Center one fixed 'arrow' element inside another fixed 'circle' element

Question

I am having trouble centering the arrow inside the red circle. How can I center one fixed element inside another fixed element?

JsFiddle: https://jsfiddle.net/sebastian3495/xtj9cga2/4/

Code

html, body {
  height: 1000px;
  width: 100%;
}
#a {
  width: 100%;
  height: 100%;
  border: 1px solid black;
  position:relative;
}
#wrapper {
  position: absolute;
  top: 50vh;
}
#b {
  width: 100px;
  height: 100px;
  border-radius: 50%;
  background: red;
  position: fixed;
}
#c {
  border: solid black;
  border-width: 0 3px 3px 0;
  position: fixed;
  width: 50px;
  height: 50px;
  transform: rotate(-45deg);
}

<div id="a">
  <div id="wrapper">
    <i id="b"></i>
    <i id="c"></i>
  </div>
</div>

Answer 1

You can simply your code like below then you can easily center your arrow and also adjust the dimension:

.arrow {
  background:red;
  width:100px;
  height:100px;
  border-radius:50%;
  position:fixed;
  top:100px;
  left:50px;
}
.arrow::before {
  content:"";
  position:absolute;
  top:50%;
  left:50%;
  width:50%;
  height:50%;
  border-top:3px solid;
  border-right:3px solid;
  /*75% instead of 50% since we need to center half the shape so 50% then 25%*/
  transform:translate(-75%,-50%) rotate(45deg);
  
}

<div class="arrow"></div>

You can still simplify more without pseudo element:

.arrow {
  width:100px;
  height:100px;
  padding:35px 35px 0 0;
  border-radius:50%;
  position:fixed;
  top:100px;
  left:50px;
  background:
    linear-gradient(#000,#000) top right/77% 3px,
    linear-gradient(#000,#000) top right/3px 77%,
    red;
  background-repeat:no-repeat;
  background-origin:content-box;
  transform:rotate(45deg);
  box-sizing:border-box;
}

<div class="arrow"></div>