{"id":117232,"date":"2026-09-29T07:05:06","date_gmt":"2026-09-29T11:05:06","guid":{"rendered":"https:\/\/bgodinspired.com\/?p=117232"},"modified":"2026-09-29T07:05:06","modified_gmt":"2026-09-29T11:05:06","slug":"why-a-day-isnt-exactly-24-hours","status":"publish","type":"post","link":"https:\/\/bgodinspired.com\/index.php\/nature-and-creation\/why-a-day-isnt-exactly-24-hours\/","title":{"rendered":"Why a Day Isn&#8217;t Exactly 24 Hours &#8211; and What Earth&#8217;s Inner Core Has to Do With It"},"content":{"rendered":"<div class='booster-block booster-read-block'>\n                <div class=\"twp-read-time\">\n                \t<i class=\"booster-icon twp-clock\"><\/i> <span>Read Time:<\/span>8 Minute, 3 Second                <\/div>\n\n            <\/div><p>Here is something almost nobody knows: a day on Earth isn&#8217;t exactly 24 hours.<\/p>\n\n<p>Most days come in a tiny bit long or a tiny bit short. We are talking about thousandths of a second, called milliseconds. You will never feel it. Your phone won&#8217;t show it. But the planet you live on does not spin like a perfect machine.<\/p>\n\n<p>And a new study says one of the reasons sits more than 5,000 kilometres (over 3,000 miles) under your feet.<\/p>\n\n<h2>Why a Day Isn&#8217;t Exactly 24 Hours<\/h2>\n\n<p>A &#8220;day&#8221; is one full spin of the Earth. On paper, that is 86,400 seconds. In real life, the spin speeds up and slows down a little, all the time.<\/p>\n\n<p>Scientists already knew about several causes:<\/p>\n\n<ul>\n<li><strong>Wind and ocean currents.<\/strong> Moving air and water push on the solid ground, and that changes the spin a little from season to season.<\/li>\n<li><strong>The Moon.<\/strong> The Moon&#8217;s pull on the oceans acts like a very gentle brake. Over millions of years, it makes days slowly longer.<\/li>\n<li><strong>Moving water and ice.<\/strong> When mass shifts around on the surface, the spin shifts a little too.<\/li>\n<\/ul>\n\n<p>But there was one pattern that was harder to explain. Over periods of roughly 10 to 70 years, the day gets a few milliseconds longer, then a few milliseconds shorter, then longer again. Weather is too fast to explain that. The Moon is too slow. Something deeper was going on.<\/p>\n\n<div class=\"convertkit-form wp-block-convertkit-form\" style=\"\"><script async data-uid=\"6491fb8269\" src=\"https:\/\/bgodinspired.kit.com\/6491fb8269\/index.js\" data-jetpack-boost=\"ignore\" data-no-defer=\"1\" data-no-optimize=\"1\" nowprocket><\/script><\/div>\n\n\n<h2>The Tug-of-War Deep Inside the Earth<\/h2>\n\n<p>Picture the Earth like a peach. There is a thin skin (the crust), a thick middle (the mantle), and a pit at the center. The pit is actually two parts: a liquid metal outer core, and a solid metal ball in the very middle called the inner core. That inner ball is roughly 2,400 kilometres across, a bit smaller than the Moon.<\/p>\n\n<p>In September 2026, two geophysicists at the University of Alberta, Huifeng Zhang and Mathieu Dumberry, published a study in the journal <em>Nature<\/em>. They looked at how the inner core and the mantle pull on each other.<\/p>\n\n<p>Here is the simple version of what they found:<\/p>\n\n<ul>\n<li>The inner core is not a perfect ball. It is slightly lumpy.<\/li>\n<li>The mantle is not evenly heavy. Some places have more mass than others.<\/li>\n<li>Because of that, the inner core and the mantle pull on each other with gravity, like two uneven weights tugging on each other.<\/li>\n<li>When the inner core turns a little faster, the mantle (and the ground you stand on) turns a little slower. When the inner core slows, the mantle speeds up. The total spin of the planet stays the same. It just gets passed back and forth.<\/li>\n<\/ul>\n\n<p>That back-and-forth, they argue, is likely the main driver of those slow, decades-long swings in the length of the day. Friction and electric-magnetic drag where the core meets the mantle push back against it, and the balance between the two sets the rhythm.<\/p>\n\n<h2>The Surprise: Earth&#8217;s Center Is Softer Than We Thought<\/h2>\n\n<p>This is the part that did not make most headlines.