Harmonic Con(game)vergenceBad Astronomy: Planetary alignments will cause earthquakes.
Good astronomy: Planetary alignments have relatively little to do with earthquakes.
NEW(and very cool)!
How it works:
Every few years, doomsayers start popping up and talking about the planets in the solar system lining up. This alignment, they claim, will cause earthquakes, floods and in some cases split the planet in two like a cleaver through a head of lettuce. The last time this happened was in 1982, which, you may remember, is notable as the date on which the world did not end. The next alignment, so they say, is in May of 2000. Many doomsayers also point out that that's the year of the millennium (they're wrong, but that's another Bad Astronomy issue altogether)! Are we doomed this time?
I have taken on one company that is trying to make a profit from peddling disaster nonsense. I have a page describing what I have done.
At first glance, these claims do seem interesting. Gravity is a long reaching force, and the planets are big. Can their influence reach across the solar system and cause all sorts of headaches here on Earth?
As always, it is not a bad idea to run a sanity check first. We have pretty good evidence that the Earth has been around a long time, like 5 billion years or so, without being cleft in two. As it turns out, planetary alignments are fairly rare. Getting more than three lined up is difficult; getting them all lined up is rare indeed. But 5 billion years is a long time! Alignments may be rare, but given enough time they do occur, and the Earth is still here. Even more, what most doomsayers say is an alignment is really more of a confluence, or loose gathering, of planets. Some say it is enough to just have them all on the same side of the Sun, which happens pretty often! This is a hallmark of crackpot science: using inflammatory words, then, when cornered, start being very vague and loose with your terms. "Alignment" sounds much better than "loose collection" and "a pattern somewhat weighted towards one side of the Sun", which are more accurate. I think we can rest assured that the Earth will not be destroyed any time soon.
Brian Monson has also worked out the positions and times of several past alignments and shows that better alignments in recent history than the one coming up in May of 2000 have occurred with no ill effects. He also has a couple of nice sky maps of these alignments on his conjunction analysis webpage. There is also an excellent page giving great detail about the upcoming "alignment" brought to you by the good folks at The Griffith Observatory, who are also good friends of these Bad Astronomy pages. Yet another page has been set up by Truman Collins as well.
But just how strong is the influence of the planets? This turns out to be a relatively easy calculation. (MATH WARNING: for those that hate math, the next few paragraphs may disturb you, but are necessary.) First, let me make something clear: there are two effects a planet can have. One is simply gravity, which basically means how hard that planet can pull on us. The other influence is tidal force, which is more complicated, but you can think of it as a stretching force rather than a simple pull. Think of it this way: a strong enough gravity could pull the Earth from its orbit, while a strong enough tide could rip it in half. Can the planets do this to us? Could they possibly send Earth flying into space, or rend us asunder (quick answer: no)? I will start with gravity, and then show why tides are even less important.
Gravity depends on two things: the mass of the object pulling on you, and its distance. The more mass something has, the stronger it pulls, and the farther away it is, the weaker it pulls. As a matter of fact, the strength depends on the square of the distance. If you double the distance, the force of gravity drops by 2 x 2=4. If you put something ten times farther away, the gravitational force drops by 10 x 10=100. You can see that gravity gets weak pretty quickly with distance.
The tidal force is much like gravity, but it drops with the cube of the distance. This makes it much less important in our case! Say you double the distance to an object. Its tidal force on the Earth drops by 2 x 2 x 2=8. If you increase its distance by a factor of ten, the tidal force drops by 10 x 10 x 10=1000! So tides are in fact much weaker than gravity. (If you want a more detailed description of tides, what causes them and how they behave, I suggest you read my web page all about tides.)
