* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  background: #05070d;
  color: white;
  font-family: Arial, Helvetica, sans-serif;
  display: flex;
}

/* MENÚ LATERAL */

.sidebar {
  width: 260px;
  min-height: 100vh;
  background: #080d1a;
  border-right: 1px solid #1e2a44;
  padding: 25px 20px;
  position: fixed;
  left: 0;
  top: 0;
  text-align: center;
}

.logo {
  width: 95px;
  margin-bottom: 15px;
}

.sidebar h1 {
  font-size: 22px;
  margin-bottom: 8px;
}

.sidebar p {
  color: #9ca7bd;
  font-size: 14px;
  line-height: 1.4;
  margin-bottom: 30px;
}

nav {
  display: flex;
  flex-direction: column;
  gap: 12px;
}
nav a {
    display: block;
    width: 220px;
    margin: 12px 0;
    padding: 16px;
    text-align: center;
    background: rgba(120,180,255,0.18);
    border: 1px solid rgba(255,255,255,0.15);
    border-radius: 18px;
    color: white;
    text-decoration: none;
    font-size: 18px;
    font-weight: 600;
    transition: 0.3s;
    backdrop-filter: blur(6px);
}


nav a:hover {
  background: #1d2b4a;
  transform: translateX(6px);
}

/* CONTENIDO */

.content {
  margin-left: 260px;
  width: calc(100% - 260px);
  padding: 25px;
}

.hero {
  text-align: center;
}

.hero h2 {
  font-size: 30px;
  margin-bottom: 6px;
}

.hero p {
  color: #aab3c5;
  margin-bottom: 18px;
}

.main-image {
  width: 100%;
  max-width: 1100px;
  border-radius: 14px;
  border: 1px solid #2b3550;
}

.grid {
  display: flex;
  gap: 22px;
  justify-content: center;
  flex-wrap: wrap;
  margin-top: 30px;
}

.card {
  background: #0b1020;
  border: 1px solid #222f4c;
  border-radius: 14px;
  padding: 15px;
  width: 220px;
  text-align: center;
}

.card h3 {
  margin-bottom: 12px;
}

.card img {
  width: 100%;
  border-radius: 10px;
}

/* MÓVIL */

@media (max-width: 800px) {

  body {
    display: block;
  }

  .sidebar {
    position: sticky;
    top: 0;
    width: 100%;
    min-height: auto;
    z-index: 999;
    padding: 10px;
    border-right: none;
    border-bottom: 1px solid #1e2a44;
  }

  .logo {
    width: 55px;
    margin-bottom: 5px;
  }

  .sidebar h1 {
    font-size: 18px;
  }

  .sidebar p {
    display: none;
  }

  nav {
    flex-direction: row;
    flex-wrap: nowrap;
    overflow-x: auto;
    gap: 8px;
    margin-top: 10px;
    padding-bottom: 4px;
  }

  nav a {
    font-size: 13px;
    padding: 8px 10px;
    white-space: nowrap;
  }

  .content {
    margin-left: 0;
    width: 100%;
    padding: 12px;
  }

  .hero h2 {
    font-size: 23px;
  }

  .hero p {
    font-size: 14px;
  }

  .main-image {
    border-radius: 10px;
  }

  .card {
    width: 100%;
  }

  .astro-card {
    width: 100%;
  }
}
<aside class="sidebar">
  <img src="img/logo.png" class="logo" alt="Cielos del Andévalo">

  <h1>Cielos del Andévalo</h1>

  <p>
    Observatorio astronómico<br>
    Valverde del Camino
  </p>

  <nav>
    <a href="index.html">Inicio</a>
    <a href="astrofotografia.html">Astrofotografía</a>
    <a href="observatorio.html">El Observatorio</a>
    <a href="sobre-mi.html">Sobre mí</a>
  </nav>
</aside>

<main class="content">

<section class="hero">
  <h2>Astrofotografía</h2>

  <p>
    
  </p>
</section>

<section class="astro-sections">

  

