Ask a Scientist

Editor’s note: The Earth Observatory no longer supports the ‘Ask a Scientist’ feature. These pages provide an archive of previous questions and answers.

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Q: Why does only one side of the moon face the earth?

--Daniel, Johannesburg, South Africa

A: Dear Daniel,

The original rotation of the moon was braked by the friction of tidal forces induced by the earth on the moon. Any time two celestial bodies (planet, moon, or star) lie in each other's gravitational fields, the side of each closest to the other experiences a stronger gravitational attraction than the side that is farthest away. When a body is rotating, the locations of maximum and minimum gravitational force continually change, driving increases and decreases in elevation of whatever solid, liquid, and gaseous surfaces there are - tides. Tides on earth are primarily driven by the moon and sun. We are most familiar with oceanic tides, but the atmosphere and solid earth also have tides. Similarly, when the moon was rotating faster than once a month (the period needed to keep the same side to the earth at all times), it experienced tides.

The IMAGE satellite program has an excellent web site that addresses various aspects of this question, as well as many other moon-related questions.

Q: I suppose that if the earth's temperature (and energy) will increase the violence of meteorological phenomena will also increase, because (tell me if I am wrong) of the delta temperature between polar regions (where the ice melting absorbs energy) and the rest of the world where temperature is increasing. But I wonder - when the ice is completely melted, will the earth reach a more stable situation?

--Diego, Rome, Italy

A: Dear Diego,

Most numerical models of climate indicate that polar regions are the most sensitive to global warming. The vast expanses of the tropical oceans already have two efficient processes in place for exporting incoming solar energy to higher latitudes - ocean currents and the Hadley circulation in the atmosphere. Ocean currents carry massive amounts of warm water toward higher latitudes and cool water back toward lower latitudes. In the Hadley circulation, heating near the Equator drives rising motion (mostly inside intense thunderstorms), which forces warmer high-altitude winds toward the Poles and cooler low-altitude winds toward the Equator (the tradewinds). Tropical land areas contribute to the Hadley circulation. At midlatitudes, large amounts of heat continue to be carried poleward by ocean currents and the familiar high and low pressure systems and fronts. Thus, the polar regions have the weakest dynamical controls on the average temperature. Even with the usual caveats about incomplete data and regional variability, the polar regions do seem to be showing average increases that are larger than in other regions.

Despite the reduction in equator-to-pole temperature gradient, there has been speculation that warming might drive more extreme events. The various concepts require considerable work before a definitive statement can be made.

The no-icecap scenario is a fascinating thought experiment, fraught with numerous uncertainties. The increase in sea level might facilitate the penetration of warm currents into the Arctic Ocean, reinforcing the loss of ice. Ice-free polar oceans might provide massive amounts of additional moisture for most of the year, driving heavy snows over bordering land areas. Changes in polar surface cover and the accompanying changes in polar cloudiness could have profound effects on the regional temperatures by modifying how efficiently heat radiates to space during the long polar nights. It is not clear that these possibilities point to more placid conditions, at least in polar regions. At lower latitudes the arguments for greater occurrence of extreme events continue to apply, if they're valid in the first place. To summarize, we are not yet in a position to know how storminess will change as the climate changes.

Q: Is there any evidence of global warming and/or climactic global changes affecting the monsoon seasons in Asia?

--Joel, Eugene, OR

A: Dear Joel,

The available datasets make it clear that the Asian monsoon system has significant fluctuations over periods of a decade or longer. However, attributing those fluctuations to particular causes is a messy business. We know that the monsoon can exhibit a wide range of behaviors for a given average state of the atmosphere. On the other hand, we know that the average state of the atmosphere has significant fluctuations. And, when the average state of the atmosphere changes, the typical progression of the monsoon likely changes. For example, during the mid-Holocene era (some 6,000 years ago) the earth's orbital parameters were slightly different than they are now, causing the seasons to be stronger than they are now. Paleoclimate data show that the main regions of monsoon precipitation shifted northward in both Asia and Africa during that time.

