The Question

Is it possible that on May 5, 2000 the alignment of the planets can have enough gravitational effect on the earth to cause it to tip slightly causing mass destruction.

I have tried to find this answer from many, but everyone refuses to respond. Please help.

The Answer

Long ago Sir Isaac Newton gave us a mathematical description of how one object affects, and is affected by, the gravitational force of another object. Many, many years of observations have proven this description to be accurate (at least for masses like those of the planets). Newton's Law of Gravitation states: The force between any two objects having masses M₁ and M₂ separated by a distance R is an attraction along the line joining the objects and has a magnitude

F = (G x M₁ x M₂) / (R x R)

G is the universal gravitational constant which has a value of 6.6732 x 10^-11 newton-meters²/kg² for all pairs of objects. A "newton" is a unit of force that physicists use. It is defined to be the amount of force needed to accelerate a 1 kg mass at 1 meter per sec². The important thing to remember here is that a newton, as a unit of force, is fairly small... like a millimeter is a small unit of distance or a microsecond is a small unit of time.

So let's examine the pull of the planets on the Earth.

We know the following:

The Earth has a mass of about 6 x 10²⁴ kg.

So... let's hypothesize that we can miraculously put all the planets at their minimum distances from Earth, all in a straight line in one direction -- so their gravitational forces add. This, of course, can never actually occur, but it will give us the maximum possible gravitational pull of the other planets on the Earth. So let's pretend that we can do this. What do we get?

If you put all the numbers in Newton's law (and make the units compatible), you get that the maximum force that all the other planets can exert on the Earth is roughly 3 x 10¹⁸ newtons. What sort of result will this have on the Earth... well, we use Newton's First Law which says that if a force F is applied to a mass M, the mass is accelerated by a value A. (This is the famous equation F=ma). A force of 3 x 10¹⁸ newtons acting on the Earth causes the Earth to accelerate by 5 x 10^-7 meters per second per second. In other words, the planets in our solar system -- aligned or not -- cannot cause a shift in the movement of the Earth which will lead to mass destruction.

To compare, let us look at the gravitational force that the Sun exerts on the Earth. The mass of the Sun is about 329,400 times that of the Earth. They are separated on average by approximately 149,000,000 km. Thus, the Sun exerts a force of ~3.5 x 10²² newtons on the Earth. So you see, that the other planets in our solar system don't matter at all compared to the Sun!

To be entirely correct, we must tell you that the laws of physics involved actually show that the Moon is the one of the most important objects to the Earth... gravitationally speaking. It is the Moon which is responsible for things like ocean tides and such. All the other planets in our solar system added together do not have as large a gravitational effect on the Earth as the Moon does.

Here is a table of tidal forces of the Sun, Moon, and Planets. With the Sun's tidal force equal to 1.00, the following values are given in Thompson (1981):

A last thought: alignments of various planets in the solar system occur all the time. It seems that every time one comes along, doom-sayers arise to cry that this is the end of life on Earth as we know it. The laws of physics, however, cannot be denied in this Universe of ours. If life on this Earth ends, it will not be because of planets aligning in our solar system.

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

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