A facsimile or analogue computer is a type of computation machine that uses physical phenomena, most commonly mechanical ones, to model the problem being solved by behaving according to the mathematical principles in questions. While the name technically applies to all analogue computers including nomograms and the slide rule, facsimiles are commonly understood to be large machines purpose-built for a specific task.

Unlike digital computers, facsimiles do not execute stepwise algorithms. Instead, the machine's internal structure is built to physically mirror the structure of the problem itself (an analogon), so that the behaviour of the machine approximates the behaviour of the system being studied. The output is read directly from the state of the machine's components rather than computed symbolically.

Facsimiles are commonly used to model a specific problem that can't be approximated through mathematics or experience. A common application of the facsimile is the tide-predicting machine, which used pulleys and wires to automatically calculate predicted tide levels for a set period at a particular location.

The most prominent modern application of facsimile technology is in orbital mechanics and spacecraft trajectory planning, where a specialised variant known as the Astrosimile is used. Astrosimiles are large-scale machines combining mechanical, electrical, and optical components to model planetary positions and orbital paths.

By tracing possible flight vectors through the simulated gravitational environment, an Astrosimile can determine optimal timing for stage separations, thruster firings, and course corrections - calculations that would be prohibitively complex to perform by hand and for which digital methods have historically lacked sufficient precision at interplanetary scales.