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Let say Mercury, Venus, the Moon were all in alignment between the Earth and Sun. They are all closest to each other in respective elliptical orbit. On the other side of the Sun you have the rest of the planets.

Would their gravity combine to have any gravitational effects on the Earth? Or are they all too far away?

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    $\begingroup$ Compared to the Sun and Moon, the other planets' gravitational influence on the Earth is minimal. Google around and you should find some hard numbers. $\endgroup$ – barrycarter Feb 26 '18 at 21:28
  • $\begingroup$ Muze, the simple answer is they have a tiny effect. Essentially "no" effect. It makes little difference how they are aligned. $\endgroup$ – Fattie Feb 27 '18 at 0:26
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TL;DR: No. Well, yes, but they are negligible.


Full answer: a conjunction like this 'nearly' happened on May 5th, 2000. NASA published an article about this event, stating:

For example, the combined gravitational effect of all the planets together is much less than the effect of the Sun or the Moon on the Earth.

and

While unusual, such alignments have happened in the past without any consequences. The planets are simply too far away to have an effect on anything here on Earth - except our imaginations.

For hard numbers, see this article by Truman Collins which is linked there:

Conclusion

What I discovered was that the tidal forces exerted on Earth from the five planets on the other side of the Sun are vastly smaller than those exerted by the Moon or the Sun. In fact, with the planets in the position they will be in on the big day, the largest tidal force from one of them will be from Jupiter, and it is about one five hundred thousandth the size of the tidal force exerted by the Moon on an average day!

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    $\begingroup$ I'd not say "no". "Yes, but the effects are very small" is more accurate. $\endgroup$ – Allure Feb 27 '18 at 1:07
  • $\begingroup$ I think it is better to point out that from the perspective of gravitational influence, there is nothing special about a conjunction, or planets "lining up". Popular media likes to use arbitrary configurations and arbitrarily accurate times to generate click bait, but in terms of gravity, the influence is always there, and it varies only very gradually. $\endgroup$ – uhoh Feb 27 '18 at 12:19
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Whether they are in a line is irrelevant.

All masses have a gravitational effect. The gravitational effect of planets causes perturbation from an elliptical orbit. The gravitational pull of Jupiter (and to a lesser extent Saturn and the other planets) causes the Earth's orbit to change from nearly circular to slightly elliptical then back to circular. The planets also cause the angle that the elliptical orbit makes to rotate around the sun. Again this mostly caused by Jupiter.

The eccentricity varies with roughly 100000 year period, and the angle varies over 112000 years.

These effects are well understood and can be observed and measured. However alignments are inconsequential. Alignments of the planets have no effect on the Earth. They do not cause earthquakes. They do not cause volcanos.

The moon does have significant effects, as it is the primary cause of tides.

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