Earth takes 24 hours to complete one rotation, right?

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It actually takes slightly less than that.

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.

Discover more astronomy experiments and science projects.

How to do it

Observing a favourite star and a bit of maths will prove Earth’s spin takes less than 24 hours.

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

Watching Vega we saw a difference of four minutes and five seconds (245 seconds).

The published figure is three minutes 56 seconds (236 seconds) short of 24 hours: that’s one rotation every 23 hours, 56 minutes and four seconds.

Try it your self. What do you get?Let us know by emailing us at contactus@skyatnightmagazine.com.

Is Earth's day actually 24 hours?

The reason for this difference is you're measuring the length of a sidereal day, not the length of the solar day.

The solar day (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.

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

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.

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 1º (matching the 360º of the orbit divided by the 365 days of the year).

This means that once Earth has rotated through 360º, it then needs to rotate 1º more to catch up with this motion.

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 360º.

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.

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The length of a sidereal day is 23 hours, 56 minutes and four seconds.

Authors

Mary McIntyre is an outreach astronomer and teacher of astrophotography based in Oxfordshire, UK.