<\/p>\n\n<p>For the math to work, the inner core has to be able to change its shape. Not quickly. Slowly, over about eight to ten years, like very stiff clay settling under pressure. The researchers describe it as deforming &#8220;viscously.&#8221;<\/p>\n\n<p>That means the solid iron ball at the center of our planet is softer than many scientists had assumed. It is still solid. It is still hotter than you can imagine. But it gives a little. And that small give is part of what lets it pass spin back and forth with the rest of the planet.<\/p>\n\n<p>The researchers are also honest about the limits. Nobody can see the inner core. Its motion has to be rebuilt from other measurements, and those rebuilds carry real uncertainty. Coverage of the study notes that using different models of the liquid core&#8217;s flow shifted some of their estimates by as much as 30 percent. So this is a strong explanation, not a closed case.<\/p>\n\n<p>If you like big questions about how our world is built, you might also enjoy how <a href=\"https:\/\/bgodinspired.com\/index.php\/bgodinspired-news\/bgodinspired-science-news\/pompeii-eruption-pliny-geology-clock\/\">a teenager&#8217;s letter about Pompeii helped scientists sharpen a clock used to date the Earth<\/a>.<\/p>\n\n<h2>How Do We Even Know the Day Is Changing?<\/h2>\n\n<p>For most of human history, the Earth&#8217;s spin <em>was<\/em> the clock. Noon was when the Sun was highest. A day was a day. There was nothing steadier to compare it to.<\/p>\n\n<p>That changed with atomic clocks. In 1967, the world&#8217;s official second stopped being a slice of the Earth&#8217;s day and started being defined by the vibration of cesium atoms. For the first time, we had a clock steadier than the planet itself.<\/p>\n\n<p>And once we had it, we could see the wobble.<\/p>\n\n<p>Since 1972, timekeepers have added 27 &#8220;leap seconds&#8221; to official clocks to keep them lined up with the Earth&#8217;s actual spin. The last one was added at the end of 2016. The system became such a headache for computers that the world&#8217;s timekeepers agreed to stop adding leap seconds by 2035.<\/p>\n\n<p>Think about that. The thing we measure our whole lives by, sunrise to sunrise, turned out to be the less dependable clock.<\/p>\n\n<p>Science keeps doing this. The better our tools get, the more we learn about the edges of what we can know, from the center of the Earth all the way out to the question of <a href=\"https:\/\/bgodinspired.com\/index.php\/bible-resources\/bible-and-science\/is-the-universe-infinite\/\">whether the universe is infinite<\/a>.<\/p>\n\n<h2>What Stays Steady When the Day Doesn&#8217;t<\/h2>\n\n<p>We build everything on the day. Work shifts. School. Medicine schedules. Birthdays. The rhythm of waking up and going to sleep. It feels like the most fixed thing in the world.<\/p>\n\n<p>And it wobbles. Because of a soft metal ball thousands of kilometres down that nobody will ever touch.<\/p>\n\n<p>There is a very old poem, written by someone sitting in the ruins of their own city, that says the faithfulness of God is new every morning. That writer had nothing steady left. Not a home, not a city, not a routine. And the thing they held onto was not that each morning would be the same. It was that something faithful would meet them in each one, whatever that morning looked like.<\/p>\n\n<p>That is a different kind of dependable. Not a machine that never changes. More like a friend who keeps showing up.<\/p>\n\n<p>So the next time a day feels too long or too short, you are not imagining all of it. The planet really does drift. The mornings keep coming anyway.<\/p>\n\n<h2>Questions People Ask About the Length of a Day<\/h2>\n\n<h3>Is a day on Earth exactly 24 hours?<\/h3>\n<p>No. A day on Earth is defined as 86,400 seconds, but the Earth&#8217;s actual spin varies by small fractions of a second. Winds, ocean currents, the Moon&#8217;s tides, shifting water and ice, and movement deep inside the Earth all speed up or slow down the planet&#8217;s rotation by milliseconds.<\/p>\n\n<h3>How does Earth&#8217;s inner core change the length of a day?