Can anyone explain how LIGO gets an a rough idea of the direction of the gravitational wave?
The idea is the same as the way your ears determine where the sound came from. You observe the signal at multiple, geographically separated locations. You also record the precise time the signal arrived at each receiving station. Because gravitational waves propagate at the speed of light, the arrival times will be different at each station. You can then align the signals, giving you the relative time delays between the stations, in turn giving you an idea of where the wave came from.
If you only have two receivers, like LIGO has, you can at best figure out the cone containing the source. This is why it was so important to have the Virgo detector as well - three detectors constrain the solution to just two directions, giving you a sporting chance of finding the source in the electromagnetic spectrum as well.
As usual, it is a little more complicated: for technical reasons, the detectors are not equally sensitive for all directions of arrival. This both makes things a bit messier, and gives some, very crude, directional information as well.
Biting the hand that feeds IT
