.loader {
  border-radius: 50%;
  height: 4em;
  width: 4em;

  // center
  margin: auto;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;

  box-shadow: -.5em 0;
  animation: moon 1s alternate infinite linear;
}

@keyframes moon {
  to {
    box-shadow: .5em 0;
  }
}