What is it?

Quite a bit more esoteric than its cousin, contact juggling is essentially the art of rolling spheres along the performer's body, primarily the arms. The contact juggler relies heavily on playing with the human senses to dazzle the audience, making the ball appear to move in a gravity-defying fashion, hanging in the air, hovering around his hands or flying between seemingly motionless fingers. Clear acrylic spheres are usually used to accomplish this illusion, although a skilled CJer can make any spherical object perform a mesmerizing dance.

What is known as contact juggling today (aka Sphereplay, Dynamic Manipulation, etc) has largely grown out of the performances of Michael Moschen, although he does not view CJ as a separate discipline himself.
There have been some references to sphere-balancing training being used by various martial arts trainees in the past, but I am not up to par with the history here.

How does this work?

• The Cradle

  Not exactly a move in itself. A position of the fingers (either all fingers flexed, middle finger lowered, or two fingers slightly spread apart) that allows the sphere to rest on the back of the hand.

• Butterflies and Windshield wipers and derived moves.

  This is perhaps the defining move, certainly the one most readily recognized from popular media (The Matrix, Labyrinth). The ball is transferred from the palm to the cradle and back again, usually over the tip of the middle finger. Quite fascinating to watch by itself if done fluidly. Also tends to be the first move taught to novices.

• Isolations

  A large family of moves where the sphere appears to stay completely still as the performer's hands move around it. Many moves can be converted into isolations. The simplest one is accomplished by grasping the sphere (something many CJers dislike) and rotating the arm around its axis.

• Palmspins

  Spinning two or more (although single-ball "spins" are also performed) spheres in one's palm. The middle, ring and pinkie fingers are normally used with the thumb serving as a guide. Practicing palmspins with smaller spheres, or Chinese meditation balls, is advised.

• Body Rolls

  Rolling the sphere along the performer's arm, neck, torso, etc. Can look spectacular, especially with the right choice of clothing. Overlaps with...

• Passes

  The connecting elements between moves, although pass combinations can look fantastic by themselves. The simplest passes are between the palms and cradles of opposite hands.

• Other

  ``Flyaways'', ``Elevators'' and everything else that defies description.

Sounds great, how do I start?

First and foremost, read the essays on contactjuggling.org. Ferret provides excellent instructions for all would-be contact jugglers. I can only add that a lacrosse ball makes an excellent practice sphere; the weight is just right, and the bounce helps while walking down the street doing half-assed butterflies.