:root{color-scheme:light;--bg:#f5f6f8;--surface:#fff;--raised:#edf0f4;--ease:cubic-bezier(.22,1,.36,1);--ink:#202c3b;--muted:#5d6b7d;--line:#dce2e9;--blue:#2366bd;--soft:#eaf1fb;--night:#101f34;--font:-apple-system,BlinkMacSystemFont,"PingFang SC","Microsoft YaHei",sans-serif}*{box-sizing:border-box}html{scroll-behavior:smooth;scroll-padding-top:85px}body{margin:0;background:var(--bg);color:var(--ink);font:15px/1.7 var(--font)}a{color:inherit;text-decoration:none}button,input,select{font:inherit}button,a,input{touch-action:manipulation}button{cursor:pointer}button:disabled{opacity:.4;cursor:default}:focus-visible{outline:3px solid #4285d6;outline-offset:4px}header{height:72px;padding:0 max(28px,calc((100vw - 1240px)/2));display:flex;align-items:center;justify-content:space-between;position:sticky;top:0;background:#f5f6f8f5;backdrop-filter:none;z-index:10;border-bottom:1px solid var(--line)}.brand{font-size:23px;letter-spacing:-1px;font-weight:500}.brand b{font-weight:500}.brand-star{color:var(--blue);margin-left:12px}nav{display:flex;gap:32px;font-size:13px}nav a:hover{color:var(--blue)}main{min-height:75vh}.wrap{max-width:1240px;margin:auto;padding:0 28px}.hero{background:var(--night);color:#f6f8fc;position:relative;overflow:hidden;min-height:530px;display:flex;align-items:center}.hero svg{position:absolute;width:66%;height:100%;right:-6%;top:0}.hero .wrap{width:100%;position:relative;pointer-events:none}.hero-copy{max-width:540px;padding:68px 0;pointer-events:auto}.eyebrow{text-transform:none;letter-spacing:0;font-size:12px;font-weight:600;color:var(--blue);margin:0 0 16px}.hero .eyebrow{color:#98b8e2}.hero h1{font-size:70px;font-weight:500;letter-spacing:-3px;line-height:1.1;margin:0 0 24px}.hero h1 small{font-size:29px;letter-spacing:0;display:block;margin-top:22px}.hero p{color:#aebed1;max-width:390px;font-size:16px;margin-bottom:32px}.button{background:var(--blue);border:1px solid var(--blue);border-radius:8px;color:white;padding:12px 20px;display:inline-flex;align-items:center;justify-content:center;gap:10px;min-height:44px;font-size:14px;transition:background .16s var(--ease),border-color .16s var(--ease),color .16s var(--ease)}.button:hover{background:#18569f;border-color:#18569f}.button.light{background:white;color:var(--ink);border-color:var(--line)}.button.light:hover{background:var(--soft)}.hero .button{background:#e9f1fc;color:#173963;border:0}.hero-note{margin-left:16px;font-size:12px;color:#9eafc4}.intro-line{display:flex;justify-content:space-between;padding:24px 0;border-bottom:1px solid var(--line);gap:20px;font-size:13px;color:var(--muted)}.intro-line strong{color:var(--ink);font-weight:500}.section{padding:64px 0 0}.section-top{display:flex;justify-content:space-between;align-items:end;gap:20px;margin-bottom:24px}h2{font-weight:500;font-size:29px;line-height:1.3;letter-spacing:-.8px;margin:0 0 10px}h3{font-weight:600;font-size:17px;margin:0 0 8px}p{margin:0 0 12px}.muted{color:var(--muted)}.small{font-size:12px}.section-top p{margin:0;color:var(--muted)}.filters{display:flex;flex-wrap:wrap;gap:6px;margin-bottom:24px}.filter{border:0;background:transparent;border-radius:6px;padding:8px 16px;color:var(--muted);min-height:40px}.filter.active{background:#e3ebf7;color:#174f95}.search{border:1px solid var(--line);border-radius:6px;background:white;padding:12px 16px;width:240px;max-width:100%}.course-grid{display:grid;grid-template-columns:repeat(4,1fr);gap:24px}.course-card{display:block;background:white;border:1px solid var(--line);border-radius:12px;overflow:hidden;transition:border-color .16s var(--ease),background .16s var(--ease)}.course-card:hover{border-color:#9aaec4;background:#fbfcfe}.thumb{height:138px;background:#edf2f8;padding:8px}.thumb svg{width:100%;height:100%}.card-text{padding:20px}.course-meta{font-size:12px;letter-spacing:0;color:var(--blue);margin-bottom:9px}.course-card h3{font-size:17px;line-height:1.6;min-height:54px}.course-card p{font-size:12px;color:var(--muted);margin:12px 0 0;display:flex;justify-content:space-between}.course-card .done{color:#25754b}.map-intro{max-width:650px}.chapter-list{display:grid;grid-template-columns:1fr 1fr;column-gap:40px;margin-top:20px}.chapter{display:flex;align-items:center;gap:14px;border-bottom:1px solid var(--line);padding:16px 0;font-size:13px}.chapter .number{font-variant-numeric:tabular-nums;color:#7e8a98;width:24px}.chapter span:nth-child(2){flex:1}.chapter a{font-size:12px;color:var(--blue)}.chapter small{color:var(--muted);font-size:12px}.about-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:40px;border-top:1px solid var(--line);padding:28px 0 60px}.about-grid strong{font-size:24px;font-weight:400;color:#95a6bb}.about-grid h3{margin-top:12px}.about-grid p{font-size:13px;color:var(--muted)}footer{border-top:1px solid var(--line);padding:24px max(28px,calc((100vw - 1184px)/2));font-size:12px;color:var(--muted);display:flex;justify-content:space-between;gap:20px}.lesson-head{padding:32px 