The fate of the Earth?
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From: dow on 8 Sep 2009 10:31 On Sep 7, 8:03 pm, Sam Wormley <sworml...(a)mchsi.com> wrote: > Yousuf Khan wrote: > > dow wrote: > >> On Sep 6, 7:22 pm, alien8er <alien8...(a)gmail.com> wrote: > >>> AIUI the timescale for the Earth becoming tidelocked is not nailed > >>> down all that well; it's entirely possible Earth will be tidelocked > >>> long before Sol goes Big Red. > > >>> Mark L. Fergerson- Hide quoted text - > > >>> - Show quoted text - > > >> Neglecting the expansion of the sun, the earth will become tidelocked > >> to the moon, so each body keeps one face toward the other. (Pluto and > >> Charon are already in this configuration). The sun will continue to > >> produce tidal friction, so the whole earth-moon system will lose > >> angular momentum. The moon will therefore spiral inward toward the > >> earth, and the rotations will speed up. Eventually, the moon will get > >> so close to the earth that it will be broken up, forming a ring. The > >> ring will spread. Material from its outer edge will be lost to space, > >> and material from the inner edge will fall into the earth. This will > >> add more angular momentum to the earth, which will then be spinning > >> very fast. When the ring has disappeared, tidal friction from the sun > >> will slow the earth's spin, until it is synchronized with its orbital > >> motion. So, over a period of many billions of years, the earth's > >> rotation will slow down, then speed up, then slow down again. > > >> dow > > > Due to the influence of the Moon in orbit around the Earth, the Earth > > will never be tide-locked to the Sun. No matter what the Moon has to go > > around the Earth, and it has a greater influence on the Earth than the > > Sun does. > > > Yousuf Khan > > "Never" is a word to be avoided sometimes. The moon is walking away from > the earth and at some point would have less influence than the sun, but > I doubt that the earth (nor the moon) will be around long enough for that > to happen.- Hide quoted text - > > - Show quoted text - Actually, it might. The moon's tidal influence on the earth is presently about three times the sun's. However, the tidal gradient is inversely proportional to the cube of the distance, so the moon would only have to spiral out to cube-root(3) of its present distance, i.e. about 1.44 times as far away as it is now, for the two tidal influences to become equal. The moon is already several times further from the earth than it was when it was formed, so a further factor of 1.44 might "only" take another billion years or two. dow
From: Sam Wormley on 8 Sep 2009 11:15 dow wrote: > > Actually, it might. The moon's tidal influence on the earth is > presently about three times the sun's. However, the tidal gradient is > inversely proportional to the cube of the distance, so the moon would > only have to spiral out to cube-root(3) of its present distance, i.e. > about 1.44 times as far away as it is now, for the two tidal > influences to become equal. The moon is already several times further > from the earth than it was when it was formed, so a further factor of > 1.44 might "only" take another billion years or two. > > dow No--the moon's tidal influence is only twice that of the sun. Furthermore, it will take longer for the moon to "walk" to your 60 x 1.44 earth radii than the time the sun will swell and likely engulf the earth-moon system. OUR RESTLESS TIDES http://co-ops.nos.noaa.gov/restles1.html
From: dow on 8 Sep 2009 14:23 On Sep 8, 11:15 am, Sam Wormley <sworml...(a)mchsi.com> wrote: > dow wrote: > > > Actually, it might. The moon's tidal influence on the earth is > > presently about three times the sun's. However, the tidal gradient is > > inversely proportional to the cube of the distance, so the moon would > > only have to spiral out to cube-root(3) of its present distance, i.e. > > about 1.44 times as far away as it is now, for the two tidal > > influences to become equal. The moon is already several times further > > from the earth than it was when it was formed, so a further factor of > > 1.44 might "only" take another billion years or two. > > > dow > > No--the moon's tidal influence is only twice that of the sun. > Furthermore, it will take longer for the moon to "walk" to > your 60 x 1.44 earth radii than the time the sun will swell > and likely engulf the earth-moon system. > > OUR RESTLESS TIDES > http://co-ops.nos.noaa.gov/restles1.html When the sun, earth and moon are aligned, i.e. at new moon or full moon, the heights of the tides are about twice as great as they are when the moon is at one of the quarter phases. But this means that the tidal effect of the moon is *three times* that of the sun. When the tides add, they are 4/3 times as high as the lunar tide alone. When they subtract, they are 2/3 times the lunar tide. So the ratio of the hughest tides to the lowest is 4/3:2/3, or 2:1 It is now thought that the sun probably will not engulf the earth and moon when it becomes a red giant. The loss of solar mass to the solar wind will reduce the maximum radius of the sun and will also cause the earth's orbit to spiral outward. Both effects will reduce the probability of the earth beig engulfed. dow
