# Making doomsayers right - a moon(s), planet alignment that matters

Considering our topic challenge, and the fantastic eclipse last Sunday a question came to me.

Could there be a stable (relatively speaking) planetary system where a(n) eclipse/alignment would actually make a noticeable difference on an earth like planet?

The eclipse/alignment should cause one or more of the following:

• Large, powerful waves that can severely damage or flood coastal areas.
• Earthquakes/tremors
• Powerful storm systems

The planet:

• should be as Earth-like as possible
• must have at least one moon (it may have more)

The questions:

• What would the planet, moon, and star sizes be?
• What would the distances between them be? (Meaning the planet and moon (or moons)
• Would eclipses occur on a regular or irregular basis?
• Pern has serious problems relating to a planet flying too close every so often. pern.wikia.com/wiki/Thread Not #hard-science though. XD – Jerenda Feb 12 '16 at 15:20
• You could have a Janus/Epimetheus orbit, where the two planets pass close enough to each other to cause earthquakes, floods, and volcanic activity. One apocalypse a year! – Xandar The Zenon Feb 12 '16 at 15:58
• It would actually be more like two apocalypses, on on each planet. Years could also go by really slowly, so you have more of a gap in between apocalypses. – Xandar The Zenon Feb 12 '16 at 16:06
• I get asking for a particular positioning of the planets but an eclipse in itself its no reason for tidal waves, earthquakes etc, its one body obscuring another. – Erik vanDoren Feb 12 '16 at 18:42
• @ErikvanDoren true but during an eclipse they are in alignment, so this is more relevant for solar eclipses, as for exerted on earth would all be in one direction. – James Feb 12 '16 at 19:03

I'm ~99% certain that the effects of a second celestial body on seismic activity on an Earth-like planet has been covered before (in that case, by a second Earth-like planet); if anyone can point me to it, that would be great. The conclusion - if I remember correctly, and I think I do - was that there wouldn't be any major effects in this area. I might have supported that conclusion, in which case I may have been wrong.

Scientific American has an interesting article on the subject. It turns out that a causal relationship between the moon and seismic activity was first postulated a long time ago. Scientific American itself published a minor story on the idea in 1855, based on the work of one Alexis Perrey. Apparently, Perrey showed three correlated relationships:

1. The frequency of earthquakes/tremors is increased during a syzygy - a time when the Earth, the Moon and the Sun are in a straight line.
2. The frequency increases during the Moon's closest approach (perigee), and decreases during the Moon's furthest approach (apogee).
3. The frequency increases when "the moon is near the meridian, than when 60° from it." I'm not entirely sure what Perrey means here, so I won't attempt an interpretation.

Perrey's work comes from "7,000 observations", which seems convincing, but it is entirely based on observations, it seems - there is no explicit theory as to why this is the case. I'm not saying that should remove credence from it, but note that no causal relationship was proven.

More recently, Straser (2010) and Vergos et al. (2015) (paywalled version; a difference version is available via ResearchGate)) investigated the problem. The former also summarized previous work on the problem, which had attempted to show a number of relationships between earthquakes and the Moon. Here are some of those works:

• Omori (1908): The rhythms of the tides can cause a rise in earthquake frequency.
• Bagby (1973): Syzygies increase earthquake frequency (this is the same as one of Perrey's conclusions).
• Kokus (2006): Changes in the Moon's motion can influence fault behavior.
• Kolvankar et al. (2010): Earthquake frequencies change according to the lunar cycle.
• Zhao (2008): The Earth can induce earthquakes on the Moon - "moonquakes".

The main point here is that tidal forces can apparently influence earthquake frequency. However, the author's conclusion was that - especially as regards his own research - links can be tenuous at times.

Vergos et al. studied an earthquake and related tremors in Greece, and established a relation between the phase angle of an earthquake ($\phi_i$) and the period of a relative tidal component ($T_d$): $$\phi_i=\left(\left[\frac{t_i-t_0}{T_d}\right]-\text{int}\left[\frac{t_i-t_0}{T_d}\right]\right)$$ Can we establish a causal relationship from all this data? Not necessarily. We have no theoretical model to explain it, either. The USGS has written some of the resultant phenomena off as coincidences (see this article). I think, however, that the evidence is compelling enough to show that some relationship might exist.

In your case, we can take advantage of syzgies. The more bodies - in this case, more moons - the greater the effects, in theory. The differential force experienced by Earth is proportional to $r^{-3}$, however, not $r^{-2}$ (see here; keep this in mind for calculations).

To answer your questions about mass and distance, I say only that it is up to you. We don't know enough to come up with accurate formulae for the effects - if they exist - so we can't know for sure what conditions are necessary to cause a given result. I can tell you that the alignment - for it is an alignment that you need, not an eclipse - would be periodic, because orbits (and therefore orbital alignments) are periodic.