0}.back{font-size:12px;color:var(--muted)}.lesson-head .eyebrow{margin:24px 0 10px}.lesson-head h1{font-size:37px;font-weight:500;letter-spacing:-1.3px;margin:0 0 12px;line-height:1.35}.lesson-head .lead{font-size:17px;color:var(--muted)}.story{border-left:3px solid #a4b9d3;padding-left:18px;max-width:880px;font-size:14px;line-height:1.9;margin-top:22px}.guess{background:#eaf0f8;border-radius:12px;padding:24px;margin-bottom:24px;display:grid;grid-template-columns:1fr auto;gap:12px;align-items:center}.guess h3{font-size:14px;margin-bottom:4px}.options{display:flex;flex-wrap:wrap;gap:8px}.option{background:white;border:1px solid #d1dce9;border-radius:6px;padding:8px 14px;min-height:42px;color:var(--ink)}.option:hover{border-color:var(--blue)}.option.chosen{background:var(--blue);color:white;border-color:var(--blue)}.feedback{grid-column:1/-1;font-size:13px;color:#35567b;margin:0;min-height:0}.workspace{border:1px solid var(--line);border-radius:12px;overflow:hidden;display:grid;grid-template-columns:minmax(0,1fr) 340px;background:white}.stage-col{min-width:0;border-right:1px solid var(--line)}.stage-title{display:flex;justify-content:space-between;align-items:center;padding:16px 24px;font-size:12px;color:var(--muted)}#scene{display:block;width:100%;aspect-ratio:720/390;background:#f5f8fc}#scene text{font-family:var(--font)}.playbar{display:flex;align-items:center;gap:8px;padding:16px 24px;border-top:1px solid var(--line)}.playbar button{font-size:12px;padding:7px 13px;min-height:38px}.step-count{margin-left:auto;font-size:12px;color:var(--muted)}.caption{padding:0 24px 24px;font-size:14px;min-height:72px}.inspector{padding:24px;min-width:0}.inspector .eyebrow{margin-bottom:12px}.formula{font:15px/1.9 ui-monospace,Menlo,monospace;background:var(--soft);padding:14px;margin:14px 0 22px;border-radius:6px;overflow-wrap:anywhere}.control{margin-top:20px}.control label{display:flex;justify-content:space-between;font-size:12px;gap:8px}.control output{color:var(--blue);font-variant-numeric:tabular-nums}.control input{display:block;width:100%;accent-color:var(--blue);margin:13px 0;height:20px}.result{border-top:1px solid var(--line);margin-top:18px;padding-top:18px}.result .value{font-size:32px;font-weight:500;letter-spacing:-1px;line-height:1.3;color:#1d5599;overflow-wrap:anywhere}.result .small{color:var(--muted);margin-top:6px}.try{padding:18px 22px;background:#f0f4f9;font-size:13px;border:1px solid var(--line);border-top:0;border-radius:0 0 8px 8px;margin:0 10px}.try b{margin-right:10px}.lesson-body{display:grid;grid-template-columns:minmax(0,1fr) 340px;gap:50px;padding-top:36px}.lesson-body h2{font-size:22px}.step-item{display:flex;gap:16px;padding:20px 0;border-bottom:1px solid var(--line)}.step-item .n{flex-shrink:0;color:var(--blue);width:24px;font-size:13px}.step-item h3{font-size:14px;margin-bottom:6px}.step-item p{font-size:13px;color:var(--muted);margin:0}.symbol{display:grid;grid-template-columns:70px 1fr;gap:10px;border-bottom:1px solid var(--line);padding:10px 0;font-size:12px}.symbol b{color:var(--blue)}details{padding:16px 0;border-bottom:1px solid var(--line);font-size:13px}summary{cursor:pointer;font-weight:500;min-height:32px}details p{margin:10px 0;color:var(--muted)}.source-link{color:var(--blue);display:inline-block;margin-top:8px}.quizzes{padding:36px 0}.quiz{padding:20px 0;border-bottom:1px solid var(--line)}.quiz h3{font-size:14px;font-weight:500;margin-bottom:14px}.quiz .feedback{margin-top:10px}.takeaway{border-left:3px solid var(--blue);padding:8px 0 8px 22px;margin:14px 0 32px}.takeaway p{font-size:18px;margin-bottom:0;max-width:800px}.lesson-bottom{display:flex;justify-content:space-between;align-items:center;gap:16px;padding-bottom:48px}.empty{padding:40px;color:var(--muted);grid-column:1/-1}.trace{border-collapse:collapse;width:100%;font-size:12px}.trace th,.trace td{text-align:left;padding:6px;border-bottom:1px solid var(--line)}.fade-in{animation:arrive .35s var(--ease)}.hero-orbit{opacity:1}.svg-reveal{animation:none}@keyframes arrive{from{opacity:0;transform:translateY(8px)}to{opacity:1;transform:translateY(0)}}@media(max-width:1000px){.course-grid{grid-template-columns:repeat(2,1fr)}.workspace{grid-template-columns:minmax(0,1fr) 290px}.lesson-body{grid-template-columns:minmax(0,1fr) 290px;gap:30px}.hero h1{font-size:60px}.hero svg{opacity:.6;right:-17%}}@media(max-width:720px){header{padding:0 20px;height:62px}.brand{font-size:20px}nav{gap:15px;font-size:12px}.wrap{padding:0 20px}.hero{min-height:510px;align-items:start}.hero-copy{padding:45px 0 170px}.hero h1{font-size:53px}.hero h1 small{font-size:24px}.hero p{font-size:14px;max-width:280px}.hero svg{width:105%;height:80%;top:34%;right:-30%;opacity:.5}.hero-note{display:none}.intro-line{font-size:12px;flex-wrap:wrap;gap:8px}.section{padding-top:40px}.section-top{align-items:start;flex-direction:column}.search{width:100%}.section-top h2{font-size:26px}.course-grid{gap:12px}.thumb{height:100px}.card-text{padding:13px}.course-card