I am asking myself this question: if a giant celestial object would approach the Earth, would Earth start to turn faster (then day duration would be diminished) or slower (day duration increased) while the object starts to become nearer and nearer? Thank you!

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    $\begingroup$ I believe it depend on which direction and speed it is coming toward the earth. Of it is a fly by then it depend otherwise, it is going to slow down if its is headed straight for the center of the earth $\endgroup$ Nov 11, 2018 at 2:27
  • $\begingroup$ hi @ipmp . Welcome new user. The tidal effects would be absolutely tiny. There are plenty of other effects as well (relativistic! quantum! etc) But this question is simply about simple Newtonian physics of bodies. It is completely inappropriate to confuse the various domains. OP is just asking about how a point mass angular momentum is affected by being moved about. $\endgroup$
    – Fattie
    Nov 11, 2018 at 7:36
  • $\begingroup$ It can be both depending if the approach is kind of prograde or or retrograde as well as depending on the actual position of the passing body. Earth has already gravitational "handles" as it is oblate and everything will depend on the relative position of the giant object. If it is massive enough it can even made its own handles and eventually things might get stripped apart. $\endgroup$
    – Alchimista
    Dec 24, 2018 at 12:24

2 Answers 2


Could be, you'd be better to ask this on the Physics site, by the way.

No, the spinning of a perfectly round object is not affected at all, it just keeps spinning identically.

Let's say ..

enter image description here

say another Sun appeared, just like that.

Yes, the path that Earth is taking would go crazy. We might fly off, head towards this new Sun, starting making a figure of 8, or some crazy path!

But the spinning as such of the Earth would be totally unchanged.

The "way we point" ("North up" so to speak) would be completely unchanged, and the speed we turn at (ie, 24 hours a turn) would be unchanged.

This might seem surprising but, well, you can learn all about it in Newtonian physics!

Once again, no change to the spin as such of the Earth. the "direction we point" (ie the axis we spin around) and the time taken is totally unchanged if (for some bizarre reason!) we are "moved around in the heavens".

Note that there would be some absolutely tiny effects due to things like relativity (have a read!), the water on our planet sloshing around, and after all our Moon could be moved, bashed, etc. These effects on the speed we are spinning would be extremely small over the short period of time you're considering, in the "sci fi" scenario you outline, as seen in the diagram.

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    $\begingroup$ Can you support "But the spinning as such of the Earth would be totally unchanged" with some science based explanation? Also, tidal distortion is a reality of planetary rotation, and to just say "The tidal effects would be absolutely tiny" and "this question is simply about simple Newtonian physics" and then give a wrong answer is unhelpful and unscientific. $\endgroup$
    – uhoh
    Nov 11, 2018 at 10:12
  • $\begingroup$ The tidal effects would be absolutely tiny. If you can prove otherwise, go for it. This question is about basic Newtonian physics. It is foolish to answer in a different domain. It's not clever or pedantic, it's just incorrect to answer in the wrong domain. $\endgroup$
    – Fattie
    Nov 11, 2018 at 10:15
  • $\begingroup$ tidal effects on Earth alone may be tiny because it is "very round" - but what about tidal effects on the Earth-Moon combo, which is much less symmetric? I suppose the moon orbit could be changed, even drastically, even be "stolen". In the long run this would influence the earth rotation because predictions how Earth rotation changes by 2.5 ms per century needs to be adjusted. Then again, all this is still either negligible or going in unpredictable directions - not to mention that the influx on the Earth orbit (as demonstrated in the highly accurate image) might be a lot more drastic $\endgroup$ Nov 11, 2018 at 14:30
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    $\begingroup$ per Wikipedia The solar tidal force is 46% as large as the lunar. Moon: 1.1 × 10−7 g, Sun: 0.52 × 10−7 g, so here you are just saying wrong stuff that no amount of bold, italics or "!!!!!!!!! Gosh!" can right. However, I sure would like to know how you were able to produce the bold italics! $\endgroup$
    – uhoh
    Nov 11, 2018 at 17:10
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    $\begingroup$ @uhoh italics is one asterisk either side of the expression, bold needs two asterisks, bold italics is three :-) $\endgroup$ Dec 12, 2018 at 9:16

If a giant object would approach the Earth, would Earth start to turn faster or slower while the object starts to become nearer and nearer?

tl;dr: Probably will slow it down a small amount, but may induce a wobble that make a simple answer impossible without further details.

The problem is complicated. I'll leave a short answer.

The question is carefully written in (at least two) ways. It is agnostic to the size of the effect, instead asking only about its sign. So arguments that the effects are too tiny to matter are moot. It also does not specify the nature, distance, or duration of the approach. That makes the question quite difficult to answer definitively.

induced tidal distortion

The most straight-forward effect we know about is slowing due to tidal forces. The Theory subsection of Wikipedia's article Tidal acceleration outlines this nicely.

Because the Earth is slightly stretchy, the Moon's or another close body's gravity induces a tidal bulge. Wikipedia's Earth tide gives several components of a thorough decomposition, but the top two are about 38 cm due to the Moon and 17 cm due to the Sun. These exist on both the near and far side of each body.

Because the Earth responds slowly, these have a "lag". The lumps exist offset from the line between the Earth and the other body, and this allows for a torque. In the case of the Earth-Moon system that torque then tends to slow the Earth's rotation.

If the body is double in mass, the induced bulge's mass will be roughly linearly proportional and since the torque proportional to both it will be quadruple, proportional to $m^2$. As shown in that subsection it is also proportional to $r^{-6}. So just for example, a planet about the size of the Earth (~80 Moons) and half the distance of the moon would have a torque 80 x 80 x 64 times larger.

However, it's hard to imagine a scenario where that could last for a thousand years to have a huge effect on Earth's rotation speed. This would be small, but definitely negative.

interaction with equatorial bulge

The body could pass perpendicular to the Earth's equator as well, and produce a really huge torque on Earth's equatorial bulge (expressed often as the $J_2$ term). That bulge is more like 30 kilometers, compared to the 30 centimeters of the induced tidal bulge by the Moon.

A perpendicular pass like that would induce a substantial wobble in the Earth's rotation during the time of passage, but I don't think it is easy to say with certainty the sign of the effect on the rotation rate with such complex motion and no further details.

  • $\begingroup$ :) Quantative hurts. So the example is an object the size of Earth, which arrives from outer space and passes us - we'll say as close as you like , 10,000 km separation. Let's say it's near us (within a million miles) for 10 minutes. No wait, let's say a day .. no, let's say a month. How much is the day slowed by tidal effects? $\endgroup$
    – Fattie
    Nov 13, 2018 at 1:39
  • $\begingroup$ @Fattie that's a detailed calculation requiring a detailed description of the particular conditions. The beauty of the question is that it does not ask for the magnitude, only the sign. $\endgroup$
    – uhoh
    Nov 13, 2018 at 1:42
  • $\begingroup$ Heh! :) I just gave you exact figures, that match the story description in the question. $\endgroup$
    – Fattie
    Nov 13, 2018 at 1:43
  • $\begingroup$ @Fattie you can't ask a new question in comments. You are not new to SE and so you already know what comments are for, and what they are not for. $\endgroup$
    – uhoh
    Nov 13, 2018 at 1:46
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    $\begingroup$ BTW it is three asterisks front and back for this :) $\endgroup$
    – Fattie
    Nov 13, 2018 at 3:12

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