How to aim: the ghost ball and everything built on it.

The ghost ball

The ghost ball is the imaginary position the cue ball must occupy at the moment of contact for the object ball to go where you want. Draw a line from the pocket back through the center of the object ball: the ghost ball sits on that line, touching the object ball. That line through the two centers is the line the object ball will travel.

To use it, sight from the object ball to the pocket to find the contact point and the line, picture the ghost ball, and then shoot the cue ball at the center of the ghost ball. Note what that means: the cue ball's aiming line does not pass through the contact point on the object ball.

The most common aiming error

Aiming the center of the cue ball at the contact point on the object ball hits the object ball too full, so cut shots come up short of the pocket (undercut). The ghost ball fixes this because it puts your aim one ball-radius farther out.

Two ways to train the picture:

• Have a friend set a real ball in the ghost-ball position, look at it, then remove it and shoot.
• Lay your cue along the object ball's path with the tip at the ghost ball's center, then pivot the cue until it lines up with the cue ball. The tangent line, where the cue ball will head after a stun hit, is at right angles to the object ball's line.

Contact-point methods

One contact-point method published on Dr. Dave's site: find the contact point on the object ball's line to the pocket, put your tip on that line half a ball-width behind the contact point, then step back so the cue runs from that spot through the cue ball's center.

The double-the-distance system gets to the same place another way. Measure from the object ball's center to the contact point and add the same distance again beyond it; that is the ghost ball's center. For thin cuts, double the distance from the contact point to the object ball's outer edge.

Fractional ball hits

Many players learn a few reference overlaps between the cue ball and object ball. With a center-ball hit and ignoring throw, they correspond to these cut angles:

On a real table the angles come out slightly smaller because of cut-induced throw (below). Hal Houle's "3-angle" system uses 15, 30 and 45 degree references; as Dr. Dave notes, the 15 and 45 values are not exact.

What aiming systems can and cannot do

Dr. Dave's site indexes many named systems, including Center-to-Edge (CTE), double-the-distance, Houle's 3-angle, bisect-pivot-and-shift, parallel lines, contact-point-to-contact-point and Pro One. His analysis is that any system built on a few fixed aim lines cannot be geometrically correct for every shot. For an object ball about 3 feet from a 5.25-inch pocket, roughly 19 distinct aim lines are needed to cover every cut angle; with only three evenly spaced lines, fewer than half of the cuts between 7.5 and 52.5 degrees go in once the object ball is more than 2 feet from the pocket.

That does not make systems useless. They can help players concentrate and aim more consistently. But most professionals aim by intuition built on ghost-ball and contact-point visualization, and Dr. Dave calls aiming while standing the most reliable method.

Why slow cuts miss thick: cut-induced throw

When the cue ball strikes the object ball at an angle, sliding friction between the balls throws the object ball slightly off the line of centers, so the cut comes out a little smaller than the geometry says. That is cut-induced throw. It is largest on slow stun shots near a half-ball hit (about 30 degrees). Dr. Dave lists two fixes: aim slightly thinner to compensate, or use enough outside english to cancel it, which the side spin page explains.