{"id":59965,"date":"2024-06-07T11:03:23","date_gmt":"2024-06-07T11:03:23","guid":{"rendered":"http:\/\/ef339d6f-fdff-406d-9840-7ead18d169fa"},"modified":"2024-06-07T11:39:40","modified_gmt":"2024-06-07T11:39:40","slug":"is-a-day-really-24-hours-calculate-the-speed-of-earths-rotation-by-observing-the-night-sky","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/is-a-day-really-24-hours-calculate-the-speed-of-earths-rotation-by-observing-the-night-sky\/","title":{"rendered":"Is a day really 24 hours? Calculate the speed of Earth’s rotation by observing the night sky"},"content":{"rendered":"

Studying the movement of the stars to work out the length of time it takes Earth to spin once on its axis. <\/p>

By <\/p>

Published: Friday, 07 June 2024 at 11:03 AM<\/p>

\n\n

Earth takes 24 hours to complete one rotation, right?<\/p>

It actually takes slightly less than that.<\/p>

You can test out how much less by studying the movement of the stars to work out the length of time it takes Earth to spin once on its axis.<\/p>

Discover more astronomy experiments<\/a> and science projects<\/a>.<\/strong><\/em><\/p>

Long-exposure images show the apparent movement of the stars in the night sky. Credit: Iain Todd<\/figcaption><\/figure>

How to do it<\/strong><\/h2>

Observing a favourite star and a bit of maths will prove Earth\u2019s spin takes less than 24 hours.<\/p>

  1. Position yourself so you can see a bright star disappear behind a chimney pot.<\/li>
  2. Note the exact time it vanishes.<\/li>
  3. Repeat this with the same star, from the same position, over the next few nights, noting how many minutes difference there is.<\/li>
  4. After a week, you\u2019ll have six values.<\/li>
  5. Add them, divide the total by six to get the average, and convert them to seconds.<\/li><\/ol>

    Watching Vega<\/a> we saw a difference of four minutes and five seconds (245 seconds).<\/p>

    The published figure is three minutes 56 seconds (236 seconds) short of 24 hours: that\u2019s one rotation every 23 hours, 56 minutes and four seconds.<\/p>

    Try it yourself. What do you get? Let us know by emailing us at contactus@skyatnightmagazine.com<\/a>.<\/strong><\/em><\/p>

    Is Earth’s day actually 24 hours?<\/strong><\/h2>

    The reason for this difference is you’re measuring the length of a sidereal day, not the length of the solar day.<\/p>

    The solar day<\/strong> (sometimes known as the synodic day) is the time it takes the Sun to appear in the same place in the sky on consecutive days. More accurately, it’s the time it takes the Sun to pass over the same line of longitude.<\/p>

    Meanwhile, the sidereal day<\/strong> is the time it takes for a distant star to appear in the same place in the sky from one night to the next.<\/p>

    So why are the sidereal and solar days different lengths? It’s because while Earth is rotating on its axis, it’s also orbiting around the Sun.<\/p>

    This means that in the time it takes Earth to rotate, the planet has moved around in its orbit, changing the angle between our planet and the Sun by about 1\u00ba (matching the 360\u00ba of the orbit divided by the 365 days of the year).<\/p>

    This means that once Earth has rotated through 360\u00ba, it then needs to rotate 1\u00ba more to catch up with this motion.<\/p>

    Meanwhile, the distant stars are far enough away that they’re effectively a fixed point in the sky, and so appear in the same place after rotating through 360\u00ba.<\/p>

    As the Sun has been humanity’s main time keeping device for much of history, it’s the solar day that was used when defining the length of a day and is exactly 24 hours long by definition.<\/p>

    The length of a sidereal day is 23 hours, 56 minutes and four seconds.<\/p> <\/body><\/html>\n

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