We do know that the tropical Indian Ocean has warmed over the last three decades, which likely has an important effect on the Asian Monsoon. We also know that the correlation between the Asian monsoon and the El Ni´┐Żo-Southern Oscillation has decreased during that time. Unfortunately, the current data records are too short to untangle the natural variability of the monsoon from the effect of climate change, whether natural or anthropogenic.

The alternative is to perform studies with numerical models of climate (similar to weather forecasting models, but designed to represent longer timescales). Results from such model studies are decidedly mixed, with tentative indications that global warming might cause the Asian summer monsoon to be somewhat stronger and the winter monsoon somewhat weaker. Even if this result holds up, the magnitude of the change is an open question.

Q: Why does the color of the moon change? What makes the moon look red sometimes?

--Eric, Bowie, Maryland

A: Dear Eric,

The atmospheric optics rules that you know about for the sun also apply to the moon (and any other object at great distances from the observer) - the sun is just a lot brighter than anything else. The tiny aerosol particles that float in the air are of such a size that they preferentially scatter shorter ("blue") wavelengths of light out of the line of sight between an observer and an object, compared to longer ("red") wavelengths. The longer the path through the atmosphere and/or the heavier the load of aerosols, the more scattering that takes place, causing the object to appear more red and less blue. When the object is directly overhead, the path through the atmosphere is as short as possible, and it gets progressively longer as the object sinks toward the horizon. Heavier aerosol loading can be caused by blowing dust, fires, ordinary pollution, and so on, as well as humid conditions that tend to plump up several classes of aerosol particles. So, just as with the sun, you see a dim, red moon on the horizon in conditions of gunky, humid air, and a bright, whitish moon well overhead on clear winter nights.

By the way, the optics rules also let the moon create the same optical phenomena as the sun, including rings around the moon, "moon dogs," and rainbows. However, since the moon is much dimmer than the sun, all of these optical effects are much dimmer.

Q: Since photosynthesis is the reaction of CO2 and H2O forming sugar and O2, isn't the rate of photosynthesis determined by the concentration of CO2 in the atmosphere? With currently increasing CO2 concentrations, shouldn't photosynthesis be speeded up? Is there evidence for this?

--Harold, Madison, Wisconsin

A: Dear Harold,

Laboratory studies show that higher concentrations of CO2 promotes plant growth, which is driven by the rate of photosynthesis, for some, but not all species of plants. For example, wheat benefits greatly, while corn sees almost no change. In the wild, the rate of photosynthesis is governed by a number of factors. Some are direct, including the concentration of CO2, the ambient temperature, and the availability of sunlight. There are other indirect factors that affect a plant's ability to support photosynthesis, including availability of moisture, temperature, genetics, soil condition (PH, salinity, water saturation, tilth, nutrient availability), disease, and damage by parasites and herbivores. If any of the factors other than CO2 is limiting a plant's photosynthesis rate, adding more CO2 will be relatively ineffective in provoking additional photosynthesis.

Q: What is the latest status of the Antarctic iceberg B-15? What effect will it have as it leaves the Ross Icesheet?


Designated "B-15" by the National Ice Center after its separation from the Ross Ice Shelf in mid-March 2000, this iceberg is one of the largest ever recorded. Despite holding together in one piece for nearly seven weeks, B-15 broke in half in early May.

Before fracturing, the berg measured 295 by 37 kilometers (170 miles long by 25 miles wide), and had a total area slightly more than 10,000 square kilometers (4250 square miles). For reference, B-15 was about as wide as Long Island, and by comparison would extend from New York City past Montauk Point to Martha's Vineyard.

Using satellite and ocean tide information, Professor Doug MacAyeal of the University of Chicago constructed some models to predict the motion of B-15 while it is in the Ross Sea. These tidal models accurately predicted the berg to rotate to a slightly NW-SE orientation. Now that it is in two pieces, the eastern portion has assumed the NW-SE orientation while the western portion is more flush against the existing iceshelf. Also, smaller bergs have calved off the main B-15 halves.

More images of the original B-15 and the current B-15A and B-15B icebergs can be viewed at the University of Wisconsin's Antarctic Meteorology Research Center iceberg images web site.