h3{font-size:14px;min-height:46px}.course-card p{font-size:12px}.course-meta{font-size:12px;letter-spacing:0}.chapter-list{grid-template-columns:1fr}.about-grid{grid-template-columns:1fr;gap:16px}.about-grid strong{float:left;margin-right:20px}.about-grid h3{margin-top:0}.about-grid p{margin-left:50px}footer{padding:20px;flex-direction:column;gap:4px}.lesson-head h1{font-size:28px}.lesson-head .lead{font-size:15px}.guess{grid-template-columns:1fr;padding:16px}.workspace{grid-template-columns:1fr}.stage-col{border-right:0}.inspector{border-top:1px solid var(--line)}.lesson-body{grid-template-columns:1fr;gap:24px}.playbar{padding:12px;gap:5px}.playbar button{padding:7px 10px}.caption{padding:0 16px 15px;font-size:13px;min-height:80px}.stage-title{padding:12px 15px}.lesson-bottom{flex-wrap:wrap}.takeaway p{font-size:16px}.try{margin:0;padding:16px}.option{font-size:13px}.lesson-head{padding-top:22px}}@media(prefers-reduced-motion:reduce){*{animation:none!important;transition:none!important;scroll-behavior:auto!important}}
/* Guided routes and reproducible edge cases. */
.path-grid{display:grid;grid-template-columns:1fr 1fr;gap:40px;margin-top:26px}.path{border-top:1px solid var(--line);padding-top:22px;position:relative}.path-number{float:right;color:#9aaabe;font-size:29px;font-weight:400}.path p{color:var(--muted);font-size:13px}.path ol{list-style:none;margin:20px 0 0;padding:0;counter-reset:route}.path li{counter-increment:route;border-top:1px solid var(--line)}.path li a{display:flex;gap:14px;padding:12px 0;font-size:13px}.path li a:before{content:counter(route,decimal-leading-zero);color:#91a1b4;font-size:12px}.path li a span{margin-left:auto;color:var(--blue)}.path li a:hover{color:var(--blue)}.chapter-links{display:flex;flex-direction:column;align-items:flex-end;gap:5px}.presets{display:flex;gap:8px;flex-wrap:wrap;margin-top:14px}.presets .button{padding:7px 12px;font-size:12px;min-height:38px}.result .value{font-size:29px}.lesson-body{padding-top:36px}@media(max-width:720px){.path-grid{grid-template-columns:1fr;gap:24px}.result .value{font-size:26px}.chapter-links a{font-size:12px;max-width:155px;text-align:right}.chapter{gap:10px}}
.stage-title{gap:10px}.stage-title>span:nth-child(2){margin-left:auto}.zoom-button{border:0;border-left:1px solid var(--line);padding:0 0 0 10px;color:var(--blue);background:transparent;min-height:32px;white-space:nowrap;font-size:12px}#diagram-modal{width:min(1040px,96vw);max-width:96vw;max-height:94vh;padding:22px;border:1px solid var(--line);border-radius:12px;color:var(--ink);background:var(--bg)}#diagram-modal::backdrop{background:#14263ccc;backdrop-filter:blur(3px)}.diagram-heading{display:flex;justify-content:space-between;align-items:center;gap:16px;margin-bottom:10px}.diagram-heading h2{font-size:20px;margin:0}.diagram-scroll{width:100%;overflow:auto;background:#f5f8fc}.diagram-scroll svg{display:block;width:100%;min-width:720px;height:auto}.diagram-heading .button{flex-shrink:0}@media(max-width:720px){#diagram-modal{padding:15px}.diagram-heading h2{font-size:16px}.stage-title{font-size:12px;gap:6px}.zoom-button{font-size:12px}}
/* Phenomenon first: a visible situation, a purposeful action, then its explanation. */
.experience-tabs{display:flex;gap:6px;align-items:center;margin:0 0 14px;flex-wrap:wrap}.experience-tabs button{border:1px solid var(--line);background:transparent;padding:12px 16px;min-height:44px;border-radius:8px;font-size:13px;color:var(--muted)}.experience-tabs button[aria-pressed=true]{background:#234d70;border-color:#234d70;color:white}.experience-tabs span{margin-left:auto;font-size:12px;color:var(--muted)}.experience-workspace{grid-template-columns:minmax(0,1fr) 280px}.experience-workspace .inspector{padding:20px}.formula-disclosure{padding:8px 0;margin-bottom:12px}.formula-disclosure summary{font-size:12px;min-height:28px;color:var(--muted)}.formula-disclosure .formula{margin:8px 0;font-size:13px}.experience-workspace .result .value{font-size:25px}.experience-workspace #step-title{line-height:1.7}.advanced-controls{margin-bottom:12px;font-size:12px}.experience-toolbar{padding:24px 28px;background:#eaf0f4;border:1px solid var(--line);border-top:0;border-radius:0 0 10px 10px;margin:0 8px}.experience-toolbar>p:first-child{font-size:14px;color:#465e71}.experience-actions{display:flex;flex-wrap:wrap;gap:8px}.experience-feedback{display:flex;flex-direction:column;gap:6px;margin-top:20px;padding:16px 0 2px;border-top:1px solid #cddbe5}.experience-feedback strong{font-size:21px;font-weight:500;color:#265a7c}.experience-feedback span{font-size:14px;color:#526d81}.experience-note{font-size:12px;color:#778a99;margin:14px 0 0}.direct-manipulation{touch-action:none;cursor:grab}.direct-manipulation:active{cursor:grabbing}.experience-workspace .caption{min-height:96px}.experience-workspace #scene{height:auto}.experience-workspace .stage-title{font-size:12px}.experience-workspace .stage-title>span:nth-child(2){max-width:46%;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}
