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Let's say Earth started rotation in the opposite direction without any catastrophic events, would a day on Earth be shorter or longer or just the same considering that speed of rotation and revolution stays the same?

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Depends on how you define "day".

There's two definitions of a "day". There's the Solar day, the time between the Sun being at its zenith. And there's the Sidereal day, the time for the Earth to rotate 360°. They are not the same. The Solar day is affected by the direction of rotation, the Sidereal day is not.

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A solar day is 24 hours. This is because the hour is 1/24th of the time it takes the Sun to return to its original position in the sky (almost, see below).

A sidereal day is a bit shorter, about 23 hours 56 minutes 4 seconds. This is because the Earth's movement around the Sun means it has to rotate a bit further to have the Sun at zenith.

If a planet both rotates and revolves in the same direction, as the Earth does, the solar day will be longer than the sidereal day. If they revolve in opposite directions, the solar day will be shorter than the sidereal day. The Sidereal day remains the same, it is only affected by how fast the Earth rotates.

Right now, the Solar day is 3 minutes 56 seconds longer than a Sidereal day. If our rotation were reversed, it would be 3 minutes 56 seconds shorter than a Sidereal day. A solar day would be 23 hours 52 minutes 8 seconds. For a total loss of 7 minutes 52 seconds.

Note: Because the Earth wobbles a bit in its rotation, and other factors, the position of the Sun in the sky is not an accurate way to define time. A second, and thus an hour, is now defined as the time it takes for 9,192,631,770 wavelengths of the light released when a Caesium 133 atom transitions between two states to go past. It does so in a very precise, fixed amount of time. The number of oscillations was chosen to keep a second a second and hour an hour, but a solar hour will fall out of sync with an SI hour.

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    $\begingroup$ It's not that a cesium atom itself transitions between two states 9192613770 times a second -- but that when/if it transitions between those two states it will emit (or absorb) radiation with that frequency. $\endgroup$
    – hmakholm left over Monica
    Commented Apr 30, 2017 at 19:37
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    $\begingroup$ @HenningMakholm I didn't want to get too deep into that unnecessary detail; point is a hour is now disconnected from the Sun. $\endgroup$
    – Schwern
    Commented Apr 30, 2017 at 20:08
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    $\begingroup$ Of course, if the earth rotated the other way all along, then that definition would be different and the solar day would still be 24 hours ;) $\endgroup$
    – Draco18s no longer trusts SE
    Commented May 1, 2017 at 14:40
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Yes...but not by a lot

This has been looked at a little over on physics SE.

Start off by imagining the earth doesn't rotate at all. Light on Earth

We get one day every 365 days with the day/night boundary moving as below: Day/night boundary

This is key. The earth is going to have a "day" whether it rotates or not. Now we choose whether we rotate with or against this day by saying whether the Earth rotates in the same direction or not. Currently we rotate with our orbit and the effect contributes to the day length. If we were to rotate against it the effect would act against the day length. As shown below:

Illustration of day length differences due to orbit or rotation So we can see an anti-clockwise rotation is added to by the orbit-day whereas the clockwise one works against the orbital "day".

The time it takes the Earth to rotate once about it's axis (regardless of direction) is called its "sidereal day". The combination of the sidereal day and the orbit day is called the "solar day".

This being said we would get shorter days only by about 8 minutes (the difference being that the rotation is about 365 times quicker than the orbit day).

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    $\begingroup$ I believe it would be 8 minutes. The Solar day is already longer than the fixed Sidereal day by 4 minutes because of the interaction between the Earth's rotation and revolution. With our rotation reversed it would be 4 minutes shorter resulting in a total change of 8 minutes. $\endgroup$
    – Schwern
    Commented Apr 30, 2017 at 18:10
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    $\begingroup$ @Schwern You're right! I had only been considering the difference between solar and sidereal rather than the sum of the two. Thanks for spotting that :) $\endgroup$
    – Lio Elbammalf
    Commented Apr 30, 2017 at 18:13
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    $\begingroup$ Do you think you could edit this to include the word "Sidereal?" That's the magic word which lets you google search everything else that one needs to know about the topic. $\endgroup$
    – Cort Ammon
    Commented Apr 30, 2017 at 19:41
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    $\begingroup$ @CortAmmon Sure, I hadn't included it because I was trying to keep things to a more intuitive explanation (hence all the pictures) but I'll work an explanation into the answer. $\endgroup$
    – Lio Elbammalf
    Commented Apr 30, 2017 at 20:14
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    $\begingroup$ Nice simple explanation $\endgroup$
    – Kilisi
    Commented Apr 30, 2017 at 22:14

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