So if we know the mass of an object and its distance, we can calculate the forces of both gravity and tides. It shouldn't be too much of a surprise to find out that the overwhelming winner in this game is the Earth's own Moon. It doesn't mass much (only about 1/80 of the Earth), but it is very close (Venus, the closest planet to the Earth, is at best 150 times farther away!). To make matters easier on us, let's say that the moon's gravitational force on the Earth is equal to 1 in whatever units gravity is measured in. That way we can see right away how strong the other planets are; a gravity of 10 means the planet pulls on the Earth 10 times as much as the Moon does. We can do the same with tides; assume that the tidal force is equal to 1 in tidal force units and see how the other planets fare. So, in units of Moon gravity and tides, below are the forces on the Earth from rest of the planets (the data for masses and distances are from the wonderful page The Nine Planets). The masses are in units of 10^22 kilograms (the Earth masses 6x10^24 kilograms, or 600 on this scale), and the distances in millions of kilometers. By the way, I used the distances of closest approach to the Earth to maximize the effect. Realistically, the force will be smaller than what is listed.
Let's look at gravity first. Right away you can see that even mighty Jupiter, king of the planets, only pulls about 0.01 (= 1%) as hard as the Moon does (just to show how I did this, Jupiter masses 27,000 times the Moon, but is 1640 times farther away. The square of 1640 is about 2.7 million, and 27,000/2.7 million=0.01). Venus is next, with only 0.6 % of the Moon's force. After that, the numbers drop a lot. The total pull of all the planets combined is 0.017, not even 2% of the Moon's pull!
That ain't much. But is it enough to destroy the Earth?
No, it isn't. Think of it this way: the Moon orbits the Earth in an ellipse, which means that sometimes in its orbit it is closer to the Earth than others. At perigee, or closest approach, it is about 363,000 kilometers away, and at apogee, or farthest point, it is about 405,000 kilometers away. If you use these numbers like we did above, you see that the Moon's own gravitational effect on the Earth fluctuates by about 25% every orbit! The Moon orbits the Earth in about a month, incidentally, so it goes from apogee to perigee every two weeks. So every 14 days we see a change in gravitational effects from the Moon more than 10 times greater than all the other planets combined! To put this in even more perspective, the force of the Moon on you is only about 0.000003 times the Earth's. For me, that means I weigh an extra 0.4 grams (0.0009 pounds) more when the Moon is under my feet versus when it's on the horizon (and therefore not contributing to the downward pull of the Earth). Not much! [Oops! (January 13, 2001): A reader pointed out to me that I blew it here. Grams measure mass, not weight. I would still mass the same amount, but my weight will increase when the Moon is under me. Converting from units of weight (pounds) to mass (grams) is only good when you are in one gravity. So really I should have reported my weight in pounds, not in grams.]
Now let's look at tides. Venus stretches us the most of the planets, simply because it is the closest on average. But look! Even Venus only stretches us 5 hundred thousandths as much as the Moon does! This is completely negligible, and the other planets have even less effect. The change in tidal force due to the Moon's elliptical orbit is hugely larger than the combined tides of all the planets. It's worth mentioning that the "alignment" in 2000 has all the planets on the far side of the Sun. This means that you can add 300 million kilometers to the above distances, and I think you can see that the numbers will drop even more. For example, Jupiter's gravity drops from 0.02 to 0.005, and Venus' tides drop by a factor of 500!
Note again that the Earth still exists. Feel better now? Of course you do. Bad Astronomy, it would seem, has a much stronger influence on our minds than our bodies.
Thanks to Bad Reader Mark Thomas for pointing out to me that this page implied that I thought the millennium came in the year 2000. The new millennium starts in the year 2001, and I'll have a page about that shortly.
Bad Addendum: There are tons of programs out there on the 'net that let anyone with a computer plot planetary positions over a long period of time. Two good places to start are The Nine Planets Planetarium Software page, and the AstroNet software list. I found two images that already show the planet positions for the two dates. These maps are courtesy of Jean-Luc Romano.
Note that in the year 2000 map, there is a loose alignment of planets, but on the opposite side of the Sun, as I noted above. I'd cancel the planetary disaster insurance if I were you.
Bad Addendum II: I might as well put all the links I have to planetary
alignments in one place: