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# Physics/solar system.

Question
QUESTION: Hi Steve,

I read that even Sun, earth and all planets are constantly moving towards the galactic center. If it is so, how does the distance between Sun and earth remain constant? Also please inform why Pluto was removed from planets list. Please explain.

ANSWER: We are orbiting the galactic center, not moving towards it.  In orbits, objects do not experience gravitation except in terms of weak tidal forces.  These are very weak force differences in comparison to our local solar system's gravitational forces, so we don't notice the effects because they are so tiny.  As far as Pluto, that was voted on by an international astronomical body because it does not actually dominate its local gravitational field.  Its moon, Charon, is nearly the size of the planet, and Pluto is absolutely tiny.  I consider it a planet, it's an arbitrary choice of a committee of astronomer-type people.  As I am not an observational astronomer in any way, I don't really understand their reasoning.  I think it should be for historical reasons, left as "a planet" and not a "dwarf planet."

---------- FOLLOW-UP ----------

QUESTION: Hi,

What is exactly galactic center? Is it a fixed point in the galaxy or does it continually move? What created galactic center?

>>In orbits, objects do not experience gravitation

If there are objects nearby the rotating object, will the object still not experience gravitation?

regards,

The galactic center is the point about which all the stars in the galaxy orbit, pulling on one another through gravitation.  It's just a geometric point that constitutes the center of mass of the galaxy.  When the galaxy was formed, there was a center of its mass (defined the usual physics way for any object, be it a baseball or a galaxy).

I mistyped that...in orbits, objects obviously orbit because of gravity.  But like I said, the local forces of gravity dominate over the tidal forces applied by the galaxy.  We orbit the center of the galaxy almost exactly the same as the Sun does (minus tiny tidal force differences), so our orbit around it is unperturbed.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.