The gravitational forces of aligned planets are too weak

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

21 June 2020

Last week, we saw that is not possible for all the planets in the solar system to fall in into a straight line. The reason is that their orbits do not lie on the same plane. They are quite close to a single plane but not close enough.

For example, there is angle of 7 degrees between the orbits of Earth and Mercury. This is the largest angular inclination. Now, that might seem small enough, but if you draw a tringle with Earth, Mercury and the Sun at the three corners, you will find that its height is about 7 million kilometers. In other words, Mercury would be 7 million km off the alignment.

The smallest inclination of orbit is that of Uranus at 0.77 degrees. Still, because of the great distances, when the Earth passes between the Sun and Uranus, it does so at about 2 million km out of alignment.

Last week, Muthoni Nyaga and Cyrus Njeru, expressed concern that, perhaps the combined gravitational pull of planets in a straight line could cause havoc. But how much is the force?

Contrary to popular belief, gravity is a very weak force. It is actually the weakest force in the universe. It only becomes significant near very massive objects.

Indeed, gravity is so weak that it took 111 years from the year Newton published his law (1687) to the time when someone figured out how to measure the constant of gravitational forces (1798). Newton knew there is a constant in his equation but he died without knowing its value!

Furthermore, the force of gravity drops rapidly as the distance between objects increases. Even though the Sun and the planets are extremely massive, they are very far apart. On Earth, for example; the gravitational pull of the sun is much weaker than that of the sun.

That might sound surprising, but think about it: if the Sun’s gravity was greater, then it would have plucked us off the earth and sucked us into itself!

On the surface of the Earth, the force of gravity felt by a one-kilogram mass is about 9.8 newtons (N). A newton is approximately the weight of one regular tomato. This is the force keeps objects on the ground. But the Sun is also trying its best to pull things to itself.

The magnitude of the Sun’s gravity at this distance (150 million kilometres) is just 0.0061N/kg. That is, the Earth’s gravitational force on its surface, is about 1,600 times that of the Sun.

If the planets were to align, the nearest one to Earth would be Venus. It’s gravitational pull on us would be just 0.00000022N/kg. The greatest planetary force comes from the largest planet, Jupiter. It is 0.00000037N/kg.

However, this is just a hundredth of the pull of the moon. The 0.000033N/kg from the Moon is what causes tides on our oceans. Increasing it to 0.0000334N/kg would not make any noticeable difference on Earth.