Say why the beacon sits beside the hydrogen line

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-          <p class="kicker"><span class="pulse"></span>Archive · made public 2219-10-05 · 1420.405 MHz</p>
+          <p class="kicker"><span class="pulse"></span>Archive · made public 2219-10-05 · 1420.55 MHz</p>
           <h1>Someone is calling from <span class="num">10.98</span> light years away.</h1>
           <p class="lede">On 2219-09-18 every pulsar-timing station in the solar system logged the same faint pattern: a voice keeping time with a pulsar. It comes from Ross 128 b, it was sent on purpose, and it is still arriving.</p>
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@@ -38,7 +38,7 @@ <h1>Someone is calling from <span class="num">10.98</span> light years away.</h1
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           <canvas data-waterfall data-line="0.62" width="280" height="140" role="img" aria-label="A live waterfall spectrogram: noise, with one narrow line that switches on and off"></canvas>
-          <figcaption><span>waterfall · 1420.405 MHz ± 600 kHz</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
+          <figcaption><span>waterfall · 1420.55 MHz ± 600 kHz</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
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@@ -88,7 +88,7 @@ <h3>Only Earth hears it in tune</h3>
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             <time datetime="2219-09-28">2219-09-28</time>
             <h3>The transmitter is on the planet</h3>
-            <p>The tone sits 0.01% above hydrogen's line because Ross 128 is coming toward us at 31 km/s, and it slides up and down by about 260 kHz every 9.87 days, exactly the time Ross 128 b takes to circle its star. The transmitter rides on that planet, and whoever built it left the tone uncorrected. The pulse timing they did correct, to the nanosecond: the timing is the message. The planet is tidally locked: one side always day, the other always night.</p>
+            <p>The tone sits beside hydrogen's line. Hydrogen is the commonest element there is, and its line is a ruler both sides know. The tone sits 0.01% above that ruler because Ross 128 is coming toward us at 31 km/s, and it slides up and down by about 260 kHz every 9.87 days, exactly the time Ross 128 b takes to circle its star. The transmitter rides on that planet. Whoever built it left the tone's Doppler shift uncorrected, so the tone itself says where it comes from. The pulse timing they did correct, to the nanosecond: the tone is only the carrier, the timing is the message. The planet is tidally locked: one side always day, the other always night.</p>
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             <time datetime="2219-10-05">2219-10-05</time>

archive/2219-signal/index.html

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-          <p class="kicker"><span class="pulse"></span>档案 · 2219-10-05 公布 · 1420.405 MHz</p>
+          <p class="kicker"><span class="pulse"></span>档案 · 2219-10-05 公布 · 1420.55 MHz</p>
           <h1>有人在 <span class="num">10.98</span> 光年外呼叫。</h1>
           <p class="lede">2219 年 9 月 18 日，太阳系里每一座脉冲星计时站都记下了同一个微弱的图案：一个声音在和一颗脉冲星合拍。它来自罗斯128 b，是有意发出的，而且还在源源不断地到来。</p>
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@@ -38,7 +38,7 @@ <h1>有人在 <span class="num">10.98</span> 光年外呼叫。</h1>
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           <canvas data-waterfall data-line="0.62" width="280" height="140" role="img" aria-label="实时瀑布图：一片噪声里，有一条细线时亮时灭"></canvas>
-          <figcaption><span>瀑布图 · 1420.405 MHz ± 600 kHz</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
+          <figcaption><span>瀑布图 · 1420.55 MHz ± 600 kHz</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
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@@ -88,7 +88,7 @@ <h3>只有地球听到的是合拍的</h3>
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             <time datetime="2219-09-28">2219-09-28</time>
             <h3>发射机在那颗行星上</h3>
-            <p>音调比氢的谱线高 0.01%，因为罗斯128正以 31 km/s 朝我们靠近；它还每 9.87 天上下滑动约 260 kHz，恰好是罗斯128 b 绕恒星一圈的时间。发射机就在这颗行星上，随它一起绕行，建造它的人没有修正音调的多普勒频移。脉冲的时刻他们却修正了，精确到纳秒：时刻本身就是信息。行星被潮汐锁定，一面永昼，一面永夜。</p>
+            <p>音调放在氢的谱线旁边。氢是宇宙里最多的元素，它的谱线是双方都认得的一把尺子。音调比这把尺子高 0.01%，因为罗斯128正以 31 km/s 朝我们靠近；它还每 9.87 天上下滑动约 260 kHz，恰好是罗斯128 b 绕恒星一圈的时间。发射机就在这颗行星上，随它一起绕行。建造它的人没有修正音调的多普勒频移，所以音调自己就说出了它从哪里来。脉冲的时刻他们却修正了，精确到纳秒：音调只是载体，时刻才是信息。行星被潮汐锁定，一面永昼，一面永夜。</p>
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             <time datetime="2219-10-05">2219-10-05</time>