@media(max-width:900px){.experience-workspace{grid-template-columns:1fr}.experience-workspace .stage-col{border-right:0}.experience-workspace .inspector{border-top:1px solid var(--line)}.experience-workspace .caption{min-height:0}.experience-tabs span{display:none}}
@media(max-width:720px){.experience-toolbar{padding:18px;margin:0}.experience-feedback strong{font-size:19px}.experience-feedback span{font-size:13px}.experience-actions .button{font-size:12px;padding:9px 12px}.experience-tabs button{padding:10px 12px;font-size:12px}.experience-workspace .caption{font-size:14px;line-height:1.9}.experience-workspace .playbar{flex-wrap:wrap}.experience-workspace .inspector .control-name{font-size:13px}}
.season-guide{margin:8px 0 18px;font-size:12px;line-height:1.7}.season-ticks{display:flex;justify-content:space-between;gap:2px}.season-ticks button{flex:1;min-width:0;padding:6px 1px;border:0;background:transparent;border-radius:5px;color:#5f788b;min-height:56px}.season-ticks button:hover{background:#e7eef7;color:var(--blue)}.season-ticks b,.season-ticks small{display:block;font-size:12px;white-space:nowrap}.season-ticks b{font-size:12px}.season-guide p{margin:10px 0 0}.season-guide #season-now{color:#24597f;font-weight:500}.season-guide>small{display:block;color:var(--muted);font-size:12px;margin-top:6px}.control output{flex-shrink:0}.experience-workspace .control label{flex-wrap:wrap}

/* Solar banner: orbit detail with a quiet, open lower edge. */
.hero{min-height:620px;align-items:center}.hero-copy{max-width:43%;padding:68px 0}.hero h1{font-size:64px}.hero h1 small{font-size:26px}
.solar-banner{position:relative;width:55%;margin-left:44%;padding:24px 28px 26px 0;z-index:1;animation:none}
.hero>.wrap{position:absolute;inset:0;display:flex;align-items:center;z-index:2}
.solar-heading{display:flex;align-items:center;font-size:12px;color:#d8e6f7;gap:16px}.solar-heading>span{white-space:nowrap}
.solar-heading i{display:inline-block;width:5px;height:5px;border-radius:50%;background:#e9c88b;margin:0 7px 2px 0}
.solar-controls{display:flex;gap:4px}.solar-controls button{display:grid;place-items:center;width:36px;min-height:36px;padding:0;border:0;border-radius:50%;background:transparent;color:#96aec7;transition:color .2s,background .2s}
.solar-controls button:hover,.solar-controls button[aria-pressed=true]{background:#253d56;color:#e7c68d}
.hero .solar-controls svg{position:static;display:block;width:18px;height:18px;opacity:1;fill:none;stroke:currentColor;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round}
.solar-time{display:flex;align-items:center;gap:12px;flex-wrap:wrap;font-size:12px;color:#7f99b5;min-height:22px}.solar-clock{font-size:12px;color:#9fb4cc;font-variant-numeric:tabular-nums}
.solar-time [data-mode]:before{content:"";display:inline-block;width:3px;height:3px;background:currentColor;border-radius:50%;margin:0 8px 3px 0}
.hero .solar-map{position:static;display:block;width:100%;height:auto;opacity:1;overflow:visible}
.solar-map text{font:12px var(--font);fill:#c6d6e8;paint-order:stroke;stroke:#101f34;stroke-width:4px;stroke-linejoin:round}.solar-map .solar-sun-label{fill:#eac991;font-size:12px}.solar-map .lunar-inset{transform:translate(460px,0)}.solar-map .lunar-inset text{font-size:12px}.solar-selection{transition:opacity .25s}
@media(min-width:1500px){.solar-banner{max-width:800px;margin-left:calc(50% - 40px)}}
@media(max-width:1000px) and (min-width:721px){.hero h1{font-size:50px}.hero h1 small{font-size:21px}.hero-copy{max-width:43%}.hero p{font-size:14px}.solar-banner{padding-right:20px}.hero-note{display:none}}
@media(max-width:720px){.hero{display:flex;flex-direction:column;min-height:0;align-items:stretch}.hero>.wrap{position:relative;order:0;display:block}.hero-copy{max-width:100%;padding:38px 0 18px}.hero h1{font-size:50px;margin-bottom:20px}.hero h1 small{font-size:23px;margin-top:16px}.hero p{max-width:340px;margin-bottom:24px}.solar-banner{order:1;width:100%;max-width:580px;margin:0 auto;padding:8px 20px 26px}.hero .solar-map{width:100%;height:auto;opacity:1}.solar-clock{font-size:12px}.solar-controls button{width:44px;min-height:44px}.solar-map text{font-size:19px}.solar-map [data-body="mercury"] text,.solar-map [data-body="venus"] text,.solar-map [data-body="earth"] text,.solar-map [data-body="mars"] text{display:none}.solar-map [data-body].is-selected text{display:block}.solar-map .solar-sun-label{font-size:18px}.solar-map .lunar-inset{transform:translate(436px,-113px) scale(1.25)}.solar-map .lunar-inset text{font-size:14px}}

.chapter-related{display:flex;flex-wrap:wrap;gap:9px;padding:0 0 28px}.chapter-related h3{width:100%;font-size:14px;color:var(--muted);margin:0 0 4px}.chapter-related .button{font-size:12px;padding:9px 12px}

/* Night sky / observing handbook.
   Art direction guided by unslop by Damir Sokolovsky (@heydamir), CC BY 4.0.
