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The inextricability of gravity and orbits

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By Joseph KramerJoseph Kramer

Astronomy Columnist

The term ‘zero gravity’ is used quite a bit in society and pop culture, but as we discussed in the last article, gravitational relationships always exist. I have been exposed to the term ‘micro gravity’ a lot recently when researching astronomy related concepts. When you are on a roller coaster and it is going down a steep hill very quickly, as the car you are sitting in separates from your body, you may describe that feeling as ‘zero G’. When you think about it, that term is inaccurate. You are falling towards the Earth at a very high rate of speed, which is a direct result of gravity. The fact that you are not being held up by a structure, does not change the gravitational relationship between you and the Earth. This same concept is applied to pilots, and even those who train for ‘low gravity’ environments in parabolic flight where they seem to float in the back of the plane. They are floating with relation to the plane that was once supporting them, but gravity is in full effect with relation to Earth, and they are falling towards it quickly.
Orbit is freefall. I love contemplating orbit, because it is weird to think of an object in orbit is falling towards its host. The current understanding of orbit is based on Einstein’s Theory of General Relativity and Kepler’s Laws of Planetary Motion.
Isaac Newton (the apple guy), used a thought experiment to explain orbit. Let’s put a cannon on a hill and fire it in a direction. It will travel a distance and then fall to the ground in an arc. When you fire it with greater force, it will travel a greater distance before arcing to the Earth. If you add progressively more and more velocity, the cannon ball will travel progressively farther and farther before falling to Earth.
If we had the engineering ability to fire it with enough velocity, the cannon ball would be fired in such a manner, that its velocity exceeds the curvature of the Earth. It would literally fall off the face of the Earth. It is still falling towards the Earth, it just passes the Earth due to the Earths limited curvature. That is Orbit.
As the object in orbit (such as the Moon) moves sideways, it is falling towards the Earth, but then the Earth curves away from the object. Due to natural laws, the object will want to fall in a straight line, but the gravity of the Earth (or whatever host) pulls the orbiting object towards it. The orbiting object is always falling towards the host, but it has enough tangential velocity that it misses the host object and will continue to orbit.
Depending on the gravitational relationships of orbiting objects with their host, they might not always remain in orbit. Here on Earth, we have satellites that re-enter Earth’s atmosphere as they come closer and closer to Earth with each pass. I am going to tell you something kind of sad.
Saturn’s rings have this type of relationship with Saturn. With every pass of the planet, the material in the rings is coming closer and closer to the planet. In a distant time, they will fall into the gas giant, and one of the most beautiful views in our entire solar system will be changed forever. Of course we as individuals will have long since passed, and hopefully by then, our predecessors will have become space faring and maybe have found views just as beautiful and equally beneficial for survival.
Due to the elliptical shape of orbits (as we have previously discussed), the speed at which an object orbits its host changes throughout its orbital period. An ellipse is essentially a circle that has had two opposing and parallel sides equally elongated. Every geometric ellipse has two focal points. These are the center points of each arc at the ends of the ellipse. If we made circles out of these arcs, the focal points (or foci) would be at the center.
When an object orbits closer to its host, its velocity increases. We can see this in our solar system as Mercury and Venus orbit the Sun much quicker than the Earth due to the smaller distance they travel. You can test this theory by tying and object to a string and spinning it around your finger. As the string wraps around your finger, and the distance between your finger and the object decreases, it will spin faster and faster.
The same concept applies to objects throughout their orbit. When the Moon is at its closest approach to the Earth, it is orbiting at a faster rate than when it is farthest away. This is not really significant in our day to day lives, just fun information and understanding to have about our universe. I really hope you can enjoy the beauty of our universe as it is an amazing and unimaginable reality that we do get to experience. Look up, you won’t regret it.

Joseph Kramer is an Army Aviation Officer. He is an amateur astronomer and contributing member of Central Arkansas Astronomical Society.