Observations

How does the world rotate?

An Earth globe by Replogle Globes

An Earth globe by Replogle Globes, picture and copyright Nick Lomb ©, all rights reserved

Powerhouse Museum Thank you everyone for a fantastic ‘Ask A Curator’ day – we’ve all had a lot fun! For our Northern Hemisphere friends, please remember we are in the future and our curators will be sleeping in a few hours time so it may take a little while for us to get back to you, but we will get back to you!
September 1 at 5:30pm •

Michael Van Tiel I think you have forgotten how the world rotates… 😉
September 1 at 5:56pm •

Powerhouse Museum That’s a question for the curator of Astronomy! (Yes, we know and were referring to our European and American counterparts)
September 1 at 6:11pm •

At the end of the recent Ask a Curator day run on the Powerhouse Museum’s Facebook page there was the amusing and light-hearted exchange above. Here are some comments:

How does the world rotate? The Earth spins around its axis once a day or every 24 hours. It rotates from west to east.

How does that relate to time? Eastern Australia is approximately 150° east of Greenwich in the UK. That means as the Sun is setting in Sydney it is early morning in London. To be more precise, the Australian Eastern Standard Time zone is 10 hours ahead of Universal Time (once known as GMT) so that at 5:30 pm AEST when the first statement was made above it was 7:30 am standard time in the UK. Similarly, the east coast of the United States is a further 5 hours behind so that the standard time in New York was only 2:30 am. Not a time that people would be posing questions to curators!

Note that the main English speaking countries such as the UK and the USA are not only to the west of Australia and hence behind in time, they happen to be in the northern hemisphere.

Is 24 hours a precise value for the Earth’s daily spin? No. It’s the average time during a year that the Earth takes to spin around its axis once with respect to the Sun. We can measure the length of a day by the interval between the Sun being due north on one day and on the next. The movement of the Earth around the Sun in the same direction as its spin lengthens this interval by a few minutes. During the year though the Earth moves at a slightly varying speed around the Sun so that the length of the day varies slightly. This is reflected in the fact that the Sun can be due north a few minutes before or after 12 noon depending on the day of the year. This variation is quantified in the equation of time that is tabulated or graphed next to sundials.

Does the Earth’s spin vary? It varies with respect to the Sun, but not with respect to distant stars. The time for the Earth to spin on its axis with respect to the stars is 23 hours 56 minutes and 4 seconds. This is known s a sidereal day and is of great importance to astronomers.

How often does the Earth spin around during one year? There are approximately 366.25 sidereal days in a year so that the Earth spins 366.25 times with respect to distant stars in a year. The Earth’s yearly motion around the Sun makes the Sun appear to make one backwards circuit of the sky during the year and hence there are only 365.25 ordinary days in a year.

I hope that this explanation has not made you dizzy!

3 responses to “How does the world rotate?

    • Hello Paul. Interesting question. At the equator the velocity of the Earth’s rotation is 1670 km/hour or 0.46 km/second from west to east. That velocity has no effect on aircraft as they are moving with respect to the Earth and share its velocity. It does have an effect on spacecraft that are being sent go into Earth orbit: launch sites are built as close to the equator as posssible and in most cases the rockets are fired towards the west [oops! that should be east -AJ,Apr2020] so as to take maximum advantage of the free boost offered by the rotation of the Earth.

    • The earth is orbiting around the sun in the same direction that it is rotating on its axis. Therefore each day (solar noon to solar noon), the earth has to rotate slightly more than 360 degrees to again be facing directly at the sun. These small rotations add up to one complete rotation over one year, so there are 366.25 rotations of earth on its axis in every 365.25 days of its orbit around the sun.

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