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Imagine for a second that Earth no longer rotates around its axis. It still revolves around the Sun.

However, would the Earth:

  • face the same way towards the Sun as it goes around it (permanent time of day everywhere, e.g. Europe always looks towards the sun, while China has permanent night)
  • not rotate at all (a day that lasts a whole year, e.g. the Earth would be to the Sun as the Moon currently is to the Earth)?

I'm trying to simulate the solar system and I need to know if I, once I rotate the Solar system (to simulate rotation of the Sun), have to rotate the Earth back for the equal angle or not.

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    $\begingroup$ The first option on your list is known as Tidal locking en.m.wikipedia.org/wiki/Tidal_locking $\endgroup$
    – Triatticus
    Apr 2, 2020 at 23:09
  • $\begingroup$ Thanks! The top right animation image perfectly describes the two options I have. $\endgroup$ Apr 3, 2020 at 0:04
  • $\begingroup$ And "tidal locking" means that the Earth is rotating. (It's rotating with a period of one year instead of one day, but it's still rotating.) $\endgroup$ Apr 12, 2020 at 21:11

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That depends on how you measure rotation, if it's not rotating in a solar frame then one side always faces the sun.

If it's not rotating in a sidereal frame then a day is a year and all the stars are fixed in the sky (not just Polaris).

Mercury spins (in a sidereal frame) only a little faster than it orbits, in a solar frame the years are longer than the days. but in a sidereal frame it rotates three times every two orbits.

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  • $\begingroup$ What does "rotating in a solar frame" actually mean? (I mean, "one side always faces the sun" is tidal locking, but that still means the Earth rotates.) $\endgroup$ Apr 12, 2020 at 21:12
  • $\begingroup$ Rotating in a solar frame means that difference faces are presented to the sun. The gravity gradient of the sun is less than that of the moon making all these scenarios unlikely in the presence of the moon, so I'm, treating this as an exercise in geometry more than in orbital mechanics. $\endgroup$
    – Jasen
    Apr 13, 2020 at 4:57
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That's up to you what "no longer rotates around its axis" means. Do you mean rotation in respect to the Sun, or in respect to the stars? The direction to the Sun changes, while the direction to the stars (approximately) doesn't, so depending on your choice you get either answer A or B.

There's no single way to decompose the movement of the Earth into its components. You can see it as the rotation that makes the Earth to face the Sun always with the same side + additional Rotation that causes solar days, or as the static situation that makes the Earth to face the far-away stars always with the same side + additional Rotation that causes star days. Which description is easier depends on what you want to use it foe, but neither is incorrect.

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Earth no longer rotates around its axis

For me this expression means no rotation in the frame of the stars. That means one day lasting one year.

We can imagine a disk-shape satelite orbiting the sun so that the plane of the orbit is the plane of the disk. If strain gages are installed to measure the stress between centre and periphery, increasing the rotation speed causes a increase in the tensile stress (or decrease in the compressive stress depending on the original stress state). The minimum of tensile stress is a measure of non rotation, and it corresponds to keep the same orientation regarding to the stars.

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I think the point your missing is that your statements

  • face the same way towards the Sun as it goes around it (permanent time of day everywhere, e.g. Europe always looks towards the sun, while China has permanent night)

  • not rotate at all (a day that lasts a whole year, e.g. the Earth would be to the Sun as the Moon currently is to the Earth)?

are completely mixed with each other: The Earth-Moon situation is the one where moon is facing earth the same side as it goes around it.

The problem is your definition of "day" when you confuse the two notions of day to mean

  1. the sun's position at the sky appearing to an observer on earth

  2. rotation with respect to a common frame of reference of both sun and earth (i.e. all other stars)

The second notion should not be considered "a day".

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