# Would we be able to feel the acceleration of a planet with a highly eccentric orbit?

Even though our Earth moves on average 67,000 miles per hour around the Sun, we cannot feel its movement since its speed is fairly constant. More precisely, its change in speed is very low and our bodies cannot detect that low of acceleration.

What if instead a hypothetical human observer were placed on a planet with a very eccentric orbit, say 0.99? Would that human be able to feel the planet's acceleration as it approached its host star, and as it receded from its host star? And if a human observer would not be able to feel any acceleration even at e=0.99, what is the minimum eccentricity, if any, that a planet must have in order for a hypothetical human observer to feel the effects of its acceleration and deceleration?

• This answer beautifully explains why acceleration isn't something that is felt. Jul 5 '16 at 6:10
• In a perfect circular orbit, the speed of the planet could be stable and difficult to notice if the life of a organism were created on earth, the organism would of adapted to any slight disturbance in acceleration and if the organism did notice a disturbance it might explain the change completely wrong calling it a earthquake Jul 5 '16 at 15:19
• and explain this disturbance with all the characteristics of a earthquake not knowing the tectonic plates are shifting due to moment in acceleration when the planet is undergoing extreme Z forces from exiting a bend in the orbit and gaining momentum. Jul 5 '16 at 15:19
• The disturbances may be noticeable at all times and the organism's calls this acceleration gravity and explains it away with the current undermined,undergoing, still in study/theory "gravity". leaving the organisms frustrated for centuries trying to explain "How is earth accelerating through space at 93 million miles but can not be noticed or felt :) Jul 5 '16 at 15:26

No.

While you're on that planet, you are also in the same orbit around the star as the planet is. As long as you're in the same orbit as your vehicle (planet, spacecraft), you don't experience any acceleration relative to the vehicle. Whether you're on the planet, or you're just tagging along on the same orbit, is the same thing.

Similar question: in a satellite in a very eccentric orbit around the Earth, is any break in microgravity experienced in various parts of the orbit? Same answer - no. The satellite and everything in it follow the same orbit, so microgravity conditions are maintained.

Now things would be different if you had a very eccentric orbit around a source of much more intense gravity, like a neutron star or a black hole. At the lowest point in the orbit, you could experience tidal forces: the things closer to center experience stronger gravity compared to things further away from center.

But for this to be experienced within a small spacecraft, or within the limits of one human body, the gradient of the field would have to be very strong indeed; the orbit would have to reach very low, very close to the neutron star or black hole.

However, what you would experience would not be orbital "acceleration", but just tidal force. Even so, in extreme cases the tidal force could be strong enough to destroy the ship or kill the passengers.

See the short story "Neutron Star" by Larry Niven for a literary depiction (fairly accurate physically) of this phenomenon. Unfortunately, the story is spoiled by the explanation above, but hopefully learning about gravity compensates for spoiling the story. :)

• Planet has size, so different parts of it want to follow different orbits, just as happens in rings. That said, any perceptible differential in gravitational acceleration would likely be masked by earthquake shaking as your planet tries to maintain equilibrium shape in discrete steps. Aug 6 '16 at 13:50
• Not really. What happens is, because the planet is not a simple dot, tidal forces intervene. The shape changes and becomes elongated, in the typical tidal fashion. There are no earthquakes involved (not at normal scales anyway), just plain old tidal forces. Aug 7 '16 at 20:26

First I think you have miss understood some basic principals. Although the Earth's speed (a scalar quantity) is nearly constant its velocity is not, acceleration is a change in velocity (a vector quantity). Due to its orbital motion the acceleration of the Earth is constantly changing.

As for the actual question, the acceleration would be so small compared to that of gravity (between the person and the planet) that the effect will not be noticeable.

• Earth, or any body in orbit (no matter how eccentric), is following a geodesic and thus does not experience any acceleration (it undergoes acceleration only in other coordinate systems). See also Einstein's thought-experiment guy that falls from the roof and does not feel gravity. Jul 5 '16 at 12:57
• @yatima2975 You're just being pedantic though. How does what you said help this answer? Jul 6 '16 at 16:22