en/index.html

@@ -39,7 +39,7 @@ <h1>Joining the conversation between the <em>stars</em>.</h1>
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-        <figcaption><span>1420.405 MHz · Ross 128</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
+        <figcaption><span>1420.55 MHz · Ross 128</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
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@@ -52,7 +52,7 @@ <h1>Joining the conversation between the <em>stars</em>.</h1>
           <h2 class="section-title" id="signal-title">A signal from Ross 128 b</h2>
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-          <p>On 2219-09-18 the pulsar-timing stations on the Moon, Earth, Mars and Ceres all logged the same narrow tone at the frequency of hydrogen. The small delays between them pointed at one star: Ross 128, a red dwarf in Virgo 10.98 light years away, with one temperate rocky planet. Its pulses kept exact time with the pulsar J1909−3744, slipped one beat ahead and fell back into step, tuned so that only Earth hears the two in tune.</p>
+          <p>On 2219-09-18 the pulsar-timing stations on the Moon, Earth, Mars and Ceres all logged the same narrow tone at 1420.55 MHz, just above the line of hydrogen. Hydrogen is the commonest element there is, and its line is a ruler any culture doing radio astronomy knows; the tone also falls inside the band used for pulsar timing, which is why the timing stations heard it first. The small delays between them pointed at one star: Ross 128, a red dwarf in Virgo 10.98 light years away, with one temperate rocky planet. Its pulses kept exact time with the pulsar J1909−3744, slipped one beat ahead and fell back into step, tuned so that only Earth hears the two in tune.</p>
           <p>It was made public on 2219-10-05. The full record, with the duet to hear from four places, is in the archive.</p>
           <p><a class="text-link" href="../archive/2219-signal/en/">Read the full record <span>→</span></a></p>
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@@ -39,7 +39,7 @@ <h1>加入群星之间的<em>对话</em>。</h1>
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-        <figcaption><span>1420.405 MHz · 罗斯128</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
+        <figcaption><span>1420.55 MHz · 罗斯128</span><time class="readout" data-story-clock>--:--:-- UTC</time></figcaption>
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@@ -52,7 +52,7 @@ <h1>加入群星之间的<em>对话</em>。</h1>
           <h2 class="section-title" id="signal-title">来自罗斯128 b的信号</h2>
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-          <p>2219 年 9 月 18 日，月球、地球、火星和谷神星上的脉冲星计时站，都记下了同一个很窄的音调，频率正是氢的谱线。各处之间细微的时间差指向同一颗星：室女座里的红矮星罗斯128，距离 10.98 光年。它有一颗温和的岩质行星。它的脉冲和脉冲星 J1909−3744 分秒不差，往前滑过一拍，又回到合拍；而且只有在地球，两者听起来才是合拍的。</p>
+          <p>2219 年 9 月 18 日，月球、地球、火星和谷神星上的脉冲星计时站，都记下了同一个很窄的音调：1420.55 MHz，紧挨着氢的谱线。氢是宇宙里最多的元素，它的谱线是任何做射电天文的文明都认得的一把尺子；这个频率又正好落在脉冲星计时的频段里，所以最先听到它的是计时站。各处之间细微的时间差指向同一颗星：室女座里的红矮星罗斯128，距离 10.98 光年。它有一颗温和的岩质行星。它的脉冲和脉冲星 J1909−3744 分秒不差，往前滑过一拍，又回到合拍；而且只有在地球，两者听起来才是合拍的。</p>
           <p>它在 2219 年 10 月 5 日公之于众。完整的记录，以及可以从四个地方去听的这段二重唱，都在档案里。</p>
           <p><a class="text-link" href="archive/2219-signal/">查看完整记录 <span>→</span></a></p>
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