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Astrophysics/Hypothetical questions about moons


Hello, Sir!

 I appreciate your taking the time to read this, I hope it isn't too far-fetched for you to bother with. I'm delving into a new genre of fiction writing for the first time and I'm struggling to make it as authentic as it can be.

 Can you tell me if the moons of other planets have effects similar to the effects that our moon has on Earth? Do they serve to stabilize or maybe give tilt to their host planets?

 Similarly, do you have any idea what would happen to Earth if we suddenly didn't have a moon? Would we fly off into space or the sun or would we remain in orbit? On a somewhat similar subject, how close can two planets of similar size come to each other without risking a collision?

 Finally, how large can a moon be in relation to its planet's size before it becomes dangerous? Or is density the determining factor in that sort of scenario?

 I genuinely appreciate any helpful thoughts you might be able to pass along, I absolutely love all things space but have no formal education on any aspect of it past grade school solar system memorization. Have a wonderful day. :-)


Well, you definitely have a lot of intriguing questions to address, and I will see what I can do to help you with your writing project.  

1) Other moons of other planets can have similar effects that our moon has on Earth provided the same parameters apply (i.e. distance to the plant, relative masses etc. )  Note I am considering extra-solar systems here for the sake of generality. Thus, a planet in the Epsilon Eridani system with a moon like ours would see similar sorts of effects, in terms of tides, eclipses, etc. assuming the parameters are the same. Obviously, if they are different then there'd be differences. For example, a moon of the same mass as our own circling another planet but ten times further away would not produce total eclipses - assuming the same stellar diameter observed from the planet. In terms of issues such as 'stabilization' or axial tilt those are not really determined by moons unless the Moon is dangerously close to the planet - which we will get to#

2)No, we would not suddenly 'fly off' into space if we didn't have a moon# The major effect would be the loss of any appreciable tides or tidal effects, since with the Moon gone the only tide-producer would be the Sun#

3# Two planets, especially of similar size, would not come into any close encounters# Planets are stabilized in their orbits with respect to their central stars #suns# by their respective gravitational potential energies #e#g# V = -GMm/r#  which are negative, and hence form gravitational "wells"# For a planet to 'escape' such a well and go off catapulting across its solar system to collide with another planet would necessitate too much energy, almost as much as a star would generate in a second#

4# The limits set on a moon's size #actually mass or density# before it becomes dangerous to the planet is determined by the Roche limit# See e#g#


The specific computation for the critical distance, given in the above article is:

d = 2#44 R2 #M 1/ M 2#^1/3

Where R2 denotes the radius #e#g# in meters# of the secondary #e#g# moon#, and M 1, M 2 are the masses of the planet and the moon, respectively#

For example,if the radius R2 = 1 737 km = 1#73 x 10^6 m and the ratio #M1/M2# = 81 then:

d = #2#44# #1#73 x 10^6 m# #81#^1/3  = #2#44# #4#32# ##1#73 x 10^6 m#

d =  18, 235 km

Note the above is somewhat idealized and simplified, i.e. ignores orbital effects, but it shows within that distance the moon-satellite is likely to disintegrate which could have negative effects for the planet and anyone living on it.  


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Philip A. Stahl


I specialize in stellar and solar astrophysics. Can answer questions pertaining to these areas, including: stellar structure and evolution, HR diagrams, binary systems, collapsars (black holes, neutron stars) stellar atmospheres and the spectroscopic analysis of stars as well as the magnetohydrodynamics of sunspots and solar flares. Sorry No homework problems done or research projects! I will provide hints on solutions. No nonsense questions accepted, i.e. pertaining to astrology, or 'UFOs' or overly speculative questions: 'traveling through or near black holes, worm holes, time travel etc. Absolutely NO questions based on the twaddle at this Canadian site: purporting to show a "new physics". Do not waste my time or yours by wasting bandwidith with reference to such bunkum.


Have constructed computerized stellar models; MHD research. Gave workshops in astrophysics (stellar spectroscopy, analysis) at Harry Bayley Observatory, Barbados. More than twenty years spent in solar physics research, including discovery of SID flares. Developed first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Developed first ever loop-based solar flare model using double layers and incorporating cavity resonators. (Paper presented at Joint AGU/AAS Meeting in Baltimore, MD, May 1994)

American Astronomical Society (Solar physics and Dynamical astronomy divisions), American Geophysical Union, American Mathematical Society, Intertel.

Papers appearing in Solar Physics, Journal of the Royal Astronomical Society of Canada, Journal of the Barbados Astronomical Society, Meudon Solar Flare Proceedings (Meudon, France). Books: 'Fundamentals of Solar Physics', 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level', 'Astronomy & Astrophysics: Notes, Problems and Solutions', 'Modern Physics: Notes, Problems and Solutions'

B.A. degree in Astronomy; M.Phil. degree in Physics - specializing in solar physics.

Awards and Honors
Postgraduate research award- Barbados government; Studentship Award in Solar Physics - American Astronomical Society. Barbados Astronomical Society award for service (1977-91) as Journal editor.

Past/Present Clients
Caribbean Examinations Council (as advisor, examiner), Barbados Astronomical Society (as Journal Editor 1977-91), Trinidad & Tobago Astronomical Society (as consultant on courses, methods of instruction, and guest speaker).

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