Who Wu is: branch, hour, trigram
Wu is the one earthly branch sitting exactly on due south. Its animal is the horse, its season high summer, its hour the middle of the day. Its element is fire, as is the whole Li group: Bing at 165, Wu at 180, Ding at 195. Wu is the centre of that group, so a reading on Wu is well inside Li.
The borders that matter are the mountain's own, 172.5 against Bing and 187.5 against Ding. A needle on either line is a riding-line reading; the stone or door is turned a degree or two back towards 180.
Wu as facing, Wu as sitting, and Heaven's Horse
Wu is met most often as the facing. A stone on Zi facing Wu, or a house with its door to the south, is the classic zuo Zi xiang Wu. That facing must answer the water and peak in front.
Sitting on Wu and facing Zi is rarer but sound when the land asks for it, say a slope falling away north. The star on Wu is Tianma, Heaven's Horse, an ordinary star of movement and speed. Master Zhang uses it only to confirm what the ground shows, say a road arriving briskly from the south. It does not lift a blocked south.
What this means on the ground
For a house, stand inside the front door with your back to it, luopan or phone held level. Take three readings a step apart, keep the middle one, and stay clear of steel. For a grave, stand behind the stone and read along its face.
The band inside Wu matters. From 172.5 to about 178 it leans towards Bing; from about 182 to 187.5 towards Ding; 178 to 182 is clean Wu. Then look out. A low open foreground, water crossing, a firm hill beyond is what a Wu facing wants. A tall block pressing in or a road aimed at the door is not.
Common mistakes
The first is true north. A phone on true north is off by the local declination, often ten degrees or more overseas, so a house really on Wu is reported on Bing or Ding. Set the phone to magnetic north.
The second is measuring from the street or the gate instead of the door. The third is taking face south from a book and turning a stone to 180 regardless of the water and peak in front. The fourth is a stone set on 172.5 or 187.5, on the line. The fifth is mixing Wu up with Bing and Ding, which carry different stars.
Questions people ask
What direction is Wu on a feng shui compass?
Due south, 180 degrees magnetic, running from 172.5 to 187.5. Wu is the horse among the twelve branches, fire by element, the centre of the Li trigram between Bing and Ding. Its opposite is Zi, due north.
Is a house facing south always good feng shui?
No. Facing Wu is common and often comfortable, but a facing is judged by what is in front of the door: the water, the foreground, the facing hill. A south door that stares at a wall or takes a road head on is not saved by the degree.
What does Heaven's Horse mean for a Wu facing?
Tianma is the star on Wu; its name means movement and speed. The Tianxing school uses it to confirm a form already seen, such as a brisk road or stream from the south. It is an ordinary star, not one of the four nobles, and never an outcome on its own.
If your door or your family stone faces south, send Master Zhang three readings and a photo of the view out, and he will tell you in a site reading what the Wu facing is answering.