How big are the gravitational forces on the Earth from the other planets?
Many people worry about the forces on the Earth from the other planets of the Solar system. They especially worry when they see three of more planets lining up with Earth and start to contemplate the end of the world. So is there any reason to be worried? Will the Earth be torn apart as all the planets align?
Well, we worked out the maximum and minimum forces on the Earth caused by the Sun, each of the Planets and the Moon at their closest and furthest points. However, how many of us actually understand forces like 3x1022 Newtons and what they mean? So we've also shown how far the Earth would move if pulled by each Planet / Moon or the Sun for one day from a standing start.
Table of Planetary Gravitational Forces on the Earth
|Object||Max force on Earth (Newtons)||Min force on Earth (Newtons)||Max distance Earth travels after a day||Min distance Earth travels after a day|
|Sun||3.66E+22||3.43E+22||22,898 km||21,417 km|
|Luna||2.21E+20||1.76E+20||138 km||110 km|
|Jupiter||2.18E+18||8.06E+17||1.36 km||503 m|
|Venus||1.33E+18||2.85E+16||831 m||17 m|
|Saturn||1.57E+17||8.15E+16||98 m||51 m|
|Mars||8.61E+16||1.59E+15||53 m||0.9 m|
|Mercury||2.20E+16||2.67E+15||13 m||1.6 m|
|Uranus||5.16E+15||3.47E+15||3 m||2.1 m|
|Neptune||2.21E+15||1.84E+15||1.3 m||1.1 m|
Gravitational forces for all planets in the Solar System as expressed as Newtons and indicating resulting movement over the period of 1 day from a standing start.
As you can see, the Sun has the biggest effect on the earth and could accelerate the earth (from a standing start) almost 23,000 km in one day. Coming in a feeble second place is the Moon with a 138 km pull. Jupiter has a hundredth the pull of the moon with Venus about half that of Jupiter. The rest of the planets come in with pulls less than a thousandth that of the Moon.
You can also see that if all the planets were aligned so that all there forces were acting together, their combined forces would always be dwarfed by the huge pull of the Sun, and also that of the Moon.
If you are thinking "Wow so that's how far the Earth could be pulled by a planet! I wonder how far it would pull just me on my own", well the answer is that if the earth were to disappear and you were left floating in empty space, then you would be pulled exactly the same distance as the earth would have been. The force on your body would be a lot smaller, but that force would only have to pull a smaller mass and so you would undergo the same acceleration and travel the same distance as in our table.
Will the alignment of all the planets cause earthquakes?
Some people believe that Earthquakes are more likely when the planets are all aligned. The table above gives some idea of the total gravitational force on the Earth from the planets. However, it doesn't tell us about the stresses the gravitational pull of the planets has on the material of the Earth itself.
The stress - or how much the earth is stretched - is due to the imbalance in gravitational force from one side of the planet to the other. As an example, a kilogram of rock near the surface of the earth nearest to the moon will experience a different force to that of a kilogram of rock near the surface of the earth furthest from the moon. That is because it will be further away from the Moon, by the diameter of the earth, and the gravitational force lessens over distance.
This imbalance in force means that the near side of the Earth to the moon is pulled more than the far side causing the Earth to be stretched.
The following table indicates the difference in force from one side of the earth to the other by each of the planets when they are at their closest to earth. Since the numbers have little meaning to most of us, we've shown what the force is when compared to that of the Moon.
Table of Planetary Gravitational Stress on Earth
|Planetary Tidal Forces expressed as fraction of the Moons Tidal Force|
Maximum Gravitational Tidal forces for all planets in the Solar System as fraction of the Moons Tidal Force.
Again it is apparent that the gravitational forces on the earth that might result in earthquakes all come from the Moon and the Sun. The stretching forces from all the other planets, even if added together, are a factor of 1000 less than that of the Moon and Sun.
It's not surprising therefore that we never worry about high tides when Venus is new (and therefore close to us) in the sky. But we do worry about Spring and Neap tides when the Sun and Moon are aligned.
The Earth is affected by all the planets in the solar system, and even all matter in the universe. However the Earth is almost completely controlled by the Sun, because it is huge, and the Moon, because it is close. All other solar system objects are gravitationaly insignificant in comparison. Even if they align in the sky, their combined gravitational might is dwarfed by minor alterations in the Moon's orbit and the Earth’s orbit about the Sun.
So next time someone tells you that the world will end when the planets align... well it might do... but not because of gravity as we know it!