js
function mouseImage(event) {
  document.getElementById("UFO").style.left= event.x -100 + "px";
  document.getElementById("UFO").style.top = event.y - 50 + "px";
}

function changeBGImage(event) {
  document.getElementById("Copyright").textContent  = "住所：地球-日本-長野県-上伊那郡箕輪町中箕輪";
  const half = document.body.clientWidth / 2;
  if (event.x >= half) {
    i = (i + 1) % 15;
  } else {
    if (i === 0) {
      i = 14;
    } else {
      i = i - 1;
    }
  }
  document.body.style.backgroundImage = "url('earth_" + i + ".jpg')";
}


css
background: url("starry_sky.jpg") center center /cover fixed;
