We have already explored some features of the Schwarzschild solution, including the tests of General Relativity that it allows. In the Schwarzschild solution the Sun is taken to be static, that is, non-rotating. In fact, however, the Sun does rotate, and this suggests the question, is there another exact solution, a generalisation of the Schwarzschild solution, describing a rotating source? And, if there is, does it suggest any additional tests of General Relativity? It turns out that a generalisation of the Schwarzschild solution does exist – the Kerr solution. This is a rather complicated solution, however; it will be discussed further in the next chapter. In this chapter we shall find an approximate solution for a rotating source (which of course will also turn out to be an approximation of the Kerr solution). The tests for this solution include a prediction for the precession of gyroscopes in orbit round the Earth (which of course also rotates). This is a tiny effect, but in April 2004 a satellite was launched to look for this precession, which goes by the names of Lense and Thirring. We shall see that there is a parallel between the Lense–Thirring effect and magnetism, just as there is between ‘ordinary’ gravity (not involving rotations) and electricity – hence the name ‘gravitomagnetism’. After a discussion of these matters the chapter finishes with a more theoretical look at the nature of the distinction between ‘static’ (Schwarzschild) and ‘stationary’ (Kerr) space-times.