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Astronomy/calculating the moons gravitational force
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Question
Is it true that the pull of the moon is greater than that of the sun on water. Does it have the same strength on a Kg of water on both side of the earth? If not what is the difference?
Hi Karl,
Yes, on water, or any other particle of Earthly mass, because it's closer. What's the formula for it? F = G m1m2/d squared?
With d squared in the denominator, 238000 miles squared is certainly
a much smaller number than 93 million miles squared, comparing
moon to sun! (While the sun is only 27 million times more massive than the moon, it's also about 391 times more distant)!
You seem to be confusing "tidal force" with gravity force....tides are not directly caused by gravity, but by 'differential' gravity, that is...the force on one side relative to the force on the other side of the Earth. (That's why there are 2 high tides per 24-25 hours, and not just one). So if you examine the ratio of the diameter of the earth to the distance to the moon, 8000 to 238000, that's certainly a much bigger number than 8000 to 93,000,000 miles.
There is why the moon's influence, it's differential gravity....
on the Earth is much more effective than the sun, the ratio's above.
8000 miles (rounded off diameter of the Earth) is a much higher fraction of 238,000 miles as compared to 8000/93 million miles.
If the moon were to magically disappear, we would still have solar
tides, but at only about 1/3 to 1/4 their current amplitude.
I'm just giving you the descriptive answer here; if this is a homework problem, I leave the actual math for you to do in the assignment.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA
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Answers by Expert:
Tom Whiting
Expertise
Astronomy has been my hobby/pasttime for over 50 years. Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions. No astrology questions please, or questions about alleged UFO picture identifications.
Experience
Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.
Organizations: President, Erie County Mobile Observers Group for over 15 years.
Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).
Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions; Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).
Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.
Education/Credentials
BS Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA
Also retired USAF pilot, 20 years.
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