   Content diagrams remain scientific views, never decorative backdrops. */
:root {
  --display: "Songti SC", "STSong", "Noto Serif CJK SC", "Source Han Serif SC", SimSun, serif;
  --surface: #fff;
  --raised: #edf0f4;
  --muted: #59687a;
}
body { font-variant-numeric: tabular-nums; }
.wrap { max-width: 1344px; padding-inline: 32px; }
header { height: 80px; padding-inline: max(32px, calc((100vw - 1280px)/2)); background: var(--bg); }
.brand { display: inline-flex; align-items: center; gap: 4px; font-size: 22px; font-weight: 500; letter-spacing: 0; }
.brand-mark { width: 18px; height: 18px; margin-left: 8px; color: var(--blue); fill: none; stroke: currentColor; stroke-width: 1.5; }
nav { font-size: 14px; gap: 32px; }
nav a { padding: 8px 0; border-bottom: 1px solid transparent; transition: color .16s var(--ease), border-color .16s var(--ease); }
nav a:hover { border-color: currentColor; }
.is-home header { color: #eef2f7; background: var(--night); border-color: #ffffff18; }
.is-home .brand-mark { color: #b4cce6; }
.is-home nav a:hover { color: #fff; }
.is-home header :focus-visible { outline-color: #d9e6f7; }
/* One composition: a large question and the actual sky calculation. */
.hero {
  display: grid; grid-template-columns: minmax(0, .85fr) minmax(0, 1.15fr);
  min-height: 680px; padding: 56px max(32px, calc((100vw - 1280px)/2)) 64px;
  gap: 0; align-items: center;
}
.hero > .wrap { position: relative; inset: auto; display: block; grid-column: 1; grid-row: 1; width: 100%; max-width: none; padding: 0; }
.hero-copy { max-width: 100%; padding: 0; }
.hero .eyebrow { color: #afc2da; font-size: 13px; margin-bottom: 32px; }
.hero h1 { font-family: var(--display); font-size: clamp(56px, 6.1vw, 88px); font-weight: 400; letter-spacing: -.03em; line-height: 1.25; margin: 0 0 32px; }
.hero p { max-width: 360px; font-size: 16px; line-height: 1.9; color: #afbed1; margin-bottom: 32px; }
.hero .button { min-height: 48px; background: #eef2f7; color: #152c47; padding: 12px 24px; border-radius: 8px; }
.hero .button:hover { background: #fff; }
.hero .button span { margin-left: 16px; }
.solar-banner { grid-column: 2; grid-row: 1; width: 100%; max-width: none; margin: 0; padding: 0; animation: none; }
.solar-heading { font-size: 13px; gap: 16px; }
.solar-time { color: #a0b2c8; }
.solar-controls button { border-radius: 8px; }
.hero .solar-map { width: 100%; }
/* Large section numbers act like chapter marks in an observing handbook. */
.eyebrow { font-size: 12px; font-weight: 400; color: var(--muted); letter-spacing: 0; text-transform: none; }
h2 { font-family: var(--display); font-size: 40px; font-weight: 400; line-height: 1.4; letter-spacing: 0; margin: 0 0 24px; }
h3 { font-weight: 500; }
.intro-line { padding: 24px 0; align-items: center; gap: 24px; font-size: 13px; }
.intro-line strong { font-weight: 400; }
.path-grid { gap: 64px; margin-top: 40px; }
.path { padding-top: 24px; }
.path-number { font: 400 56px/1 ui-monospace, Menlo, monospace; color: #a6b3c2; }
.path h3 { font-size: 22px; max-width: calc(100% - 88px); }
.path > p { min-height: 44px; padding-right: 80px; }
.path ol { margin-top: 24px; }
.path li a { min-height: 52px; align-items: baseline; gap: 16px; }
.path li a::before { font: 400 12px/1.7 ui-monospace, Menlo, monospace; color: var(--muted); }
.section-top { gap: 32px; align-items: end; margin-bottom: 32px; }
.section-top p { font-size: 15px; }
.search { min-height: 48px; width: 288px; padding: 12px 16px; background: transparent; border-radius: 8px; }
.search::placeholder { color: var(--muted); opacity: 1; }
.filters { display: flex; gap: 8px; padding-bottom: 16px; margin-bottom: 32px; border-bottom: 1px solid var(--line); }
.filter { padding: 8px 16px; border-radius: 8px; }
.filter:hover { color: var(--ink); background: var(--raised); }
.filter.active { color: #174f95; background: #e8edf4; box-shadow: inset 0 0 0 1px #cbd6e3; }
.course-grid { grid-template-columns: repeat(3, minmax(0,1fr)); gap: 32px; }
.course-card { display: flex; flex-direction: column; border-radius: 8px; background: transparent; border-color: #cfd8e3; }
.course-card:hover { background: #fff; border-color: #879eb8; }
.thumb { height: 224px; padding: 16px; background: #e9eef3; }
.card-text { display: flex; flex-direction: column; flex: 1; padding: 24px; }
.course-meta { font-size: 12px; color: var(--muted); margin-bottom: 16px; line-height: 1.7; }
.course-card h3 { font-size: 19px; font-weight: 500; line-height: 1.7; min-height: 64px; margin-bottom: 24px; }
.course-card p { font-size: 12px; margin-top: auto; padding-top: 16px; border-top: 1px solid var(--line); gap: 8px; }
#map .map-intro { max-width: 760px; font-size: 15px; line-height: 1.9; }
.chapter-list { column-gap: 64px; margin-top: 32px; }
.chapter { min-height: 64px; gap: 20px; padding: 20px 0; }
.chapter .number { font: 400 18px/1.7 ui-monospace, Menlo, monospace; width: 32px; color: #65788e; flex-shrink: 0; }
.chapter-links { gap: 8px; }
.chapter-links a { border-radius: 4px; }
.chapter-links a:hover, .source-link:hover { text-decoration: underline; text-underline-offset: 4px; }
.about-grid { margin-top: 32px; padding: 32px 0 80px; gap: 48px; }
.about-grid strong { font: 400 40px/1 ui-monospace, Menlo, monospace; color: #a6b3c2; }
.about-grid h3 { font-size: 20px; margin: 24px 0 12px; }
.about-grid p { font-size: 14px; line-height: 1.9; }
footer { padding: 32px max(32px, calc((100vw - 1280px)/2)); font-size: 12px; }
