Imagine beams of light in your backyard patio keeping the bugs away or a beam across your bedroom window, keeping the mosquitoes from entering your room.
During radiation therapy, you're positioned on a table and a large machine moves around you, directing radiation beams to specific areas of your body in order to target your cancer.
Find this orientation by looking through the crystal at the sky at a time when you can see the sun, mark the sun's direction on the crystal, and your mark will always point towards the sun when you match the beams from even a tiny patch of blue in an otherwise overcast sky.