body {
    margin: 0;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: black;
}

.container {
    font-size: 10px;
    width: 40em;
    height: 40em;
    position: relative;
    display: flex;
    align-items: center;
    justify-content: center;
}

.sun {
    position: absolute;
    width: 10em;
    height: 10em;
    background-color: yellow;
    border-radius: 50%;
    box-shadow: 0 0 3em white;
}

.earth,
.moon {
    position: absolute;
    width: calc(var(--diameter) * 1em);
    height: calc(var(--diameter) * 1em);
    border-width: 0.1em;
    border-style: solid solid none none;
    border-color: silver transparent transparent transparent;
    border-radius: 50%;
    animation: orbit linear infinite;
    animation-duration: calc(var(--duration) * 1s);
}

@keyframes orbit {
    to {
        transform: rotate(1turn);
    }
}

.earth {--diameter: 30; --duration: 36.5;}
.moon {--diameter: 8; --duration: 2.7; top: 0.3em; right: 0.3em;}

.earth::before,
.moon::before {
    content: '';
    position: absolute;
    width: calc(var(--diameter) * 1em);
    height: calc(var(--diameter) * 1em);
    background-color: var(--color);
    border-radius: 50%;
    top: calc(var(--top) * 1em);
    right: calc(var(--right) * 1em);
}

.earth::before {--diameter: 3; --color: dodgerblue; --top: 2.8; --right: 2.8;}
.moon::before {--diameter: 1.2; --color: white; --top: 0.8; --right: 0.2;}