<\/h3>\n<p>A 2026 study in the journal Nature by Huifeng Zhang and Mathieu Dumberry of the University of Alberta found that the Earth&#8217;s inner core and mantle pull on each other through gravity because neither is perfectly even. When the inner core spins slightly faster, the mantle and surface spin slightly slower, and the reverse. This exchange changes the length of the day by a few milliseconds over periods of roughly 10 to 70 years.<\/p>\n\n<h3>Is Earth&#8217;s inner core solid or soft?<\/h3>\n<p>Earth&#8217;s inner core is solid iron and nickel, but a 2026 study in Nature suggests it is softer than many scientists assumed. The study found the inner core can slowly change shape over about eight to ten years, which helps explain how it exchanges spin with the rest of the planet.<\/p>\n\n<h3>Why were leap seconds added to clocks?<\/h3>\n<p>Leap seconds were added to official clocks because atomic clocks keep steadier time than the Earth&#8217;s rotation. Since 1972, 27 leap seconds have been added to keep atomic time lined up with the Earth&#8217;s actual spin. International timekeepers have agreed to stop adding leap seconds by 2035.<\/p>\n\n<h3>Are days on Earth getting longer or shorter?<\/h3>\n<p>Both, depending on the time scale. Over millions of years, the Moon&#8217;s tidal pull slowly makes days longer. Over years and decades, days can get slightly longer or shorter because of weather, oceans, ice, and the movement of the Earth&#8217;s core. The changes are measured in milliseconds.<\/p>\n\n<h2>What Do You Think?<\/h2>\n\n<p>Does it change anything for you to learn that the day itself isn&#8217;t perfectly steady, or is it just a fun fact? What is one thing in your life that feels completely fixed, but probably isn&#8217;t? Share your answer in the comments.<\/p>\n\n<div class=\"convertkit-form wp-block-convertkit-form\" style=\"\"><script async data-uid=\"6491fb8269\" src=\"https:\/\/bgodinspired.kit.com\/6491fb8269\/index.js\" data-jetpack-boost=\"ignore\" data-no-defer=\"1\" data-no-optimize=\"1\" nowprocket><\/script><\/div>\n\n\n<h2>Share This<\/h2>\n\n<p><strong>Post 1:<\/strong> Wild fact of the day: a day on Earth isn&#8217;t exactly 24 hours. A new study says part of the reason is a soft, lumpy metal ball at the center of the planet tugging on everything above it. https:\/\/bgodinspired.com\/?p=117232<\/p>\n\n<p><strong>Post 2:<\/strong> I always thought the day was the one thing you could count on. Turns out the Earth&#8217;s spin drifts by milliseconds, and official clocks have had 27 leap seconds added to keep up with it. The mornings keep coming anyway. https:\/\/bgodinspired.com\/?p=117232<\/p>\n\n<p><strong>Post 3:<\/strong> Scientists just learned that Earth&#8217;s inner core is softer than they thought, and it may be changing the length of our days. Read this and tell me it doesn&#8217;t make you look at your alarm clock differently: https:\/\/bgodinspired.com\/?p=117232<\/p>\n\n<p><em>Source: Zhang, H. and Dumberry, M., <a href=\"https:\/\/doi.org\/10.1038\/s41586-026-10999-2\" target=\"_blank\" rel=\"noopener\">Nature, 2026<\/a>. Coverage via <a href=\"https:\/\/phys.org\/news\/2026-09-earth-deep-secret-day-isnt.html\" target=\"_blank\" rel=\"noopener\">phys.org<\/a>.<\/em><\/p>        <div class=\"booster-block booster-reactions-block\">\n            <div class=\"twp-reactions-icons\">\n                \n                <div class=\"twp-reacts-wrap\">\n                    <a react-data=\"be-react-1\" post-id=\"117232\" class=\"be-face-icons un-reacted\" href=\"javascript:void(0)\">\n                        <img decoding=\"async\" src=\"https:\/\/bgodinspired.com\/wp-content\/plugins\/booster-extension\/\/assets\/icon\/happy.svg\" alt=\"Happy\" title=\"\">\n                    <\/a>\n                    <div class=\"twp-reaction-title\">\n                        Happy                    <\/div>\n                    <div class=\"twp-count-percent\">\n                                                    <span style=\"display: none;\" class=\"twp-react-count\">0<\/span>\n                        \n                   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