TIL How To Read A Turntable

A strobe light flashes rapidly. Most commonly at 100 (50Hz version) or 120 times a second (60Hz version).
The platter or disc is marked with dots or lines. A flash illuminates each individual line as it passes. When the speed is accurate, the motion of the marks is "frozen" by the strobe light. Slight speed inaccuracy shows as a gradual drift in the markings.