    <a href="#">
      Entrar
    </a>
  </div>

</section>

</main>

</body>
</html>
/* AJUSTE PROFESIONAL SIN ROMPER LA WEB */

.sidebar {
  background: linear-gradient(180deg, #071326, #020814);
}

nav a {
  max-width: 220px;
  margin: 0 auto;
}

.content {
  padding-top: 30px;
}

.hero h2 {
  font-size: 34px;
}

.main-image {
  display: block;
  margin: 20px auto;
  max-width: 1150px;
}

.grid {
  margin-top: 35px;
}

.card {
  max-width: 420px;
}

@media (max-width: 800px) {
  .sidebar {
    padding: 14px 10px;
  }

  .logo {
    width: 70px;
  }

  .sidebar h1 {
    font-size: 24px;
  }

  nav a {
    max-width: none;
  }

  .hero h2 {
    font-size: 25px;
  }
}/* TOQUE VISUAL PROFESIONAL */

body {
  background:
    radial-gradient(circle at top right, rgba(45, 95, 160, 0.25), transparent 35%),
    radial-gradient(circle at bottom left, rgba(20, 60, 120, 0.2), transparent 40%),
    #020814;
}

.sidebar {
  box-shadow: 0 0 30px rgba(0,0,0,0.4);
}

.logo {
  filter: drop-shadow(0 0 12px rgba(120,180,255,0.35));
}

.sidebar h1 {
  letter-spacing: 0.5px;
  text-shadow: 0 0 12px rgba(120,180,255,0.35);
}

.hero {
  background: rgba(8, 16, 32, 0.55);
  border: 1px solid rgba(255,255,255,0.08);
  border-radius: 22px;
  padding: 25px;
  box-shadow: 0 0 35px rgba(0,0,0,0.35);
}

.main-image {
  box-shadow: 0 0 35px rgba(80,140,220,0.25);
}

.card {
  background: rgba(8, 16, 32, 0.75);
  border: 1px solid rgba(120,180,255,0.18);
  box-shadow: 0 0 25px rgba(0,0,0,0.35);
  transition: 0.3s;
}

.card:hover {
  transform: translateY(-4px);
  border-color: rgba(120,180,255,0.45);
}

.card h3 {
  color: #dbe8ff;
}/* TARJETAS ASTROFOTOGRAFÍA PROFESIONALES */

.astro-sections {
  display: flex;
  gap: 24px;
  flex-wrap: wrap;
  justify-content: center;
  margin-top: 40px;
}

.astro-card {
  width: 300px;
  height: 180px;
  border-radius: 22px;
  text-decoration: none;
  color: white;
  display: flex;
  align-items: flex-end;
  justify-content: center;
  padding: 25px;
  background: linear-gradient(180deg, rgba(10,20,40,0.15), rgba(2,8,20,0.95));
  border: 1px solid rgba(120,180,255,0.25);
  box-shadow: 0 0 30px rgba(0,0,0,0.45);
  transition: 0.3s;
}

.astro-card h3 {
  font-size: 28px;
  text-shadow: 0 0 12px black;
}

.astro-card:hover {
  transform: translateY(-6px);
  border-color: rgba(120,180,255,0.7);
  box-shadow: 0 0 35px rgba(80,140,220,0.35);
}

.astro-gran-campo {
  background:
    linear-gradient(180deg, rgba(10,20,40,0.1), rgba(2,8,20,0.95)),
    url("img/gran-campo.jpg") center/cover;
}

.astro-cielo-profundo {
  background:
    linear-gradient(180deg, rgba(10,20,40,0.1), rgba(2,8,20,0.95)),
    url("img/cielo-profundo.jpg") center/cover;
}

.astro-sistema-solar {
  background:
    linear-gradient(180deg, rgba(10,20,40,0.1), rgba(2,8,20,0.95)),
    url("img/sistema-solar.jpg") center/cover;
}