/* Lesson pages read as a field notebook around the same interactive experiment. */
.lesson-head { padding: 48px 0 32px; }
.lesson-head .eyebrow { margin: 32px 0 16px; }
.lesson-head h1 { font-family: var(--display); font-size: 48px; font-weight: 400; letter-spacing: 0; line-height: 1.5; margin-bottom: 20px; }
.lesson-head .lead { font-size: 18px; }
.story { max-width: 920px; border-color: #a4b9d3; margin-top: 24px; padding-left: 24px; color: #465568; }
.guess { padding: 24px; background: #edf0f4; border: 1px solid var(--line); border-radius: 8px; margin-bottom: 24px; }
.experience-tabs { gap: 8px; margin-bottom: 16px; }
.experience-tabs button { border-radius: 8px; }
.experience-tabs button[aria-pressed=true] { color: #174f95; border-color: #bdcddd; background: #e8edf4; }
.workspace { border-radius: 12px; }
.stage-title { min-height: 56px; padding: 16px 24px; }
.inspector, .experience-workspace .inspector { padding: 24px; }
.inspector .eyebrow { line-height: 1.8; }
.playbar { gap: 8px; flex-wrap: wrap; padding: 16px 24px; }
.playbar button { min-height: 40px; padding: 8px 12px; }
.caption { padding: 0 24px 24px; line-height: 1.9; }
.control { margin-top: 24px; }
.control label { font-size: 13px; align-items: baseline; }
.control input { margin: 12px 0; height: 24px; }
.result { margin-top: 24px; padding-top: 24px; }
.result .value { font-size: 32px; letter-spacing: 0; margin-top: 8px; }
.experience-toolbar { padding: 24px; background: #edf0f4; border-radius: 0 0 8px 8px; }
.experience-feedback { padding-top: 16px; gap: 8px; }
.experience-feedback strong { font-weight: 500; color: #265a7c; }
.experience-feedback span { color: #465568; }
.experience-note { color: var(--muted); }
.option { border-radius: 8px; min-height: 44px; }
.option, .experience-tabs button { transition: background .16s var(--ease), border-color .16s var(--ease), color .16s var(--ease); }
.formula { padding: 16px; border-radius: 8px; }
.chapter-related { gap: 8px; padding-bottom: 32px; }
.season-guide > small { font-size: 12px; }
.lesson-body h2 { font-family: var(--font); font-weight: 500; }
.quizzes h2 { font-size: 32px; }
.takeaway p { font-family: var(--display); font-size: 24px; line-height: 1.8; }
:focus-visible { outline: 2px solid #2366bd; outline-offset: 4px; }
.hero :focus-visible { outline-color: #d9e6f7; }
@media (min-width: 1500px) { .solar-banner { margin-left: 0; max-width: none; } }
@media (max-width: 1100px) and (min-width: 721px) {
  .hero { min-height: 600px; grid-template-columns: minmax(0, .9fr) minmax(0, 1.1fr); }
  .hero h1 { font-size: 56px; }
  .hero-copy { max-width: 100%; }
  .hero p { max-width: 300px; font-size: 15px; }
  .course-grid { grid-template-columns: repeat(2, minmax(0,1fr)); gap: 24px; }
}
@media (max-width: 720px) {
  header { height: 64px; padding-inline: 20px; }
  .brand { font-size: 18px; }
  .brand-mark { margin-left: 4px; }
  nav { font-size: 12px; gap: 16px; }
  .wrap { padding-inline: 20px; }
  .hero { display: flex; flex-direction: column; align-items: stretch; min-height: 0; padding: 40px 20px 24px; gap: 32px; }
  .hero > .wrap { padding: 0; order: 0; }
  .hero-copy { padding: 0; }
  .hero .eyebrow { margin-bottom: 24px; font-size: 12px; }
  .hero h1 { font-size: clamp(44px, 11.5vw, 64px); line-height: 1.3; margin-bottom: 24px; }
  .hero p { max-width: 400px; font-size: 15px; margin-bottom: 24px; }
  .solar-banner { order: 1; width: 100%; padding: 0; margin: 0; max-width: none; }
  .hero .solar-map { width: 100%; }
  .intro-line { font-size: 12px; gap: 8px; padding: 20px 0; }
  h2, .section-top h2 { font-size: 30px; line-height: 1.5; margin-bottom: 24px; }
  .path-grid { grid-template-columns: 1fr; gap: 32px; margin-top: 24px; }
  .path-number { font-size: 48px; }
  .path h3 { font-size: 20px; }
  .path > p { padding-right: 64px; }
  .section-top { gap: 16px; margin-bottom: 24px; }
  .search { width: 100%; }
  .filters { gap: 8px; margin-bottom: 24px; }
  .course-grid { grid-template-columns: repeat(2, minmax(0,1fr)); gap: 20px; }
  .thumb { height: 160px; padding: 8px; }
  .card-text { padding: 20px; }
  .course-card h3 { font-size: 17px; min-height: 56px; margin-bottom: 20px; }
  .course-meta { margin-bottom: 12px; }
  .chapter { gap: 12px; padding: 16px 0; }
  .chapter .number { font-size: 16px; width: 24px; }
  .chapter-list { grid-template-columns: 1fr; }
  .about-grid { gap: 32px; padding-bottom: 48px; }
  .about-grid strong { float: none; font-size: 32px; }
  .about-grid h3 { margin: 16px 0 12px; }
  .about-grid p { margin-left: 0; }
  footer { padding: 24px 20px; }
  .lesson-head { padding-top: 24px; }
  .lesson-head .eyebrow { margin-top: 24px; }
  .lesson-head h1 { font-size: 32px; }
  .lesson-head .lead { font-size: 16px; }
  .story { padding-left: 20px; }
  .guess { padding: 20px; }
  .experience-tabs button { padding: 12px; }
  .stage-title, .experience-workspace .stage-title { font-size: 12px; padding: 12px 16px; gap: 8px; }
  .playbar { padding: 12px 16px; }
  .caption { padding: 0 16px 20px; }
  .inspector, .experience-workspace .inspector { padding: 20px; }
  .experience-toolbar { padding: 20px; margin: 0; }
  .takeaway p { font-size: 22px; }
  #diagram-modal { padding: 16px; }
}
@media (max-width: 440px) {
  .solar-map text { font-size: 22px; }
  .solar-map .lunar-inset text { font-size: 18px; }
  .filters { display: grid; grid-template-columns: 1fr 1fr; }
  .filter { text-align: left; }