Another approach:

It's not a moon that's causing the eclipse. Rather, it's a large planet that occasionally passes very close to the world in question. There will be stability issues here but so far they have been countered by the fact that the worlds are in resonance. The perihelion for the world getting beat up (the other world suffers also but figure it's a gas giant) has been very slowly decaying due to these encounters, as it decays the encounters get closer and closer (and thus more damaging) until eventually you either get a major orbital disruption or else it's destruction.

• This doesn't meet the focus of the question, nor does it satisfy the requirements of the hard-science tag. – HDE 226868 Sep 30 '15 at 0:14
• @HDE226868 He wants something astrological that actually matters--this would. It meets that part of it. And why can't you have a orbit that causes a close encounter? If it wasn't a resonance orbit it would certainly result in the planet being flung away (and will in time anyway) but I doubt he needs something that lasts more than historical time. – Loren Pechtel Sep 30 '15 at 1:23
• Sure, it meets that requirement, but it doesn't back itself up like it should. – HDE 226868 Sep 30 '15 at 1:27

Rather than using the more conventional approach of abusing gravitational effects to produce our doomsday, this answer relies on solar radiation and optics. There is one caveat - it requires a very weird moon which, while scientifically possible, must be an artificially created structure.

Approach:

Make the moon a spherical lens. During an 'eclipse', the moon will focus the sun's rays to a single point on the earth's surface. This will cause rapid concentrated heating, leading to drastic weather changes (as well as melting any location unfortunate enough to fall under the focal point).

This will only occur during a perfectly aligned total lunar eclipse; an imperfect alignment will cause the focal beam to miss the earth.

While this is perhaps somewhat outside of the intended scope of the question, it does fit within the spirit of the question - a celestial alignment causing doomsday-like effects.

Other than the composition of the moon, the solar system is similar to ours for the purposes of interplanetary distances and eclipse frequency.

The Lens:

The effective focal length of a lens is:

$$EFL = \frac{nD}{4(n-1)}$$

(source)

Then, let: $$D = Distance\;from\;earth\;to\;moon = 384,400\,km$$ $$EFL=Diameter\;of\;moon = 3,474\,km$$

Putting this into the equation, this gives an index of refraction of approximately n = 1.0023. We can achieve something close to this by using benzene gas as our refractive material (n = 1.0018). To get a bit closer to this value of n, we can increase the distance to 49,3774 km or decrease the diameter to 2704 km.

This leaves us with a moon comprised of a solid transparent shell filled with benzene gas or similar for our lens.

Note that this means the moon will be much lighter than our moon, so the planet would not likely experience any tides.

Effects:

During an 'eclipse', the moon lens will focus (most of) the sunlight passing through it to a small point on the earth's surface.

At earth's orbit, the power density of sunlight is approximately 1.36 kW/m2 (source).

Given a diameter of 3,474 km (r = 1.737 x 106 m), the cross-sectional area of the moon will be:

$$\pi r ^2 = 9.4787 \times 10^{12}\,m^2$$

This means that we will have 1.289x1016 Watts passing through the lens.

If we assume a totality/alignment of about 100 minutes (6000 s) (source), this gives a total energy output of about 7.734x1019 Joules over the course of the eclipse.

While the location of the focal point will (rapidly) move across earth's surface during the eclipse, this is still enough energy to cause plenty of damage. For instance, if the focal point spent most of its time over ocean, it would boil away somewhere around 3 x 1016 g of water. Given that a typical hurricane produces about 2.1 x 1016 g of rain (source), this should be sufficient to produce some spectacular storms.

For some more context on how much energy we are seeing at the focal point, I took a look at https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy).

Notably, the focal point outputs an equivalent amount of energy to the Hiroshima bomb every microsecond.

It is possible.

• The planet should have more than one moon, revolving in the same plane, at different distances from the planet. The moons should be as large and close to the planet as practically possible, without messing things up.

• The star should be as heavy as practically possible without messing things up.

• The oceans should be very deep (~5 km average depth as compared to ~3 km on earth).

When/if all the moons get in-line with the star, this compound eclipse would have horrible consequences. We are talking tsunamis (tidal effect), raging storms (tidal effect on the atmosphere) and earthquakes (tidal effects on the crust) here.

• Hard Science!!! – James Sep 29 '15 at 19:59
• It is. There are planets with more than one moon. And the moons can line up (even if they exist in different interfering orbital planes). – Youstay Igo Sep 29 '15 at 20:03
• @YoustayIgo James is referencing the hard-science tag he has applied to the question. It requires much more rigorous detail in an answer than you have here so far. If this is actually a solution to the question there should be math and science (at the very least some citations) to back it up. – Avernium Sep 29 '15 at 21:08
• @Avernium thanks for clearing up what I meant. Youstay, apologies for not elaborating, but yes the hard science tag comes with certain expectations of citation and/or calculation. – James Sep 30 '15 at 13:51
• Could have been done when I was writing that answer, but too much work and hassle now. Can't be bothered to go on quoting planetary system and statistical analysis now. – Youstay Igo Sep 30 '15 at 13:53