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The Planets' Influence on the Earth's Motion (thing) by IWhoSawTheFace Sat Jan 21 2012 at 21:28:04
linked by Oolong

Newton's law

          m⋅mE 
   FG = G ----  nt
           d2

where G is the gravitational constant, 6.67 x 10-11 nt⋅m2/kg2, m is the mass of the other celestial body, mE is the mass of the earth, 5.98 x 1024 kg, and d is the closest distance between the earth and the other body.

Gravitational Field Strength: Gravitational field strength is the gravitational force (due to another planet in the solar system) divided by the Earth's own mass. P = FG / mE, measured in Newtons per kilogram.

          m 
   P = G ---  N/kg
          d2
N.B. The gravitational force between two objects is a vector, and the gravitational force on many planets (and sun and moon) on the earth is a vector field, with a net direction and magnitude. Therefore, the gravitational field strength is also a vector field. However, the quantity P used here is just the magnitude of the field.

We'll use this quantity to compare the effects of the relative amount of influence of the Earth's motion by the bodies in our solar system.


                                             Maximum
                                          Gravitational
                    Orbital  Closest      Field Strength
            Mass    Radius   Distance     acting on Earth
Body        (kg)      (m)      (m)       (N/kg)   (ratio)
=================================================================
Sun      1.97E+30  1.50E+11  1.50E+11   5.81E-03  100
Moon     7.35E+22  3.84E+08  3.84E+08   3.30E-05    0.6
Jupiter  1.90E+27  7.80E+11  6.30E+11   3.18E-07    0.0055
Venus    4.87E+24  1.08E+11  4.20E+10   1.83E-07    0.0032
Saturn   5.69E+26  1.43E+12  1.28E+12   2.30E-08    0.0004
Mars     6.42E+23  2.28E+11  7.80E+10   7.01E-09    0.00012
Mercury  3.31E+23  5.85E+10  9.15E+10   2.62E-09    0.00005
Neptune  1.03E+26  4.51E+12  4.36E+12   3.60E-10    0.00001
Uranus   8.71E+25  2.88E+12  2.73E+12   7.76E-10    0.00001
=================================================================

All the planets are in a roughly circular orbit around the sun. Since the orbits have different radii, each planet has a shortest distance to the earth with a frequency that varies with difference in radii. The "closest distance" column is the difference in radii (Δr = |rEARTH - rOTHER| - that's the closest distance any planet can come to the earth.

The last column is gravitation field strength normalized to 100. The sun's effect was arbitrarily scaled to 100, and all other potential values are shown scaled to that number to give you an idea of how small the effects are of the perturbations on our planet.

The results are what you expect: The sun has the most influence by a far amount. The other planets perturb our orbit only weakly. The moon has the largest effect on the earth. It is so heavy relative to the earth that the moon doesn't actually revolve around the earth. Instead the earth and the moon revolve around a center-of-mass point located about 1000 km below the surface of the earth. A good illustration of that motion can be found here.

The sun is 390 times further away than the moon, but it has 27 million times more mass. Its influence on the earth is over 100 times stronger.

The sun has over a thousand times more mass than the next closest planet, the gas giant Jupiter. Because of this, the solar system can be thought of as having one gigantic central mass, whose little planets affect its orbit hardly at all.

The planets have even less of an effect on the earth's motion - miniscule compared to the nearby moon. Jupiter and Venus have about equal effect, but the gravitational field strength that they exert on the earth is 100-200 times weaker than the moon's. Other planets have even weaker influences.



Sincerest thanks to Oolong for pointing out problems with my terminology. There's an important difference between gravitational field strength and potential.

tide (idea) by Oneiromancer Wed Jul 31 2002 at 23:55:13
linked by Oolong

Tide as a general physical phenomenon is due to the spreading out of a central attractive force a la a gravitational field. Let's take the Earth-Moon system as an example.

The part of the Earth that is closer to the Moon is more strongly attracted to the Moon than the rest of the Earth because it's closer. Similarly, the far side of the Earth is less attracted because it is further away. If we set aside the net force on the Earth by cancelling it out with the centrifugal force*, the unevennesses of the gravitational field and the centrifugal force each contribute to a stretching force: the near side pulls towards the Moon, the far side pushes away.

Since the tidal effect is proportional to the change in the gravitational force, it varies as the gradient of the force (for objects small compared to the orbital radius). This gives it an inverse cube law.

Also, since the tidal effect is proportional to the change in the gravitational force over the entire object, its strength is proportional to the size of the object in question. Thus, lakes do not exhibit large tides, while the oceans do.

Since the stretching effect works simultaneously in opposite directions, when the Moon is full (and thus opposite the Sun), its tide and the Sun's tide are actually in agreement, even though they are opposites! This, and the more obvious case of a new moon, cause the stronger spring tide. When the Moon and Sun are maximally out of alignment (at right angles) the tidal effect is weakest - a neap tide.

Also note that if an object is not perfectly spherical (especially if one axis is longer than the others), it can become tidally locked, meaning the same side always faces the other orbiting body. One such example is the Moon, which is tidally locked to the Earth.

This rule also applies to the electrical force, but only in an insulator. In a conductor, all the charges rush to the nearer side, screwing up the balance of attraction force versus centrifugal force.

* In an orbital frame of reference the centrifugal force exists.

dimethyl sulfide (thing) by Oolong Thu Jan 19 2012 at 12:16:52

Cloudy issues

In a country dominated by desert, the lush tropical rainforests of northern Queensland stand out. This fantastically rich ecosystem is located just inland from the Great Barrier Reef, and maintained by something like three metres of rainfall per year - as much as eight metres, some years. It is still too early be sure, but new research by Graham Jones and his team at Australia's Southern Cross University suggests a surpsigingly intimate connection between this rainfall and the nearby reefs.

