Here's the 'superalignment' I'm referring to:

Planetary Superalignment

We've all heard the stories about 'mystical planetary alignments' that will increase/decrease the effective surface gravity experienced on Earth (one debunked here on snopes), sometimes referred to as 'Zero G Day'.

What I'm wondering is: what would be the maximum possible effect on a given weight (ratio of 'normal' weight to 'alignment' weight)?

  1. Noon at a new moon, Venus and Mercury between the Earth and the Sun, Mars, Jupiter, Saturn, Uranus and Neptune across the sun in roughly a straight line (maximum lightness).
  2. Midnight during the same alignment (maximum heaviness - almost the same ratio, but 2 Earth radii further away from the planets and sun).

Also, how often (if ever) could this happen?


I have calculated the resulting effects of this 'superalignment':

Planetary Superalignment Calculator

The result is that with the planets and our moon aligned as much as they can be to have their forces be additive, their gravity culminates in a $\pm0.06\%$ difference. Since I weigh 90kg, I would weigh 89.94 kg at noon and 90.05 kg at midnight.

Now, the last part of this question remains - would this superalignment, or something approximating this superalignment, ever occur, and if so would it be on a repetition and how often?

  • 1
    $\begingroup$ You can calculate this yourself. Use Newton's formula for the gravitational force and plug in some numbers for the minimum distances between the planets and the moon. You will find that the effects are very small. Having said that, the long term dynamics of the solar system depends very delicately on these small forces adding up resonantly over millions and hundreds of millions of years. The cumulative effects are large and it is predicted that the solar system is most likely unstable. en.wikipedia.org/wiki/Stability_of_the_Solar_System $\endgroup$
    – CuriousOne
    Dec 24, 2014 at 9:25
  • $\begingroup$ I'm not after minimum distance of each planet summed; I'm looking for maximum cumulative gravitational effects (Jupiter, for example, would be across the Sun). Also, how would it be possible to answer how often (if ever) said alignment with as much error margin is necessary would occur? $\endgroup$
    – Ehryk
    Dec 24, 2014 at 9:29
  • 1
    $\begingroup$ So what stops you from calculating it yourself? A simple Excel spreadsheet could tell you a good approximation to the max. and there are plenty of digital orreries available to calculate the future constellations. $\endgroup$
    – CuriousOne
    Dec 24, 2014 at 9:59
  • 1
    $\begingroup$ With the latest edit to this question, making it about whether the alignment could happen at all, it's a perfectly good question, but the stuff about how large the gravitational effect would be is now pretty irrelevant. So I'd suggest taking that out entirely. $\endgroup$
    – David Z
    Dec 25, 2014 at 1:49
  • 1
    $\begingroup$ Hi Ehryk. If you haven't already done so, please take a minute to read the definition of when to use the homework-and-exercises tag, and the Phys.SE policy for homework-like problems. $\endgroup$
    – Qmechanic
    Dec 25, 2014 at 8:04

1 Answer 1


SciShow covered this and their sources are in the video description. It probably can't happen, but it depends on your tolerances.

Planetary alignments have a "quality" which is how far apart in the sky the planets are allowed to be and still considered in "alignment". All planets lined up in a nice straight line from the Earth to the Sun? Almost impossible due to the planets' movement above and below the plane of the ecliptic. How about lining up in just two dimensions? Also almost impossible, there are too many moving parts.

This source from the National Solar Observatory calculated the next time the planets will be aligned within 30 degrees is March 20, 2673 and the last time was Jan 1, 1665. There's details in the article.

As for the effect on gravity, it will be inconsequential compared to the pull of the Moon and Jupiter. Phil Plait did the math (so did NASA, but only for tides) and Jupiter only has 1% the gravitational influence as the Moon with the rest of the planets rapidly falling off into inconsequential amounts. The position of the Moon and Jupiter will swamp any other gravitational considerations.

  • $\begingroup$ Thanks for the info, I'll review the details when I get a chance. I'm beginning to realize that what I'm after is something different than the typical definition of planetary alignment, and includes a solar and lunar alignment in the same gravitational direction, in other words Mars through Neptune are across the Sun. $\endgroup$
    – Ehryk
    Dec 25, 2014 at 20:33
  • $\begingroup$ @Ehryk The odds won't be very different. You want 8 orbital bodies to be in certain positions in their orbits, it doesn't matter much what those positions are unless they're in orbital resonance (only a problem with Pluto and Neptune). Due to the overwhelming gravitational influence of Jupiter and the Moon, the position of the other planets will have no appreciable effect. $\endgroup$
    – Schwern
    Dec 25, 2014 at 21:45
  • $\begingroup$ I want 8 planets, a star, and the Earth's moon to be in alignment. 10 bodies. $\endgroup$
    – Ehryk
    Dec 25, 2014 at 22:02
  • $\begingroup$ @Ehryk With a period of 28 days, less than a third of the next fastest, Mercury, the Moon doesn't make much difference. The Sun doesn't count, it's stationary relative to everything else. $\endgroup$
    – Schwern
    Dec 25, 2014 at 22:21
  • $\begingroup$ The sun does count. Many 'planetary alignments' are based on aligning 'in the sky', and being collinear without solar inclusion. Here's an example of such an alignment: modernsurvivalblog.com/wp-content/uploads/2011/05/… $\endgroup$
    – Ehryk
    Dec 25, 2014 at 23:54

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