# What makes the "new moon" a moon? **URL:** \ **Category:** Factual Questions\ **Created:** [October 5, 2024, 4:51pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477 "2024-10-05T16:51:16Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [October 5, 2024, 4:51pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/1 "2024-10-05T16:51:17Z")
The new moon won’t even make one complete orbit around the Earth so what makes it a new moon of the Earth. It can’t be that it’s affected by the Earth’s gravity – everything is. Is the local part of it’s orbit concave towards the Earth? Is the gravitational force on it more than the sun’s or anything else’s? Exactly why is it a moon now? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 5, 2024, 5:13pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/2 "2024-10-05T17:13:56Z")
Asteroid 2024 PT5 is not a moon; this is just bad pop-sci journalism making clickbait headlines. An actual moon would be in a stable closed orbit of its planet by definition. Charon (the ‘moon’ of dwarf planet Pluto) is also arguably not a moon; Pluto-Charon are a doublet which orbit around their common barycenter which is external to both bodies. Stranger ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [October 5, 2024, 8:59pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/3 "2024-10-05T20:59:08Z")
![](https://media1.tenor.com/m/KR0rX24lv5sAAAAC/thats-no-moon-obi-wan-kenobi.gif) ---
**Author:** ![I\_Love\_Me\_Vol.I](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/i_love_me_vol.i/32/521_2.png) [@I\_Love\_Me\_Vol.I](https://boards.straightdope.com/u/I_Love_Me_Vol.I)\ **Post date:** [October 6, 2024, 11:43am UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/4 "2024-10-06T11:43:42Z")
> [@Stranger\_On\_A\_Train](#): > > Asteroid 2024 PT5 is not a moon; this is just bad pop-sci journalism making clickbait headlines. An actual moon would be in a stable closed orbit of its planet by definition. Au contraire, mon frère… [It’s officially a moon](https://www.earth.com/news/its-official-earth-now-has-two-moons-captured-asteroid-2024-pt5/) > Over the past few weeks, you’ve probably heard somewhere that Earth was going to get a second moon. Well, **we can now confirm that Earth now officially has two moons** _[Bolding mine]_ So that’s that, IT’S OFFICIAL! Since you can’t say something’s official unless it really is (I read that somewhere on the web) I for one, am convinced. The quote is from [earth.com](http://earth.com), and you can’t have a name like [earth.com](http://earth.com) without being the official arbiter of all things Earthy. 'Nuff said. …_But_, the second paragraph paints a different picture: > On Sunday, September 29, our planet captured the **tiny asteroid** named 2024 PT5, turning it into a **temporary mini-moon**. _[Bolding mine]_ Oh. Earth pulled a space rock in to its orbit and it’s tiny and it’s temporary. That’s not what your headline implied, now is it? ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [October 6, 2024, 1:59pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/5 "2024-10-06T13:59:37Z")
I found this thread really confusing until I got to the first reply and realized that the OP wasn’t asking about the “[new moon](https://www.timeanddate.com/astronomy/moon/new-moon.html)” phase of The Moon. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 6, 2024, 6:46pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/6 "2024-10-06T18:46:32Z")
> [@I\_Love\_Me\_Vol.I](#): > > Earth pulled a space rock in to its orbit and it’s tiny and it’s temporary. That’s not what your headline implied, now is it? It is _apparently_ captured because the instantaneous eccentricity of the object drops below unity, i.e. if you made an orbital determination based upon measurements made close about that region it would appear to be an ellipse. A paper on the topic published by American Astronomical Society states that [“Near-Earth objects (NEOs) that follow horseshoe paths, and approach our planet at close range and low relative velocity, may undergo _mini-moon events_ in which their geocentric energy becomes negative for hours, days or months, but without completing one revolution around Earth while bound,”](https://iopscience.iop.org/article/10.3847/2515-5172/ad781f); I’ve never seen the term “geocentric energy” (and did not find in any any reference on my shelf including Vallado, Prussing & Conway, Danby, BMW, SME:SMAD, Murray and Dermott, or even Belbruno’s _Capture dynamics and Chaotic Motions in Celestial Mechanics_) and can only infer that it refers to the the characteristic energy, C3. To the extent that this is true, it is because that while the object is in the Earth’s sphere of influence for some significant amount of time it is really in orbit of the Sun and just performing an incidental (and obviously unpowered) swing-by maneuver about the Earth which never forms a closed or crossing trajectory even in the Earth-centered reference frame. Nor have I ever seen the term “mini-moon events” in literature previously (it appears to be a neologism by the authors) but upon reviewing the International Astronomical Union policy on the naming of