One of the great uncertainties in the scientific debate around climate change is the exact nature of the interaction between cloud cover, rising temperatures and increasing pollution. Broadly speaking, the more clouds there are, the higher the Earth's albedo – that is, the more radiation gets reflected straight back into space – and the cooler the Earth gets. On the other hand, uncondensed water vapour and high ice-crystal clouds have the opposite effect, bouncing infrared radiation back towards the ground in the same way that carbon dioxide does.

Warmer seas lead to higher evaporation rates, increasing levels of both clouds and vapour, and it is not yet clear which effect will win out. If the increased albedo caused by clouds turns out to dominate then we have a negative feedback, putting a brake on climate change. If instead the warming effect of the vapour dominates, we are looking at a positive feedback loop, meaning that any heating of the Earth will tend to be self-reinforcing. There is a danger of runaway climate change if this and other self-catalysing mechanisms – the disappearance of polar ice caps (decreasing the Earth's albedo), the melting of the permafrost (releasing stored methane), and so on – prove to overwhelm any negative feedback effects. My brother made an excellent animation about positive feedback loops and tipping points in climate change, with extensive scientific references to back it up, which has been translated into 22 languages by volunteers.

Jones' research gives us yet another source of positive feedback to worry about. Algae, along with other marine organisms, produce a gas called dimethyl sulfide (DMS), which encourages clouds to form. Water vapour in the atmosphere will usually only condense into droplets and form clouds when it has something to stick to, such as specks of dust or soot – or the sulfate aerosols created by DMS when it oxidises in air. The same principle has been exploited with some success in cloud seeding experiments, and if the presence of algae-rich coral and other sources of DMS does indeed cause the increased cloud cover expected, its implications for the local and global climate could be profound. Jones suggests that the presence of coral reefs may help explain the lusciousness and location of rainforests in Queensland and around the world, though this is a long way from being confirmed by research.

At one time it was hoped that rising sea temperatures might lead to more productive algae, increasing the overall production of DMS and creating a negative feedback loop to help slow down climate change. Unfortunately, Jones' research suggests that the opposite is likely to be true. When temperature rises by just 2°C, some coral algae stop producing DMS at all – and of course with less DMS, the cloud cover will be reduced, more sun will get through, temperatures will rise, and levels of DMS will drop.

Perhaps in the future, a better understanding of the mechanisms involved might help us to mitigate climate change either through a global engineering project to increase the planet's albedo, or at least helping us to ensure local levels of rainfall. Until then, the prospect of algae choking in warmer seas is just one more cloud on the climate horizon.

References


I originally wrote this when I was applying to work at The Economist in 2010, so I made some effort to write in their house style. I'm not sure how well I succeeded; they didn't give me a job, in any case.

Science to write by Oolong Sat Feb 08 2003 at 22:56:06
(test)

SCIENCE NEEDING NODING

The Content Rescue Team : Nodes meets the Science Index...

(but not yet the BioTech list)

You are invited to write up anything on the list that you can get your head round enough to explain to other people. Add to the list any scientific subjects which are still in need of a good noding. If you see something listed which you're convinced has been done pretty comprehensively, feel free to remove it - you should probably also use the 'Sciencify!' button to place them on the E2science page. Please consider all additions and subtractions carefully; /msg me if you're not sure about something.

- Oolong

Key:
(E) = Empty
(O) = Nodeshell only
(W) = Webster 1913 only
(S) = Short
(S?) = Short but might be enough. I need feedback on these.
(M) = Messy / Needs reorganization/consolidation
(?) = Other: Will have a comment

Mathematics
Physics
Chemistry
Earth Science
Life Sciences (Including Biology and Medicine)
Animals
Taxonomy Social Science
Other Science n' Technology
People

Compiled by Oolong on Februrary 9, 2003, by taking the Science and Technology part of the CRT list and subtracting 'Human anatomy and bits of the body', 'Computer Hardware', 'Computer Software', 'Computing - Games and toys' and 'Computing - other', then adding 'Chemistry', 'Social Sciences' and 'Earth Sciences'. Topics marked (s), (w) or (0) in the Science Index were added to appropriate categories, and various existing entries were moved.

Updated: March 10, 2005 by lifix: Cleaned up the list, and added topics from the e2science index.

Updated: April 3, 2006 by Oolong; striking out some holes which have been filled. 

apocrine gland (thing) by decoy hunches Wed Jan 18 2012 at 2:28:11
linked by Oolong

Apocrine glands are glands which produce sweat. Most modern mammals have apocrine glands and use them to communicate. Located in between the skin and subcutaneous fat, these glands are tiny spirals that secrete sweat into the hair follicles found around the aerolas, armpits and genitals. In addition to these there are specialized apocrine glands around the eyelids called Moll's glands.

The secretions themselves are milky, viscous and do not develop an odor until coming into contact with bacteria located on the skin's surface. It is likely that the vomeronasal organs can detect the scent of these secretions before the bacteria have a chance to do their thing. These secretions contain pheromones which allow for easier identification of the mammals in question—identification in terms of lover-lover, parent-child, etc.

Once when my sister was detoxing, she stank superbad and after I complained she blithely informed me that humans are biologically programmed to find the body odor of siblings abhorent in order to prevent incest. Said explanation was accompanied by her raised arms, much to my dismay.

Interestingly enough, apocrine glands are in fact merocrine and not apocrine. (They are merocrine because the secretions pass through an intermderiary tissue (ie hair) before leaving the body.) They were first thought to be apocrine and the name has stuck like bad BO.

 

 

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