From: Sam Wormley on 8 Sep 2009 14:46 Sam Wormley wrote: > dow wrote: >> On Sep 8, 11:15 am, Sam Wormley <sworml...(a)mchsi.com> wrote: >>> dow wrote: >>> >>>> Actually, it might. The moon's tidal influence on the earth is >>>> presently about three times the sun's. However, the tidal gradient is >>>> inversely proportional to the cube of the distance, so the moon would >>>> only have to spiral out to cube-root(3) of its present distance, i.e. >>>> about 1.44 times as far away as it is now, for the two tidal >>>> influences to become equal. The moon is already several times further >>>> from the earth than it was when it was formed, so a further factor of >>>> 1.44 might "only" take another billion years or two. >>>> dow >>> No--the moon's tidal influence is only twice that of the sun. >>> Furthermore, it will take longer for the moon to "walk" to >>> your 60 x 1.44 earth radii than the time the sun will swell >>> and likely engulf the earth-moon system. >>> >>> OUR RESTLESS TIDES >>> http://co-ops.nos.noaa.gov/restles1.html >> >> When the sun, earth and moon are aligned, i.e. at new moon or full >> moon, the heights of the tides are about twice as great as they are >> when the moon is at one of the quarter phases. But this means that the >> tidal effect of the moon is *three times* that of the sun. When the >> tides add, they are 4/3 times as high as the lunar tide alone. When >> they subtract, they are 2/3 times the lunar tide. So the ratio of the >> hughest tides to the lowest is 4/3:2/3, or 2:1 > > http://hyperphysics.phy-astr.gsu.edu/HBASE/tide.html > > âF_sun = 0.00017 * F_sun = 0.03 x F_moon > > âF_moon = 0.068 * F_moon > > The tidal effect on the earth of the moon is about 2.27 times the > tidal effect on the earth of the sun. > > > > >> >> It is now thought that the sun probably will not engulf the earth and >> moon when it becomes a red giant. The loss of solar mass to the solar >> wind will reduce the maximum radius of the sun and will also cause the >> earth's orbit to spiral outward. Both effects will reduce the >> probability of the earth beig engulfed. > > Please cite your sources for "It is now thought". > >
From: dow on 8 Sep 2009 16:36
On Sep 8, 2:38Â pm, Sam Wormley <sworml...(a)mchsi.com> wrote: > dow wrote: > > On Sep 8, 11:15 am, Sam Wormley <sworml...(a)mchsi.com> wrote: > >> dow wrote: > > >>> Actually, it might. The moon's tidal influence on the earth is > >>> presently about three times the sun's. However, the tidal gradient is > >>> inversely proportional to the cube of the distance, so the moon would > >>> only have to spiral out to cube-root(3) of its present distance, i.e. > >>> about 1.44 times as far away as it is now, for the two tidal > >>> influences to become equal. The moon is already several times further > >>> from the earth than it was when it was formed, so a further factor of > >>> 1.44 might "only" take another billion years or two. > >>> Â Â Â Â Â Â dow > >> Â Â No--the moon's tidal influence is only twice that of the sun. > >> Â Â Furthermore, it will take longer for the moon to "walk" to > >> Â Â your 60 x 1.44 earth radii than the time the sun will swell > >> Â Â and likely engulf the earth-moon system. > > >> Â Â OUR RESTLESS TIDES > >> Â Â Â http://co-ops.nos.noaa.gov/restles1.html > > > When the sun, earth and moon are aligned, i.e. at new moon or full > > moon, the heights of the tides are about twice as great as they are > > when the moon is at one of the quarter phases. But this means that the > > tidal effect of the moon is *three times* that of the sun. When the > > tides add, they are 4/3 times as high as the lunar tide alone. When > > they subtract, they are 2/3 times the lunar tide. So the ratio of the > > hughest tides to the lowest is 4/3:2/3, or 2:1 > > Â Â http://hyperphysics.phy-astr.gsu.edu/HBASE/tide.html > > Â Â Â âF_sun = 0.00017 * F_sun = 0.03 x F_moon > > Â Â Â âF_moon - 0.068 * F_moon > > Â Â Â The tidal effect on the earth of the moon is about 2.27 times the > Â Â Â tidal effect on the earth of the sun. > > > > > It is now thought that the sun probably will not engulf the earth and > > moon when it becomes a red giant. The loss of solar mass to the solar > > wind will reduce the maximum radius of the sun and will also cause the > > earth's orbit to spiral outward. Both effects will reduce the > > probability of the earth beig engulfed. > > Â Â Please cite your sources for "It is now thought".- Hide quoted text - > > - Show quoted text - Mass of sun = 2.707 e 7 times mass of moon (using figures in Rubber Bible) Radius of earth's orbit = 389.4 times radius of moon's orbit. (ditto) Tidal effect of moon on earth = 389.4^3 / 2.707e7 = 2.181 times tidal effect of sun. This assumes both orbits are circular, of course. In reality, they are not, so the ratio varies considerably. When earth is at aphelion and the moon is at perigee, the sun is 418.6 times further from earth than the moon is, which makes the tidal ratio 2.709:1, so in this situation it is closer to 3:1 than 2:1. Neither of us should be too dogmatic! Honestly, I can't recall where I read that the earth is likely to escape being engulfed by the sun, but I have read it somewhere. Of course, if I am thnking it, then "it is now thought". dow |