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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.14.2 Why does the Moon always show the same face to the Earth? Next Document: E.14.4 What was the origin of the Moon? See reader questions & answers on this topic! - Help others by sharing your knowledge Author: Richard A. Schumacher <schumach@convex.com>, Michael Dworetsky <mmd@zuaxp0.star.ucl.ac.uk>, Joseph Lazio <jlazio@patriot.net> Yes, at a rate of about 3--4 cm/yr. The tidal bulges on the Earth (largely in the oceans), raised by the Moon, are rotated forward (ahead of) the Earth-Moon line by the Earth's rotation since it is faster than the Moon's orbital motion. Using a similar picture as from the previous question, we'd see (looking down from the north pole): Earth Moon ____ / ) ___ ^ / / / \ | (____/ \___/ Moon's orbit & Earth's rotation (Ocean) Tidal bulge this way *greatly* exaggerated. The gravity from these leading and trailing bulges impels the Moon mostly forward along the direction of its motion in orbit (the Moon's orbit is not exactly in the plane of the Earth's equator). This force transfers momentum from the rotating Earth to the revolving Moon, simultaneously dragging the Earth and accelerating the Moon. In addition to causing the Moon to recede from the Earth, this process also causes the Earth's rotation to slow and days to become longer (at a rate of about 0.002 seconds every century). Eventually the result will be that the Earth will show only one face to the Moon (much like the Moon only shows one face to the Earth). A lower limit to how long it will take for the Earth and Moon to become tidally locked is 50 billion years, at which point the month and the Earth's "day" will both be approximately 50 (of our current) days long. However, this estimate is based on the assumption that liquid water seas would be present on Earth's surface to provide the tidal interactions necessary. But as the Sun evolves, the seas will evaporate and tidal interactions will be much slower (solid planet distortions only). The oceans will evaporate about 1--2 billion years from now, so the actual time will probably be much longer. Considerably more detail on the evolution of the Earth-Moon system can be found in an article by J. Burns in the book _Planetary Satellites_ (ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952). It is also interesting to consider what would happen if a satellite orbits its planet *faster* than the planet rotates. This is not the case for the Earth and Moon, but it is true for Mars and Phobos. In this case, Phobos also raises (crustal) tides on Mars. But now, Phobos is in front of the tidal bulge, so the gravitational action of the tidal bulge slows Phobos and Phobos moves *inward*. Thus, at some point in the future, Phobos will hit Mars. The most recent estimate is that the impact will occur in 40 million years, by A. T. Sinclair (1989, Astronomy & Astrophysics, vol. 220, p. 321). User Contributions:Comment about this article, ask questions, or add new information about this topic:Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.14.2 Why does the Moon always show the same face to the Earth? Next Document: E.14.4 What was the origin of the Moon? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Section - E.14.3 Is the Moon moving away from the Earth? (and why is Phobos moving closer to Mars?)
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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Section - E.14.3 Is the Moon moving away from the Earth? (and why is Phobos moving closer to Mars?)
( Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page
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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP