Voltages relative to earth
It is very common in electrical circuits to have an earth connection. This earth connection has no effect on potential differences across components, however it does affect the values of the potentials or voltages at points in the circuit.
The earth is a reference point and considered to be at zero volts. Potential differences between earth and the negative terminal of the supply result in negative voltages and potential differences between earth and the positive terminal result in positive voltages. It should be noted that due to static build up on the airframe, the earth connection (airframe) of an airborne aircraft is unlikely to be at zero potential with respect to the ground
You should also note that earth connections, for example to the chassis of an equipment or the airframe of an aircraft, are often used as the current return lead in an electrical circuit.