celestial bodies, the IAU gives no strict definition of what comprises a moon (which it refers to as a “planetary satellite”) so I suppose someone could refer to this as a ‘mini-moon’ even though that defies the general understanding of the term. I still think that headline is pop-science nonsense, but the [earth.com](http://earth.com) article is decently researched, well-written for general public consumption, and mostly other than the statement that “Earth will have a tiny companion circling around it.” It is also clearly generated by an actual human writer instead of the plausible nonsense of an LLM. So, there is that, at least. Stranger ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 6, 2024, 10:43pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/7 "2024-10-06T22:43:11Z")
> [@Stranger\_On\_A\_Train](#): > > I’ve never seen the term “geocentric energy” (and did not find in any any reference on my shelf including Vallado, Prussing & Conway, Danby, BMW, SME:SMAD, Murray and Dermott, or even Belbruno’s _Capture dynamics and Chaotic Motions in Celestial Mechanics_) I’d say that it’s pretty clear that it means potential energy relative to the Earth plus kinetic energy relative to the Earth. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 6, 2024, 11:43pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/8 "2024-10-06T23:43:49Z")
Well, if all they care about is whether it be positive or negative, it doesn’t really make a difference what they mean since it can be multiplied by some constant, the mass, etc., without changing the sign. ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [October 6, 2024, 11:47pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/9 "2024-10-06T23:47:03Z")
Same. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 8, 2024, 8:36am UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/10 "2024-10-08T08:36:15Z")
> [@OldGuy](#): > > The new moon won’t even make one complete orbit around the Earth The diagrams I have been seeing appear to show just a little over one complete circuit around the earth. I’m not sure any of it at all technically qualifies as an ‘orbit’, but it does go around at least once. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 8, 2024, 6:11pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/11 "2024-10-08T18:11:31Z")
> [@Mangetout](#): > > The diagrams I have been seeing appear to show just a little over one complete circuit around the earth. I’m not sure any of it at all technically qualifies as an ‘orbit’, but it does go around at least once. Although from the surface of the Earth it might appear to make a complete orbit, it never actually crosses its own geocentric path: [The embedded link just shows the Wikipedia article but if you actually click the link it sends you directly to the animation of the trajectory from a geocentric frame.] > **[2024 PT5](https://en.wikipedia.org/wiki/2024_PT5#/media/File:Animation_of_2024_PT5_around_Earth_-_Close_approach.gif)** > > 2024 PT5 is a near-Earth object roughly 11 meters (36 ft) in diameter discovered by ATLAS South Africa, Sutherland on 7 August 2024, the day before approaching Earth at 568,500 km (353,200 mi). > The object orbits the Sun but makes slow close approaches to the Earth–Moon system. Between 29 September (19:54 UTC) and 25 November 2024 (16:43 UTC) (a period of 1 month and 27 days) it will pass just outside Earth's Hill sphere (roughly 0.01 AU \[1.5 million km; 0.93 million mi\]) at a low relative ve... Stranger ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [October 8, 2024, 6:18pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/12 "2024-10-08T18:18:20Z")
> [@Stranger\_On\_A\_Train](#): > > Asteroid 2024 PT5 is not a moon; this is just bad pop-sci journalism making clickbait headlines. And, there is no definition about what makes a “moon” size-wise. Earth must have thousands of baseball sized or smaller things in orbit, Saturn clearly has millions. I think the definition should be “If the atmosphere was clear, one could see it with the naked eye”, which leaves in “the hurtling moons of Barsoom/mars”. > [@I\_Love\_Me\_Vol.I](#): > > Au contraire, mon frère… [It’s officially a moon](https://www.earth.com/news/its-official-earth-now-has-two-moons-captured-asteroid-2024-pt5/) Who is the office/authority to declare it a moon? The articles writer, it seems. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 8, 2024, 7:01pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/13 "2024-10-08T19:01:05Z")
From the Wikipedia article on [Temporary Satellite](https://en.wikipedia.org/wiki/Temporary_satellite): “Earth has at least one temporary satellite 1 m (3.3 ft) across at any given time, but they are too faint to detect by current surveys.” (Based on simulations.) ---
**Author:** ![outlierrn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/outlierrn/32/3005_2.png) [@outlierrn](https://boards.straightdope.com/u/outlierrn)\ **Post date:** [October 8, 2024, 8:32pm UTC](https://boards.straightdope.com/t/what-makes-the-new-moon-a-moon/1008477/14 "2024-10-08T20:32:51Z")
Now I have a mental image of dr evil as a man in the moon saying 'I will call him mini me. ’