The first part of May also saw the calving of three large icebergs from the Ronne Ice Shelf of Antarctica. The bergs were designated A43A, A43B, and A44 by the National Ice Center. With the loss of these three icebergs, the front of the Ronne Ice Shelf returns to about the same position it occupied in the 1950s.

According to Dr. Ted Scambos of the National Snow and Ice Data Center, "The recent calving of the large bergs from the Ross Ice Shelf and the Ronne Ice Shelf reflects the normal cycle of Antarctic ice shelves slowly advancing and periodically calving new bergs. These events are not necessarily indicative of regional warming as are the events we see along the Antarctic Peninsula where ice sheets are tending to disintegrate rather than calve large bergs."

For more information on these icebergs and where they may move next, and on iceshelves in general, see the following websites:
The National Snow and Ice Data Center's iceshelves pages
The National Ice Center (Click on "Icebergs")

Q: I am having trouble visualizing why high noon can vary at my location as much as a half hour during a calendar year. The reasons given, namely the tilt of the axis and the elliptical nature of the Earth's orbit, seem to be insufficient to account for that large a variation. Also the extremes of the fluctuation do not seem to align with perigee or apogee or equinoxes or solstices. Is there more to it or am I missing something?

--Anssi, Vancouver, BC, Canada

A: Dear Anssi,

In Chapter 1 of this science mystery, our perceptive reader noticed that the time that a sundial shows ("Sun time") varies in the course of a year as much as 15 minutes ahead or behind the time that a clock shows. In Chapter 2, a little library work showed him that the variations in Sun time are due to two separate effects, namely the tilt (or "obliquity") of the Earth's axis and the elliptical orbit that the Earth takes around the Sun. For the Earth's present orbital parameters both effects are important, with the first effect being slightly larger. Now in Chapter 3 he is trying to puzzle out how this all fits together. The short answer is that his sources are correct and he is correct -- it's not obvious. The first key insight is that even though these effects are small, the changes are cumulative. So, changing individual Sun days by a few seconds shifts the relationship between Sun and clock time by minutes over several weeks.

The rest of the story gets more technical, but there's a surprise ending! It takes 23 hours, 56 minutes, and 4 seconds for the Earth to revolve exactly once on its axis (a "sidereal day"). During this day, the orbital motion of the Earth makes it appear that the Sun has moved (mostly) eastward in the sky, requiring another 3 minutes, 56 seconds of rotation to again point at the Sun (on the average). Because of the Earth's tilt, part of this apparent motion is north-south, reducing the amount of extra rotation needed to account for the eastward motion. Apparent north-south motion is largest at the equinoxes and zero at the solstices, so the Sun appears to move eastward the fastest at the solstices and the slowest at the equinoxes. The variations in orbital velocity produced by an elliptical orbit cause faster apparent eastward motion at perigee (closest approach) and slower at apogee (largest separation). A second key insight is that fast changes in apparent Sun position mean a longer Sun day, since it takes a little longer for the Earth to rotate the extra distance to face the Sun, and periods with slow changes have shorter Sun days.

The final chapter for this mystery depends on the key insight that the obliquity effect provides two speed-up/slow-down cycles each year, since there are two solstices in a year, while the elliptical orbit effect only has one. The winter solstice and perigee happen to nearly coincide (December 21 and 31, respectively), providing reinforcing longer-Sun-day effects and the largest cumulative slowdown compared to clock time at that season. Six months later there is near-balance between the summer equinox's increase and the apogee's decrease in Sun day length. So, at the end of the story the seemingly obscure variations in sundial readings actually provide a tangible illustration of the Earth's motions in space. Cool!

By the way, these variations in Sun time and the Sun's annual north-south migration are frequently combined into the "analemma," sometimes called "the astronomer's odd figure-8," that appears on many globes and sundials. You can read more about Sun time by visiting web sites such as or

Q: In God's grand design, doesn't "global warming" make good sense? We raise the co2 level, the warming raises the growing seasons to use the co2 and produces the corresponding O2. I know this is overly simplistic but is there a positive scientific perspective? Warm air holds more moisture which will water the greater number of plants and so on.

