The moon has a rich symbolic significance in yoga mythology. In hatha yoga, for example, the sun and the moon represent the two polar energies of the human body. In fact, the word hatha itself is often divided into its two constituent syllables, "ha" and "tha", which are then esoterically interpreted as signifying the solar and lunar energies respectively.


 In Sanskrit ardha means "half," and chandra means "moon," thus, this is the "half moon" posture. This pose is also one the postures that are are sequenced in surya-namaskar (the Sun Salutation).

(are-dah chan-DRAHS-anna)
ardha = half
candra = glittering, shining, having the brilliancy or hue of light (said of the gods); usually translated as "moon

Physical benefits:

This pose is a highly effective strengthener for the legs and ankles. You have to sort of squint to see the half moon; try drawing a half-circle from the raised top hand through the lifted foot to the standing foot and supporting hand.


Trims fat all over the body" thighs, hips, waist, arms and abdomen.
Improves and strengthens every muscle in the central part of the body, especially the back and abdomen.
Increases flexibility of the spine
Corrects bad posture
Promotes proper kidney function
Helps cure enlargement of the liver and the spleen
Helps cure dyspepsia, and constipation
Helps to eleviate lower back pain, bronchial distress, scoliotic deformities, tennis elbow, frozen shoulder

Precautions - Fatigue, high blood pressure.


Step by Step

•  Perform Utthita Trikonasana to the right side, with your left hand resting on the left hip. Inhale, bend your right knee, and slide your left foot about 6 to 12 inches forward along the floor. At the same time, reach your right hand forward, beyond the little-toe side of the right foot, at least 12 inches.

• Exhale, press your right hand and right heel firmly into the floor, and straighten your right leg, simultaneously lifting the left leg parallel (or a little above parallel) to the floor. Extend actively through the left heel to keep the raised leg strong. Be careful not to lock (and so hyperextend) the standing knee: make sure the kneecap is aligned straight forward and isn't turned inward.

•  Rotate your upper torso to the left, but keep the left hip moving slightly forward. Most beginners should keep the left hand on the left hip and the head in a neutral position, gazing forward.

•  Bear the body's weight mostly on the standing leg. Press the lower hand lightly to the floor, using it to intelligently regulate your balance. Lift the inner ankle of the standing foot strongly upward, as if drawing energy from the floor into the standing groin. Press the sacrum and scapulas firmly against the back torso, and lengthen the coccyx toward the raised heel.

•  Stay in this position for 30 seconds to 1 minute. Then lower the raised leg to the floor with an exhalation, and return to Trikonasana or warrior 2. Then perform the pose to the left for the same length of time.                                  (exerpts from Yoga journal)
  

Many beginning students have difficulty touching the floor with their lower hand, even when resting it on the fingertips. These students should support their hand on a block. Start with the block at its highest height and, if your balance is steady and comfortable, lower it down first to its middle height, then finally if possible to its lowest height.

Modifications and Props:
 
Balance is always tricky in this pose for beginners. A wall is a useful prop, which you can use in one of two ways. Stand with your back to the wall, one leg's length away from the wall. Exhale and bend forward into a standing forward bend, then inhale and raise your left leg parallel to the floor and press the left sole against the wall. Start with your toes turned toward the floor. Exhale again and rotate your torso to the left; at the same time, turn the left leg and foot until the inner foot is parallel to the floor. Rest your left hand on the left hip. The pressure of the raised heel against the wall will help you maintain your balance. You can also perform the pose with your back to, and leaning against, the wall.

Variations   
To increase the challenge of this pose, raise the lower hand away from the floor and rest it on the standing thigh. Balance solely on the standing leg for 15 to 30 seconds.