, it is a container for the power that is raised within it, to keep that energy from dissipating before it is used for spellwork. Third, it is a protection against any negative outside influences -- not demons or any such thing, but rather any energies that might have a negative effect on the work we are doing in circle. A good analogy is the way that a surgical theatre in a hospital is sterilized prior to and during an operation.
The circle of power is actually a sphere of power. The circle is the point at which the sphere intersects the ground or the floor; it actually continues below the floor and upward into the air above the heads of those within the circle.
You need to be able to raise and hold energy in order to cast an effective circle. A good exercise for this is to sit or stand quietly, in a place where you will not be disturbed, and concentrate on your breathing. With each breath in, imagine that you're pulling up energy from the ground beneath your feet and storing it in a "battery" just behind your solar plexus. Do this for ten breaths. Then, continuing to concentrate on your breathing, imagine that with each breath out you are returning that energy to the earth, until the "battery" is empty. Practice this exercise until you can easily fill and empty your energy battery.
The next step is to practice directing that energy after you have raised it. Perform the exercise of filling your energy battery; then put your palms of your hands together in front of you, and slowly pull them apart, visualizing the energy in your battery flowing up your arms and into that space between your palms to form a growing sphere of energy. Hold your hands about six inches apart, concentrating on maintaining the sphere, for the span of several breaths; then bring your hands slowly together again, reducing the size of the sphere by drawing the energy back into yourself. When you are done, concentrate on draining energy back into the earth with each breath until the battery is empty again.
When you cast a circle, you will be raising and directing energy in a similar way, but instead of forming the sphere from the inside out, you will be creating it the way you would create a circle with a pencil on a piece of paper. Before casting a circle you will have gone through the steps of ritually purifying yourself and grounding and centering; then, going to the eastern edge of where you want the circle to be, you will trace an Invoking Earth Pentagram in the air in front of you, then point the tip of your athame at the ground and visualize the energy flowing up through you and out through the tip of the athame. Walking slowly clockwise (or deosil), you will trace the edge of the circle on the ground where your athame is pointing, until you come back to the east edge (or Quarter) and complete the circle, tracing another Invoking Earth Pentagram to seal the circle.
To uncast the circle, you repeat the process in reverse, using Devoking Earth Pentagrams and walking counter-clockwise (or widdershins). You have the option of either visualizing the energy of the circle flowing back through you and down into the earth, or simply dissipating.
Here is a picture of the Invoking and Devoking Earth Pentagram:

I hope that helps. :-) If you have any further questions, please feel free to ask.
Blessings,
Prairie Crow
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