Sable

You'll be sent here by the Mechanist in Marrow Bone Station, but you can head here at any point. The Watch is located in the bottom right corner of the map, and has a large statue in front of it you can see for a good distance to locate the building. Presumably, though I'm not positive, this puzzle needs to be done during the daytime.

The puzzle room is a giant orrery of sorts (not the first, smaller one) with 4 platforms with arches on the upper sides of the room. Head to the one above the Mask door (across from where you entered) and hit the switch on the floor there to turn the device on. Use the wingnut-looking switches on the other 3 platforms to align the moving planets with the dashed outlines that appear on the orrery. After the 3 planets are aligned, head back to the door-side platform. Below where the switch is are a number of pedestal things, the middle of which has a small sun you pick up. Put it into the slot under the symbol that is currently lit by the sun on the large band under the orrery, and you're done.

(Don't miss the chest on the lower floor in the center of the next room)

This achievement is available to unlock as soon as you enter the south eastern area of the map (The Wash). This area has white sand and a lot of huge skeletons. The Watch is a huge domed building which can be seen from a distance. It can easily be seen from the settlement of Marrow Bone Station. You can also get a quest from the local machinist connected to the Watch.

The Watch is located in the bottom eastern part of The Wash and is seen on the map as a large red structure.

Entering the building is not hard and the first two rooms you go through have nothing of interest when it comes to the achievement.

The third room is huge and in here you need to solve the puzzle.

• The first step is to make your way to the top level in the room. Up there you can find a few raised platforms. One of them have a button in the floor. When you step on this the roof of the dome is moved and there is a small cutscene. You can now interact with the mask on the same platform and a text is availabe. This gives you hints on what to do.

• The second step is to move to the other platforms on the same level and interact with the machines on them. If done right the stone arch above these platforms should turn from red to blue. The moving orbs should also turn blue. When you move the last one (there are three) there is another cutscene.

• It is now time to move to the floor below and make your way to the big stone mask. In front of this mask there is a daise that produces small suns. Take these and place them within the small altars with sigils on top of them. When they are all activated you need to wait. The sun moving across the stone slab in front of the sigils will eventually open the door. If you solve the puzzle at night you will have to wait for the sun to come up and then move along the stone slab.

• When the last stone marking is lit up the mask/door will open up behind you and the achievement will unlock! If you have the quest from the machinist this will also then be updated.

Video for those that like those better.