A trackball is a pointing device consisting of a ball housed in a socket containing sensors to detect rotation of the ball about two axes—like an upside-down mouse with an exposed protruding ball. The user rolls the ball with the thumb, fingers, or the palm of the hand to move a cursor. Large tracker balls are common on CAD workstations for easy precision. Before the advent of the touchpad, small trackballs were common on portable computers, where there may be no desk space on which to run a mouse. Some small thumbballs clip onto the side of the keyboard and have integral buttons with the same function as mouse buttons. The trackball was invented by Tom Cranston and Fred Longstaff as part of the Royal Canadian Navy's DATAR system in 1952, eleven years before the mouse was invented. This first trackball used a Canadian five-pin bowling ball.

When mice still used a mechanical design (with slotted 'chopper' wheels interrupting a beam of light to measure rotation), trackballs had the advantage of being in contact with the user's hand, which is generally cleaner than the desk or mousepad and doesn't drag lint into the chopper wheels. The late 1990s advent of scroll wheels, and the replacement of mouseballs by direct optical tracking, put trackballs at a disadvantage and forced them to retreat into niches where their distinctive merits remained important. Most trackballs now have direct optical tracking which follows dots on the ball. Some mice, in place of a scroll wheel, acquired a small trackball between the buttons, useful in maps and other circumstances calling for scrolling in two dimensions.

Special applications

Large tracker balls are sometimes seen on computerised special-purpose workstations, such as the radar consoles in an air-traffic control room or sonar equipment on a ship or submarine. Modern installations of such equipment may use mice instead, since most people now already know how to use one. However, military mobile anti-aircraft radars and submarine sonars tend to continue using trackballs, since they can be made more durable and more fit for fast emergency use. Large and well made ones allow easier high precision work, for which reason they are still used in these applications (where they are often called "tracker balls") and in computer-aided design.

Trackballs have appeared in computer and video games, particularly early arcade games (see a List of trackball arcade games) notably Atari's Centipede and Missile Command. "Football", by Atari, was the first arcade game to use a trackball, released in 1978 - though Atari spells it "trak-ball". Console trackballs, once common in the early 1980s, are now fairly uncommon: the Atari 2600 and 5200 consoles had one as an optional peripheral, with a joystick as standard. The Bandai Atmark, a Japanese console introduced in 1995 had a trackball as standard for its gamepad. Trackballs are also preferred by many so-called professional gamers, who value their consistency highly. A trackball requires no mousepad and enables the player to aim swiftly (in first person shooters). Trackballs remain in use in pub golf machines (such as Golden Tee) to simulate swinging the club.

Ergonomics

People with a mobility impairment use trackballs as an assistive technology input device. Access to an alternative pointing device has become even more important for them with the dominance of graphically-oriented operating systems. There are many alternative systems to be considered. The control surface of a trackball is easier to manipulate and the buttons can be activated without affecting the pointer position.

Trackball users also often state that they are not limited to using the device on a flat desk surface. Trackballs can be used whilst browsing a laptop in bed, or wirelessly from an armchair to a PC playing a movie.

Trackballs are generally either symmetrical in design, with the ball operated by the fingers, or asymmetrical, with the ball operated by the thumb. Many users favour one format or another, for reasons of comfort, precision, or because it reduces strain on one part of the hand/wrist. Only the symmetric format can be used by both hands. Asymmetric or "handed" trackballs are not generally available in left-handed configurations, due to small demand.

Some computer users prefer a trackball over the more common mouse for ergonomic reasons. There doesn't seem to be conclusive evidence of one being better than the other in terms of comfort. Users are encouraged to test different devices, and to maintain proper posture and scheduled breaks for comfort. Some disabled users find trackballs easier since they only have to move their thumb relative to their hand, instead of moving the whole hand, while others incur unacceptable fatigue of the thumb. Elderly people sometimes have difficulty holding a mouse still while double-clicking; the trackball allows them to let go of the cursor while using the button.

At times when a user is browsing menus or websites rather than typing, it is also possible to hold a trackball in the right hand like a television remote control, operating the ball with the right thumb and pressing the buttons with the left thumb, thus giving the fingers a rest.

Mobile phones

Some mobile phones, such as the T-Mobile Sidekick 3, T-Mobile Sidekick iD, T-Mobile Sidekick LX, T-Mobile Sidekick Slide, BlackBerry Pearl, Curve and Bold, now feature trackballs. These miniature trackballs are made very small to fit within the thickness of a mobile device, and are controlled by the tip of a finger or thumb.

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