The opened bottom half of the unit sphere has a narrower groove than the top half. Take 2 loosely threaded nuts bolts & place the nut ends into the groove at the so they are 90 degrees apart.
Then, gently finger tighten the nut-bolts & then press the 2 halves together. The top half has a guide pin which you can use to line up with an indent on the bottom half.
Click on the Details Tab below for assembly instructions.