It is an occupational hazard of an astronomer to be asked about the supposed catastrophe(s) in the year 2012. Usually people want to know about the effects of the alignment of the planets in that year. Sometimes, prompted by the movie 2012, they mention neutrinos. Even if they don’t think the world will end, they are surprised when I tell them that there is no planetary alignment in 2012, and uncomprehending when I point out that it would not make a bit of difference if there were.

Here is a movie of where the 8 major planets are from 2000 through 2050 (click the link to open and play the movie; use the controls to scroll through and pause on any year, displayed at lower left).
Motion of the Planets from 2000 to 2050
Notice how closely spaced the four inner planets (Mercury, Venus, Earth, and Mars) are compared to the outer four. Notice also the frenetic pace of the inner planets. Clearly it’s not too rare for Earth and its terrestrial neighbors to be roughly lined up simply because they are all orbiting the Sun relatively quickly. And just as clearly, with its 165-year orbital period, Neptune (the triangle on the right side of the movie) is not very frequently in line with the rest of the planets. There is a much more impressive alignment of the planets, in fact, in 2010 than in 2012.

And just what are the implications of a planetary alignment? For the outer planets, it means skygazers will have a nice view of several planets each night, as they will all be up in the night sky at about the same time. That’s about it. If we tally up the ways planets could interact with us here on Earth, we come to a pretty short list:
– they reflect sunlight toward the Earth
– they have a gravitational effect on the Earth
Their magnetic fields do not extend to the Earth which is, anyway, enclosed in its own relatively strong magnetic field. The amount of sunlight coming to us from the planets is obviously puny and generally less than many stars.

For their gravitational influence, we can do a simple comparison. The gravitational acceleration exerted by an object on you or me is proportional to the mass of that object divided by the square of the distance between you or me and that object. So, for Jupiter, the most massive (and relatively nearby) planet, the gravitational acceleration you feel due to Jupiter’s presence is proportional to Jupiter’s mass (1.9 times 10^27 kg) divided by 3.9 times 10^25 meters squared. That gives us 48 in our units (where we are not worrying about the universal constant of gravitation since it will drop out when we make our comparison). Let’s compare that to, say, the acceleration you feel due to the gravitational influence of your spouse. If your spouse or significant other is a rather svelte 110 pounds (50 kg) and is sleeping 1 meter away from, then the gravitational acceleration you feel from that person is 50 divided by 1 squared, or 50, roughly the same value as the entire planet Jupiter. To pick a more dramatic example, when you stand next to your car, it is exerting a far greater influence on you (about 40 times greater) than all of the planets in the solar system. But then, you knew that, didn’t you?

The two astronomical objects that do produce a noticeable gravitational effect down here on the surface of the Earth are, not surprisingly, the Moon and the Sun. And when they line up it does have a measurable impact: the so-called “spring tides” or “full Moon tide” and “new Moon tide”. The gentle rising of the ocean up the beach every six hours or so is due to the tidal force of the Moon, and tides are simply due to the difference in the Moon’s gravitational pull across the body of the Earth. The Sun, although more massive, has a somewhat smaller effect on tides than the Moon because it is so much further away. But when the Sun and Moon are aligned (at full Moon and new Moon), their tidal effects combine and ocean tides are higher than usual. In the units we computed gravitational acceleration above, where Jupiter and the person standing next to you both rate about a 50, the Moon’s effect is about 460, 000 and the Sun’s is 9 billion. The Earth does go around the Sun after all. (Tidal force depends on the derivative of gravity and so gets weaker with distance faster than gravity, hence the weaker tidal influence of the Sun than the Moon.)

So catastrophes in 2012 are likely to be restricted to homebound, terrestrial causes (hurricanes, earthquakes, volcanoes, famine, drought, floods, and elections). The only sky-based catastrophe possible would be the impact of a comet. A catastrophic asteroid impact in 2012 is unlikely because we have much better advance warning for asteroids than for comets due to the shapes of their orbits (comets can sneak in from the outer solar system, while we have observed most dangerous asteroids).

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JC

Planetary Scientist and Asst. Professor of Physics at University of Central Florida; Movie Buff; Trekkie; Jethro Tull fanatic; part-time actor, piano player, writer; and full-time husband and father.

One thought on “”

  1. That explains the formidable gravitational pull between my backside and the couch. Never should have got a sectional.

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