Myst

While solving the Mechanical Age puzzle, you will find yourself rotating the entire fortress using a set of controls atop the age's elevator (while at the highest level, push the middle elevator button, shaped like a minus sign, then hop out before the lift descends.) To solve the puzzle correctly, you need to rotate the large gear mechanism in front of you (and thus the fortress) 90 degrees clockwise from the start, then 180 degrees from start, and then return the fortress to the original position (360 degrees from start.)

To see the skeleton, rotate the fortress 270 degrees (3/4 of a unit circle) from the original position, or just 90 degrees from the second Myst book puzzle solution. Timing for this is subjective since the left lever adjusts the rotational speed, so instead go by angle. It helps that the fortress will only settle on one of the four previously mentioned positions so if you're not precise, the mechanism will move forward or backwards to the nearest stopping point.

Go down the elevator and go into Sirrus' bedroom. It is the more foppish of the two bedrooms with a toy bird and, more importantly, a telescope. You should see a skeleton image in the telescope's viewfinder and the achievement will pop without you interacting with anything in the room.

Note: some have commented on the achievement not popping for them. I had it unlock on my first try, but to be safe, make a manual save before attempting this. I attempted this while going through a random run and I do not know if it is affected by completing the good white page ending or not.

And yes I know that the fortress rotates anticlockwise, however the gear mechanism you control rotates clockwise.

EDIT: Browsing online forums, no one seems to have found a cause for the achievement not unlocking. Folks having continued issues might consider reaching out to the publisher on this. For myself, I had already beaten the game once, then started a new random game. I completed the Selenetic Age first, grabbing both colored pages in the age. I then traveled to the Mechanical Age, rotated the fortress around to 90 then 180 degrees in order to solve the Myst age linking book puzzle. I also grabbed the red and blue pages in the age. Then, before going back to Myst, I rotated the fortress to the 270 degree position, approached the telescope, and popped the achievement.

This popped by rotating the mechanical house 180 degrees from the starting point by holding the lever approx 10 seconds. I did not need go down to the main level and look through the telescope which is how you would actually see the skeleton. This seems to be bugged.

Go to the mechanical age. Go into the house, past either room find the hallway with the unusable elevator. Before traveling the hall, hit the button to lower the ramp, and aligning the gap in the circles so the elevator door is revealed. Go back up the stairs and press the button again to raise the ramp. Take the elevator to the top floor. Before exiting the elevator, press the middle button and go out onto the landing. After the beeping stops, the elevator will go halfway down and you will be presented with 2 levers. Use the left lever to unlock the right lever. Hold the right lever for 10 seconds. Let go, and it should pop. If not, go down the elevator and go into the room with the telescope to look for the skeleton.

Hey Guys I figured I'd help out with how I got this one since it is buggy. The game is still relatively new so It's bound to happen. Also, this was the last achievement that I was working on. I tried multiple methods(i.e. making a save at the beginning of the Mechanical level, starting a new game on classic, starting a new game on random), but this one worked for me.

I started up a new game, finished it with the good ending (turning in the white page on classic). This should take ~ 2 minutes so you can double up with the Speedy achievement if you want.

Start up another new game on classic, and traverse your way back to the mechanical level and rotate the fortress 270 degrees (I didn't hold for 10 seconds cause that is basically a full unnecessary rotation). But if you do have "Display Gameplay Context Subtitles” enabled as stated by some in the comments section, if you pull on the left lever it will say “Spin Sound Effect" which is the correct amount of rotation. From there, go to the telescope and look at the lens and it should pop.

Edit (This was played on the PC version.) Also note when doing the rotation of the elevator in the basement, there were certain times when the opening was lined up correctly but the glass door did not open. The game should have auto-saved and you should be able to re-load it to fix that bug.