Planetary alignments (in astronomy called ‘conjunctions‘) excite the public, and raise inevitable questions in their minds. Astronomers, especially those involved in public outreach programs, are used to answering these questions. However, when believers in various mystical systems use alignments as evidence for their own beliefs, many astronomers are at a loss to explain why the mystics are wrong.
There was a spectacular alignment in May of 2000 which raised all kinds of doomsday theories. Several authors were calling it “The Great Shift”. There are many ‘alignments’ that go unnoticed by us on Earth because they appear too close to the sun for us to see, which raises the question: Why is it only the alignments we can see that seem to have any effect?
The alignment argument
The argument for a planetary alignment goes something like this: Somehow, the planets will align, which will cause something else to happen, which will cause death and destruction on Earth.
There are several problems with this argument.
Alignments are not rare
First of all, alignments where the five naked-eye planets cluster together in the sky within a circle 25 degrees or less in diameter occur once every 57 years, on average. There was one in in 1962 which sent many people into a panic. There was another in 2000. They are fairly common. The next time it will happen is September 8, 2040. The 2040 grouping will include Mercury, Venus, Mars, Jupiter, Saturn and the crescent Moon. They will appear in the sky in a cluster well east of the Sun in the early evening.
Alignments do not cause problems
Some sites claim that ‘unique alignments will cause sunspots and massive earthquakes’. The problem is that planetary alignments do not cause sunspots, and do not cause earthquakes.
Alignments do not cause earthquakes
The ‘earthquakes’ claim appears to be that the gravitational influence of the non-existent alignments will cause earthquakes on Earth. However, the moon is much closer to us than any other astronomical object, so even though it is much smaller and lighter, it completely swamps out any gravitational or tidal effects caused by the Sun, Jupiter, Saturn, or any other object. The Sun does cause tides on the Earth, and when these tides are combined with the moon tides (at the full and new moons) then they are called ’spring tides’. When they are counter to the moon’s tides (at the first and last quarter moons) then they are called ‘neap tides’. The primary influence on the Earth’s tides is the Moon. Other planets are too small and too far away to cause any noticeable effect.
Here is what Astronomer Phil Plait has to say on this subject:
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 2x2=4. If you put something ten times farther away, the gravitational force drops by 10x10=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 2x2x2=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 6×10^24 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.
Planet Mass (10^22 kg) Distance Gravity (Moon=1) Tides( Moon=1) Mercury 33 92 0.00008 0.0000003 Venus 490 42 0.006 0.00005 Mars 64 80 0.0002 0.000001 Jupiter 200,000 630 0.01 0.000006 Saturn 57,000 1280 0.0007 0.0000002 Uranus 8,700 2720 0.00002 0.000000003 Neptune 10,000 4354 0.00001 0.000000001 Pluto ~1 5764 0.0000000006 0.00000000000004 Moon 7.4 0.384 1.0 1.0
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!
So Dr. Plait is pretty clear on this issue: there is no way that a planetary alignment of any kind can cause earthquakes. The premise (that planetary alignments can cause earthquakes) is incorrect.
Recently someone familiar with astrology told us that astrologers consider anything within 30 degrees an alignment. Maybe this is the basis for the claim? Then why are we hearing that there will be an exact alignment?
Conclusion: The claims of the consequences of alignments in general are false. We have shown that planetary alignments occur frequently with no ill effects, and are insignificant events to everyone except astronomers and rocket scientists. They do not cause noticeable effects on the Earth or on the Sun.