# Moon, or Planet? **URL:** \ **Category:** Factual Questions\ **Created:** [February 10, 2000, 3:09am UTC](https://boards.straightdope.com/t/moon-or-planet/7046 "2000-02-10T03:09:00Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [February 11, 2000, 4:48am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/21 "2000-02-11T04:48:00Z")
Whoa. This is getting out of hand. As long as everybody else is slinging disinformation… Mercury _does not_ show the same face to the Sun all the time, disappointing a lot of science fiction fans. Pluto is farther away from the Sun than Neptune–for a while, scientists thought it was closer, but it is now known that that is erroneous. Saturn has a gazillion moons. Some places in the world have virtually no tides for days. The Earth could fit into the great red spot on Jupiter. The diameter of the Sun is only about a hundred times greater than the Earth’s diameter. The moon does not rotate only once every 29.5 days–it’s more like 27.4 days. The diameter of Mars is half that of the Earth’s. \---------------- rocks\\Sure would be nice to have a preview feature\ ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [February 11, 2000, 4:56am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/22 "2000-02-11T04:56:00Z")
Recently an asteroid was discovered to have a satellite of it’s own. The smallest body known to have a satelite, in fact. Some time in the next few months, though another object is going to acquire a satellite, this time an artificial satellite. The NEAR spacecraft is slowly approaching Eros, or asteroid 433. It should become the first artificial satellite of an asteroid on Valentines day. Read about it at [Astronomy Picture of the Day](http://antwrp.gsfc.nasa.gov/apod/). (You might have to check previous to get back to 2/10/2000) \

**Tris** \ \\ \*\*\*\ Y\\\*\*\* ---

**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [February 12, 2000, 6:29am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/23 "2000-02-12T06:29:00Z")
> [@](#): > > Pluto is farther away from the Sun than Neptune–for a while, scientists thought it was closer, but it is now known that that is erroneous. Errr… For a while, it _was_ closer. (January 1979 thru February 11, 1999, to be specific). Its orbit is a lot more eccentric than the rest. And the superior planets most definitely display retrograde motion. * * * John W. Kennedy “Compact is becoming contract; man only earns and pays.” – Charles Williams ---
**Author:** ![Atrael](https://avatars.discourse-cdn.com/v4/letter/a/6a8cbe/32.png) [@Atrael](https://boards.straightdope.com/u/Atrael)\ **Post date:** [February 11, 2000, 7:24pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/24 "2000-02-11T19:24:00Z")
Wait wait wait…now you’re confusing **me**. Let’s start with a few difinitions here. From Webster’s: > [@](#): > > Mass:the property of a body that is a measure if it’s inersia, that is commonly taken as a measure of the amount of material it contains and causes it to have wieght in a gravitaional field, and that along with leangth and time constitutes one of the fundamental quantities on which all physical measurements are based > [@](#): > > Gravity:the gravitational attraction of the _mass_ of the earth, the moon, or a planet for bodies at or on its surface > (emphasis mine) > [@](#): > > Orbit:a path described by one bady in its revolution about another (as by the earth about the sun) Now by these definitions, the reason the earth _orbits_ the sun, is because the sun has a greather **mass**. Likewise the reason the moon orbits the earth, is because the earth has a greater mass. As I understand it, the wobble in the earth’s rotation (I don’t remember the name right now) is caused by the slight influence of the moons gravitaional pull. That slight pull, is enough to cause the earth to wobble back and forth upon it’s axis. Or, it’s _center of gravity_ So that’s why this bit here confuses me: > [@](#): > > Quite simply: the earth and moon revolve about their common center of gravity. > > Do a little thought experiment: Two bodies of equal mass will revolve about a point halfway between them. Label them body A and body B. Now start shrinking the mass of B. As you do, the point about which they revolve will move toward the center of body A. When B becomes massless, it will actually be orbiting about the exact center of A. This is an impossibility, in order for any body to _orbit_ another, one (by definition) **must** have a greater mass than the other. ---
**Author:** ![Atrael](https://avatars.discourse-cdn.com/v4/letter/a/6a8cbe/32.png) [@Atrael](https://boards.straightdope.com/u/Atrael)\ **Post date:** [February 11, 2000, 7:38pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/25 "2000-02-11T19:38:00Z")
Forgive the typos in my post…my boss wanted me to attend a meeting, and I wanted to get that in before I left. ---
**Author:** ![APB9999](https://avatars.discourse-cdn.com/v4/letter/a/8e8cbc/32.png) [@APB9999](https://boards.straightdope.com/u/APB9999)\ **Post date:** [February 11, 2000, 8:30pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/26 "2000-02-11T20:30:00Z")
Atrael, I was sloppy in conflating center of gravity and center of mass, though I don’t think it really changes anything. There’s nothing in the definitions you quote that says that an orbit is only defined for bodies that differ greatly in mass. If you had two bodies of equal mass they could very well orbit each other, i.e. they would move in a circle around their center of mass. Since the two bodies are of equal mass, that center would be halfway between them. If one (A) was of greater mass than the other (B), the center of mass would be closer to the heavier body, or more properly to the center of mass of that heavier body. Let’s call the center of mass of the pair of bodies COM, and the center of mass of bodies A and B the comA and comB respectively. If the bodies are different enough in mass it is possible for the distance between the COM and comA to be less than the radius of body A. Above, AWB suggested that this be adopted as the difference between a planet/moon system and a binary planet system: A planet/moon system are different enough in mass, and the planet has a large enough radius, that the distance between the COM and the comA is less than the radius of A. Body A would then be the planet and B the moon. In binary planets, the COM is in the empty space between the bodies. Seems perfectly sensible to me. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [February 11, 2000, 9:30pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/27 "2000-02-11T21:30:00Z")
> [@](#): > > Originally posted by **\APB9999\** : > **Above, AWB suggested that this be adopted as the difference between a planet/moon system and a binary planet system: A planet/moon system are different enough in mass, and the planet has a large enough radius, that the distance between the COM and the comA is less than the radius of A. Body A would then be the planet and B the moon. In binary planets, the COM is in the empty space between the bodies. Seems perfectly sensible to me.** ## I see one problem. We would lose one of the nine planets that way. \ Wouldn’t Jupiter be a binary sun? rocks    \\Sure would be nice to have a preview feature\ ---
**Author:** ![APB9999](https://avatars.discourse-cdn.com/v4/letter/a/8e8cbc/32.png) [@APB9999](https://boards.straightdope.com/u/APB9999)\ **Post date:** [February 11, 2000, 9:42pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/28 "2000-02-11T21:42:00Z")
By the way, [this](http://www.seds.org/billa/tnp/data1.html) link will give you the masses of all the solar system’s major bodies. The earth is 81 times as massive as the moon. It’s the acceleration constant at the surface that is 1/6 that of earth’s at the surface (the numbers don’t match because the bodies have different radii). [Previous page](https://boards.straightdope.com/t/moon-or-planet/7046.md?page=1)