--Al, Fort Wayne, Indiana

A: Dear Al,

Let's start with the really big picture. What are the direct climatic changes due to increasing average global temperature--generally referred to as "global warming"? Scientists are fairly confident that most areas would see increased temperatures year-round, although some areas will actually have decreased temperatures due to particular local/regional circulations. If temperatures rise unchecked, storm tracks and global air and ocean circulation patterns will certainly change, but there is very little agreement about what precisely to expect. More hurricanes or fewer? Enhancement or suppression of the Asian Monsoon? More floods and droughts or fewer?

At the next level of detail, warmer summers and winters will have a wide variety of effects. In our homes and offices, for instance, less energy would be used for heating, and more for cooling. Large regions of Canada and Russia might achieve a sufficiently long growing season to allow for more crop farming. At the same time, tropical pests would move into higher latitudes. In fact, ecosystems in general would go through periods of massive adjustments. The navigation season would lengthen for waters that experience seasonal ice, such as the Great Lakes and the margins of the Arctic Ocean. The snow season would be shorter, reducing water storage and winter recreation opportunities in the mountains. Major melting of glaciers and icecaps would occur. The swimming pool industry would boom and the market for snow shovels, road salt, and parkas would decline. It has been suggested that warming will promote more rain in moist areas, but even if true, the increased evaporation that warming should cause might actually lead to drier conditions. And how important might the rainfall and evaporation changes be compared to the unknown changes in large-scale storm patterns?

The chain of connections goes on. One major issue is that higher temperatures combined with the melting icecaps and glaciers would cause a rise in sea level, estimated to be 0.5 meters in the next 100 years (and more after that) if global warming continues as expected. People living in coastal areas worldwide will have to respond, most notably in low-lying areas such as Florida; Venice, Italy; the Netherlands; and the hundreds of low islands and atolls in the global oceans.

Are these effects "good" or "bad"? Some are pretty obvious, but these types of judgements are a societal issue, not scientific. Scientists can say that global warming is a package deal--you get everything listed above plus a whole lot more that we still lack understanding of. Many of the effects that the warming will bring will last a long time once it gets started and there's no way to turn it off once the chain of events begins. So one might conclude these are "bad" effects because today's ecosystems and human activities are highly attuned to the current climatic state. In short, the potential for negative effects in significant parts of the global community makes it hard to speak favorably of global warming.

Q: Is it possible to flow water to the Sahara Desert from the Atlantic and, therefore, change the climate for good in Africa?

--Isidro, Barcelona, Spain

A: Dear Isidro,

There is archeological evidence which tells us that the Sahara was relatively green during the last Ice Age. This evidence includes rock paintings of animals that are typical of a modern-day savannah. Since the last Ice Age, the Sahara has become one of the great deserts of the world because the atmospheric circulation patterns changed as the ice sheets retreated to the extreme polar regions. It is highly likely that a single project to soak the desert with water would only make a difference for a few years, during which time the atmospheric circulations would again dry out the area. Alternatively, you could pump water for as long as you wished to sustain lush vegetation. In either case, the financial, political, and technical hurdles seem beyond solution: Remove the salt from millions of tons of ocean water, pump it thousands of kilometers across many international boundaries, and distribute it more or less evenly on millions of hectares of land. Compared to these strenuous efforts, the natural process for producing rainfall that most of us enjoy seems positively simple!

Q: Where does the heat required to activate the "warm pool" in the Indonesian area come from? Since the sun doesn't change its radiation patterns to specifically enhance the temperature of the Indonesian area in its daily transit, it seems likely that some other source of energy must exist. Perhaps subterranean volcanoes?

--Robert, USA

A: Dear Robert,

The heat that warms the ocean around Indonesia comes from the sun. Areas along the equator receive abundant exposure to the sun's rays, which eventually are stored as heat in the ocean. Unlike the eastern Pacific, which usually has strong upwelling of cold, deep currents (driven by strong trade winds) that keep the surface waters cool, the Indian Ocean and Western Pacific warm pool don't have similar upwelling mechanisms for cooling the surface waters as efficiently, so the waters are much warmer there.

While there are thermal vents on the Indian Ocean floor, the amount of heat they release is negligible compared to the heat from the sun's rays.