<p>There is <strong>no gravitational phenomenon associated with <em>exact</em> alignment</strong>. (moreover, exact alignment never happens, there is always a small deviation).</p>

<p>The directions of tidal forces are aligned and hence sum-up for alignment, but this maximum is quite broad and nothing spectacular happens near exact alignment.</p>

<p>You need close to exact alignment (depending on the sizes of the objects) to get an <strong>eclipse</strong> of one sort or another (a Solar complete eclipse is quite spectacular). This presumably requires the closest to exact alignment of all possible effects, but is not gravitational.</p>

<p><strong>added in edit</strong></p>

<p>If the planetary orbits were <em>exactly</em> co-planar, then <em>exact</em> alignment of the three bodies would occur regularly. However, the <em>chance</em> for the planetary orbits to be <em>exactly</em> (to infinite precision) co-planar is zero. This is just simple probability of continuous (as opposed to discrete) random variables: The <em>probability density</em> $p(x)$ for a continuous variable $x$ is finite, hence finding a value between $x$ and $x+\delta x$ has probability $p(x)\delta x$, which goes to zero as $\delta x\to0$. The chance of hitting <em>close to exact</em> alignment is the smaller the closer you want it to be.</p>

CC BY-SA 3.0
2014-09-11T19:16:47.150Z
2014-09-12T09:01:50.020Z
2014-09-12T09:01:50.020Z
Walter
<p>Referring to your #gravity tag this would only create a peak in the planets tidal forces. But this would be a visible effect in case the planets would be big and close enough... Last night happened that with Sun-Earth-Moon although they were not aligned as you say. </p>

<p>To sum up: <strong>a proper alignment would increase the effect of tidal forces.</strong></p>

CC BY-SA 3.0
2014-09-12T01:51:50.147Z
Sure, but the effect is infinitesimal - impossible to detect except if you are really looking for it. TLDR: nothing.
Florin Andrei
CC BY-SA 3.0
2014-09-10T08:14:28.963Z
2014-09-10T08:14:28.963Z
Joan.bdm
<p>Imagine a line passing through the cores of two planets (in the same planetary system) and their star. Have we ever witnessed this - either with both planets on the same side of the star or on opposite sides? If so, was there a unique phenomenon associated with the event?</p>

CC BY-SA 3.0
2014-09-10T07:51:47.630Z
It seems odd no one has made a planetary-system tag.
Honest Abe
CC BY-SA 3.0
2014-09-10T23:13:54.803Z
A lot of the "stellar-system" questions fit that tag.
HDE 226868
CC BY-SA 3.0
2014-09-11T10:39:22.457Z
https://en.wikipedia.org/wiki/Syzygy_(astronomy)
TildalWave
CC BY-SA 3.0
2014-09-11T14:42:59.180Z
Superior and inferior conjunctions, transits: http://en.wikipedia.org/wiki/Conjunction_%28astronomy%29 http://eclipse.gsfc.nasa.gov/transit/transit.html
Wayfaring Stranger
CC BY-SA 3.0
2014-09-20T03:48:57.160Z
Isn't this pretty much what happens during a solar or lunar eclipse, except that the moon and the Earth are closer than planets would be? The tides are a bit higher, but not much else happens.
Marc
CC BY-SA 3.0
2014-09-20T18:42:33.570Z
@Marc The key difference is that the Earth orbits the Sun and the Moon orbits the Earth. I'm interested in observations regarding two planets that both orbit the same star. I didn't have high hopes that anyone had actually witnessed and analyzed this event (yet).
Honest Abe
CC BY-SA 3.0
2014-09-21T23:26:18.930Z
I don't see any reason to expect *more* effect from to widely separated planets in direct alignment with their star than you'd see with two bodies in a double planet in direct alignment. Everything is the same, except the gravitational forces between the two planets in a double planet system would be much, much greater, and all you get is a minor increase in tides.
Marc
<star><planet><gravity><planetary-systems>
any unique phenomenon when the cores of two planets are aligned with their star's core?
Honest Abe
# any unique phenomenon when the cores of two planets are aligned with their star's core?

(Asked by: username_0 on 2014-09-10)

Imagine a line passing through the cores of two planets (in the same planetary system) and their star. Have we ever witnessed this - either with both planets on the same side of the star or on opposite sides? If so, was there a unique phenomenon associated with the event?

**Tags:** star, planet, gravity, planetary-systems
> **Comments:**
> **username_0**: It seems odd no one has made a planetary-system tag.  
> **username_1**: A lot of the "stellar-system" questions fit that tag.  
> **username_2**: https://en.wikipedia.org/wiki/Syzygy_(astronomy)  
> **username_3**: Superior and inferior conjunctions, transits: http://en.wikipedia.org/wiki/Conjunction_%28astronomy%29 http://eclipse.gsfc.nasa.gov/transit/transit.html  
> **username_4**: Isn't this pretty much what happens during a solar or lunar eclipse, except that the moon and the Earth are closer than planets would be? The tides are a bit higher, but not much else happens.  
> **username_0**: @Marc The key difference is that the Earth orbits the Sun and the Moon orbits the Earth. I'm interested in observations regarding two planets that both orbit the same star. I didn't have high hopes that anyone had actually witnessed and analyzed this event (yet).  
> **username_4**: I don't see any reason to expect *more* effect from to widely separated planets in direct alignment with their star than you'd see with two bodies in a double planet in direct alignment. Everything is the same, except the gravitational forces between the two planets in a double planet system would be much, much greater, and all you get is a minor increase in tides.  

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## 2 answers

### Answer by username_5 on 2014-09-11. Score: 4

There is **no gravitational phenomenon associated with *exact* alignment**. (moreover, exact alignment never happens, there is always a small deviation).

The directions of tidal forces are aligned and hence sum-up for alignment, but this maximum is quite broad and nothing spectacular happens near exact alignment.

You need close to exact alignment (depending on the sizes of the objects) to get an **eclipse** of one sort or another (a Solar complete eclipse is quite spectacular). This presumably requires the closest to exact alignment of all possible effects, but is not gravitational.

**added in edit**

If the planetary orbits were *exactly* co-planar, then *exact* alignment of the three bodies would occur regularly. However, the *chance* for the planetary orbits to be *exactly* (to infinite precision) co-planar is zero. This is just simple probability of continuous (as opposed to discrete) random variables: The *probability density* $p(x)$ for a continuous variable $x$ is finite, hence finding a value between $x$ and $x+\delta x$ has probability $p(x)\delta x$, which goes to zero as $\delta x\to0$. The chance of hitting *close to exact* alignment is the smaller the closer you want it to be.


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### Answer by username_6 on 2014-09-10. Score: 4

Referring to your #gravity tag this would only create a peak in the planets tidal forces. But this would be a visible effect in case the planets would be big and close enough... Last night happened that with Sun-Earth-Moon although they were not aligned as you say.

To sum up: **a proper alignment would increase the effect of tidal forces.**

> **Comments:**
> **username_7**: Sure, but the effect is infinitesimal - impossible to detect except if you are really looking for it. TLDR: nothing.  

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