  .course-grid { grid-template-columns: 1fr; }
  .course-card { display: grid; grid-template-columns: 104px minmax(0,1fr); }
  .thumb { height: 100%; min-height: 176px; padding: 4px; }
  .card-text { padding: 20px 16px; }
  .course-card h3 { min-height: 0; font-size: 17px; margin-bottom: 16px; }
  .course-meta { margin-bottom: 12px; }
}
/* Learning order and browser-local learner records. */
[hidden] { display: none !important; }
.header-actions { display: flex; align-items: center; gap: 24px; min-width: 0; }
.learner-trigger { max-width: 184px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; padding: 8px 12px; min-height: 40px; border: 1px solid var(--line); border-radius: 8px; color: inherit; background: transparent; font-size: 13px; }
.is-home .learner-trigger { border-color: #ffffff30; }
.learner-trigger:hover { background: var(--raised); color: var(--ink); }
.stage-guide { display: grid; grid-template-columns: repeat(3,minmax(0,1fr)); gap: 32px; margin: 32px 0; }
.stage-guide > a { border-top: 1px solid var(--line); padding-top: 24px; }
.stage-guide span { font: 24px/1 ui-monospace,Menlo,monospace; color: var(--muted); }
.stage-guide h3 { margin: 20px 0 12px; font-size: 20px; }
.stage-guide p { color: var(--muted); font-size: 14px; }
.stage-guide small { color: var(--blue); font-size: 13px; }
.stage-guide > a:hover h3 { color: var(--blue); }
.learning-next { border: 1px solid var(--line); border-radius: 8px; padding: 24px; display: flex; align-items: center; justify-content: space-between; gap: 24px; }
.learning-next h3 { margin: 8px 0; }
.learning-next .button { flex-shrink: 0; }
.learning-next p { margin: 0; }
.route-disclosure { margin-top: 24px; }
.level-tabs { display: flex; flex-wrap: wrap; gap: 8px; margin: 0 0 24px; }
.level-tabs button { border: 1px solid var(--line); border-radius: 8px; background: transparent; color: var(--muted); min-height: 44px; padding: 8px 16px; }
.level-tabs button[aria-pressed=true] { color: var(--ink); background: #e7edf4; border-color: #a8bdd4; }
.catalog-note { font-size: 13px; margin: -12px 0 24px; }
.quiz-personal { border-top: 1px solid var(--line); padding: 20px 0; }
.quiz-personal p { color: var(--muted); font-size: 14px; }
#learner-dialog { width: min(560px, calc(100vw - 32px)); max-height: 90vh; overflow: auto; border: 1px solid var(--line); border-radius: 16px; padding: 24px; background: var(--bg); color: var(--ink); }
#learner-dialog::backdrop { background: #101f34b8; }
#learner-dialog h2 { font-family: var(--font); font-size: 24px; }
#learner-dialog p { font-size: 14px; overflow-wrap: anywhere; }
.learner-list { display: grid; gap: 8px; margin: 24px 0; }
.learner-list button { text-align: left; background: white; color: var(--ink); border: 1px solid var(--line); border-radius: 8px; padding: 12px 16px; }
.learner-list button[aria-pressed=true] { background: #e7edf4; border-color: #a8bdd4; }
.learner-list span, .learner-list small { display: block; overflow-wrap: anywhere; }
.learner-list small { margin-top: 4px; font-size: 12px; color: var(--muted); }
#learner-create { border-top: 1px solid var(--line); padding-top: 24px; }
#learner-create label { display: block; margin-bottom: 8px; }
#learner-create > div { display: flex; gap: 8px; }
#learner-create input { min-width: 0; flex: 1; border: 1px solid var(--line); border-radius: 8px; padding: 8px 12px; background: white; color: var(--ink); }
#learner-create .button { flex-shrink: 0; }
#learner-storage-note { margin: 24px 0 0; }
@media(max-width:900px) {
 .stage-guide { gap: 24px; }
 .header-actions { gap: 16px; }
 .learner-trigger { max-width: 128px; }
}
@media(max-width:720px) {
 .stage-guide { grid-template-columns: 1fr; gap: 24px; }
 .stage-guide > a { display: grid; grid-template-columns: 40px 1fr; column-gap: 16px; padding-top: 20px; }
 .stage-guide span { grid-row: span 3; font-size: 20px; }
 .stage-guide h3 { margin: 0 0 8px; }
 .stage-guide p { margin-bottom: 12px; }
 .learning-next { align-items: start; flex-direction: column; padding: 20px; }
 .level-tabs { display: grid; grid-template-columns: repeat(2,minmax(0,1fr)); gap: 8px; }
 .level-tabs button { padding: 8px; font-size: 13px; }
 .catalog-note { margin-top: -8px; }
 .learner-trigger { max-width: 112px; font-size: 12px; padding: 8px; }
 #learner-dialog { padding: 20px; }
 #learner-create > div { flex-direction: column; }
}
@media(max-width:600px) {
 .header-actions { gap: 12px; }
 header nav a[href="#map"] { display: none; }
 .learner-trigger { max-width: 104px; }
}

.topic-select { display: none; }
@media(max-width:720px) {
 .filters[role=group] { display: none; }
 .topic-select { display: flex; align-items: center; gap: 16px; margin: 0 0 24px; font-size: 14px; }
 .topic-select span { color: var(--muted); }
 .topic-select select { flex: 1; min-width: 0; min-height: 44px; border: 1px solid var(--line); border-radius: 8px; padding: 8px 12px; color: var(--ink); background: var(--bg); }
}
/* A subject catalogue with real coverage, distinct from suggested learning depth. */
.start-guidance { margin: 24px 0; color: var(--blue); font-size: 15px; }
.start-guidance a:hover { text-decoration: underline; text-underline-offset: 4px; }
.catalog-heading { display: flex; justify-content: space-between; align-items: baseline; gap: 24px; margin: 8px 0 24px; }
.catalog-heading p { margin: 0; }
.catalog-heading .source-link { margin: 0; white-space: nowrap; font-size: 13px; }
