Adam observes the moon every other day for one month. He notices that over this time period, the moon looks slightly different in how much it's lighted. What accounts for the difference in the appearance of the moon over this time period?
1) the angle of the moon's orbit around Earth
2) the speed of the moon's orbit around Earth
3) the position of the moon in its orbit around Earth
4) the distance between Earth and the moon in its orbit around Earth