| |
In lugnet.space, Lindsay Frederick Braun writes:
>
> Hi,
>
> Long time no see, John! Have you just been lurking about?
I've posted a few times this year, but mostly I've been lurking. And trying
to build something that truly satisfies me. I keep buying more parts and
experimenting... patience now...
> (I'm finally back from Europe myself.)
Welcome home. Is the dissertation in the can?
> Coming to Brickfest?
Travelling to DC tries my five year-old's patience sorely. We've done it a
few times, and he's always cranky. I will, however, take him to the LEGO
Life On Mars exhibit truck when it visits Baltimore next week.
> How's JHU?
I like it well enough here. I have a boss who treats me well. Science
treats me however it wants to treat me. One day my experiments work,
another day they don't. Ugh.
> In lugnet.space, John J. Ladasky, Jr. writes:
> >
> > > In lugnet.space, Jason J. Railton writes:
> > >
> > > Actually, I seem to remember that the moon's pull on the tides is mutual
> > > (the moon is affected by the gravity of water on the Earth), and because
> > > tidal waters drag across the surface (thus slowed by friction), this is
> > > gradually decelerating the moon's orbit. So, it's orbit is very slowly
> > > shrinking...
> >
> > Actually, you have this backwards. The friction of Earth's oceans against
> > its solid parts is slowing the Earth's rotation down. This translates into
> > a loss of angular momentum for the Earth. But angular momentum must be
> > conserved. The angular momentum is transferred to the moon, so the moon is
> > actually gradually moving *farther* from the Earth. IIRC the increasing
> > separation of the Earth and Moon has been measured quite accurately by
> > bouncing lasers off of the mirrors left behind by the Apollo missions.
>
> Weren't there other methods used recently as well? I'm not
> sure that any would be as accurate as a laser, given that
> the international meter standard is based on the speed of
> light (as of the 1980s, I think). Somehow using radar sticks
> in my mind, but that might just be a holdover from earlier
> measurement in the 1940s and 1950s.
I don't know anything about other methods of measuring the Earth-Moon
distance, besides laser ranging. I do remember hearing that Earth-based
radar was used to infer the existence of ice at the lunar poles, a finding
which was later corroborated by the Lunar Prospector mission in 1998. Could
you be thinking of this?
> > When the Earth has slowed enough so that its period of rotation equals the
> > period of the moon's revolution, there will be no more tidal friction. The
> > Earth will cease to slow, and the moon will cease to move farther away.
> > Interestingly, at this point one side of the Earth will always point towards
> > the moon -- just as, right now, one side of the Moon is always pointed
> > towards the Earth. This state of affairs is known as "tidal locking." When
> > this finally happens, one Earth day will be somewhat longer than 28 current
> > Earth days.
>
> I wasn't aware both faces had to be locked for the term
> "tidal lock" to be valid.
It's not a requirement. Sorry if I was unclear. The Moon is already
tidally locked to the Earth. Eventually the Earth will also be tidally
locked to the moon.
> For example, I've heard the
> statement made that Mercury is tidally locked to the Sun--
> true in that the same face of Mercury is sunward, but not
> true for the Sun, if you can really call that a "face".
Actually, Mercury is weirder than that. It orbits the Sun in 88 days, but
its period of rotation is 59 days. Mercury is in what is called a 3:2
resonance. For every two of its revolutions around the Sun, Mercury rotates
three times around its axis.
> And locking is not fixed tight--both Mercury and the moon,
> like Jupiter's satellites, and presumably Pluto and Charon
> (which *are* a tidally-locked double planetoid system, like
> Hector in the Belt) too, "librate"--they basically wobble.
> But whether this is the settling of a golf ball in the cup
> or it's being powered from outside, I don't know offhand.
>
> > This tidal locking will take a pretty long time. In fact, some recent
> > studies suggest that increasing solar radiation will cause Earth's oceans to
> > evaporate in the next 500 million to 1 billion years, sooner than tidal lock
> > is expected to be achieved. Tidal lock can also occur with an ostensibly
> > solid body (e.g., Jupiter's moons), but it's a slower process.
>
> Of course, this does assume that no weird momentum-altering
> things happen (collisions, the unexpected expulsion of a
> gaseous shell from the Sun, etc). But all of those kinds of
> things might make our discussion a little bit, um, "academic."
> Not that there's anything wrong with that.
>
> Re: the oceans evaporating: I wonder if we can look at solar
> output in past aeons?
They're trying to infer this from paleoclimatological data and models of
stellar evolution. I don't believe there is a way to measure the Sun's
historical output directly.
> It may be that Earth was only warm
> enough for multicellular life at a certain point
The Earth's surface, maybe. We have every reason to believe that the
thermal deep-sea vents have been around for a long time.
> --and that it
> may be different enough *now* that if one brought, say, an
> eryopsid labyrinthodont (big, giant, mega-amphibian) to the
> present day, it would cook or suffocate somehow. I know that
> there's a lot of work being done on the sheet-of-ice planet
> idea--where only the equator regions were ice-free, sort of
> a super Ice Age.
>
> rambling,
Feel free! But maybe we should FUT .off-topic.geek?
> LFB
--
John J. Ladasky Jr., Ph.D.
Department of Biology
Johns Hopkins University
Baltimore, MD 21218
|
|
Message is in Reply To:
195 Messages in This Thread:
(Inline display suppressed due to large size. Click Dots below to view.)
- Entire Thread on One Page:
- Nested:
All | Brief | Compact | Dots
Linear:
All | Brief | Compact
|
|
|
|