.topic-overview { display: grid; grid-template-columns: repeat(3,minmax(0,1fr)); gap: 0 32px; border-bottom: 1px solid var(--line); }
.topic-card { display: flex; gap: 16px; padding: 32px 0; border-top: 1px solid var(--line); }
.topic-number { font: 13px/1.8 ui-monospace,Menlo,monospace; color: var(--muted); flex-shrink: 0; }
.topic-card h3 { font-size: 21px; line-height: 1.6; margin-bottom: 12px; }
.topic-card p { font-size: 14px; color: var(--muted); line-height: 1.9; }
.topic-card-meta { display: block; color: var(--blue); font-size: 13px; margin-top: 20px; }
.topic-card:hover h3 { color: var(--blue); }
#catalog-results { margin-top: 24px; }
#catalog-results .catalog-note { margin: 0 0 24px; }
.topic-page { padding-top: 40px; padding-bottom: 64px; }
.topic-head { padding: 32px 0; max-width: 900px; }
.topic-head h1, .topic-page > h1 { font: 400 48px/1.5 var(--display); margin: 0 0 24px; }
.topic-description { font-size: 18px; line-height: 1.9; color: var(--muted); margin-bottom: 20px; }
.topic-start { display: flex; align-items: center; justify-content: space-between; gap: 32px; padding: 24px; border: 1px solid var(--line); border-radius: 8px; margin: 0 0 40px; }
.topic-start h2 { font-family: var(--font); font-size: 20px; line-height: 1.7; margin-bottom: 8px; }
.topic-start p { font-size: 14px; color: var(--muted); margin: 0; }
.topic-start .button { max-width: 40%; flex-shrink: 0; }
.topic-tools { display: flex; flex-wrap: wrap; align-items: start; justify-content: space-between; gap: 16px; }
.topic-tools .search { width: 256px; }
.topic-tools .level-tabs { margin-bottom: 16px; }
.topic-course-group { margin-top: 32px; padding-top: 24px; border-top: 1px solid var(--line); }
.topic-group-heading { display: flex; align-items: baseline; gap: 16px; margin-bottom: 24px; }
.topic-group-heading > span { font: 14px/1.7 ui-monospace,Menlo,monospace; color: var(--muted); }
.topic-group-heading h2 { font-family: var(--font); font-size: 24px; font-weight: 500; margin: 0; }
.topic-group-heading small { margin-left: auto; color: var(--muted); font-size: 13px; white-space: nowrap; }
.topic-other { display: flex; flex-wrap: wrap; gap: 16px 32px; margin-top: 64px; border-top: 1px solid var(--line); padding-top: 24px; }
.topic-other h2 { width: 100%; font: 500 20px/1.7 var(--font); margin-bottom: 8px; }
.topic-other a { color: var(--blue); font-size: 14px; }
.topic-empty { max-width: 880px; padding: 32px 0; border-top: 1px solid var(--line); }
.topic-empty h2 { font-size: 32px; }
.topic-empty p { line-height: 1.9; margin-bottom: 24px; }
.topic-empty h3 { margin-top: 32px; margin-bottom: 16px; }
@media(max-width:1000px) { .topic-overview { grid-template-columns: repeat(2,minmax(0,1fr)); } }
@media(max-width:720px) {
 .catalog-heading { flex-direction: column; align-items: start; gap: 12px; }
 .topic-overview { grid-template-columns: 1fr; }
 .topic-card { padding: 24px 0; }
 .topic-card h3 { font-size: 21px; }
 .topic-card-meta { margin-top: 12px; }
 .topic-page { padding-top: 24px; }
 .topic-head { padding: 24px 0; }
 .topic-head h1, .topic-page > h1 { font-size: 32px; margin-bottom: 16px; }
 .topic-description { font-size: 16px; }
 .topic-start { flex-direction: column; align-items: start; padding: 20px; gap: 20px; margin-bottom: 32px; }
 .topic-start .button { max-width: 100%; }
 .topic-tools { flex-direction: column; align-items: stretch; }
 .topic-tools .search { width: 100%; }
 .topic-tools .level-tabs { margin: 0; }
 #topic-result-count { margin-top: 20px; }
 .topic-course-group { margin-top: 24px; }
 .topic-group-heading { gap: 12px; }
 .topic-group-heading h2 { font-size: 20px; }
 .topic-other { gap: 16px 24px; margin-top: 48px; }
}

/* Equations are reading content, not clipped captions in the experiment. */
.formula-link{display:inline-block;color:var(--blue);font-size:13px;padding:8px 0;text-decoration:underline;text-underline-offset:4px}
.formula-reading{scroll-margin-top:100px;min-width:0}
.formula-reading .formula-intro{font-size:14px;color:var(--muted);line-height:1.9}
.formula-reading .step-item>div{min-width:0;flex:1}
.equation-scroll{max-width:100%;min-width:0;overflow-x:auto;background:#f0f4f7;border-left:2px solid #8ea9be;margin:14px 0;padding:16px 18px;overscroll-behavior-x:contain}
.equation-scroll:focus-visible{outline:2px solid var(--blue);outline-offset:3px}
.equation-text{width:max-content;min-width:100%;white-space:pre;font:17px/2.1 ui-monospace,Menlo,"Noto Sans Math",monospace;color:#183d59;font-variant-numeric:tabular-nums}
.formula-chain{margin-top:28px}
.formula-reading-note{font-size:14px;line-height:1.9;padding:4px 0 16px;color:var(--muted)}
.formula-live{border-top:1px solid var(--line);padding:22px 0}
.formula-live dl{margin:12px 0;font-size:13px}
.formula-live dl>div{display:grid;grid-template-columns:minmax(0,1fr) minmax(0,1fr);gap:16px;padding:8px 0;border-bottom:1px solid var(--line)}
.formula-live dt,.formula-live dd{margin:0;overflow-wrap:anywhere}
.formula-live dd{color:var(--blue);text-align:right;font-variant-numeric:tabular-nums}
#formula-output{font-size:15px;line-height:1.9;overflow-wrap:anywhere}
.symbol{grid-template-columns:minmax(80px,.4fr) minmax(0,1fr)}
.symbol b,.symbol span{overflow-wrap:anywhere}
.formula-disclosure .formula{white-space:pre-wrap;font-size:14px;line-height:1.9}
@media(max-width:720px){.equation-text{font-size:15px}.equation-scroll{padding:14px 12px}.formula-reading .step-item{gap:10px}}