The Center button centers your selected satellite in the sky chart. Use this button if your satellite has moved off screen, and you want to re-center it in the field of view. The selected satellite will stay centered if you zoom in or out, or animate the sky chart using the Time Flow controls.
If your device's motion sensors are turned on, the Center button changes name to Locate. When you tap Locate, an arrow appears, leading you toward your selected satellite. Follow the arrow with your device to center the satellite on screen. When the satellite is centered, and your phone will be pointing toward the satellite in the sky.