Motion of Moon

4.1 The Phases of the Moon

• The Moon orbits the Earth roughly once a month.

  Looking down on the Earth and Moon from above the Earth’s north pole, we see that its revolution is in the same direction as the Earth’s rotation (and also the Earth’s revolution around the Sun).

• The Moon shines by reflected sunlight.

  Therefore, at any time only one half of the Moon, the side facing the Sun, is illuminated.
  
  The dividing circle between the light side and the dark side is called the terminator.

• The illuminated side of the Moon is not necessarily the half which faces the Earth.

  Depending on the relative positions of the Sun, Moon, and Earth, we see different fractions of the Moon illuminated.

  These are called the phases of the Moon.

  

• At new moon, we can’t see any of the illuminated half of the Moon; at full moon we can see all of it.

• Halfway in between new and full moon, we see half of the illuminated half of the Moon, or a quarter of the Moon.

  First quarter occurs as the Moon moves from new to full; third or last quarter occurs as the Moon moves from full to new.

• Between the new and quarter moons, only a small fraction of the Moon is illuminated; we call this a crescent moon.
  
  Between the quarter and full moons a larger fraction of the Moon is illuminated; we call this a gibbous moon.
  
  When the Moon moves from new to full, it becomes more illuminated, and we say that it is waxing. When it moves from full to new, it becomes less illuminated, and we say that it is waning.
• On any particular night, the Moon will essentially be motionless.

  As can be seen from the diagram above, a full moon must therefore rise around 6 P.M., be overhead at midnight, and set around 6 A.M.

  A first quarter moon must rise around noon, be overhead around 6 P.M., and set around midnight.

  Crescent moons are overhead during the day, but they are generally only visible near sunrise/sunset (both because of their small illumination and the brighter light from the Sun).

  Question: if it’s 3 A.M. and the Moon is rising, what phase is it?

  

• The synodic month is defined as the time it takes for the Moon to return to the same position relative to the Sun, e.g. from full moon to full moon.

  The synodic month is equal to 29.5 days.

• The sidereal month is defined as the time it takes for the Moon to return to the same position relative to the stars; it is equal to 27.3 days.

  The sidereal month is shorter than the synodic month because of the revolution of the Earth around the Sun, as can be seen at the right.

  The Moon doesn’t have to travel as far around its orbit to line up with the same distant star.

  (Note: the motion of the Earth around the Sun is exagerated in this picture to clarify the positions.)

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4.2 Eclipses (Discovering the Universe, 5th ed., §1-9) The Moon’s orbit is inclined at an angle of 5° to the Earth’s, so it is usually either above or below the plane of the ecliptic. The points where it crosses the ecliptic are called the ascending and descending nodes, depending on whether it is moving north or south, respectively. The line connecting the two nodes is called the line of nodes. Question: a line is also the intersection of two planes; what are those two planes for the line of nodes? Sometimes, when the Moon passes through the ecliptic, it will happen to be full (directly opposite the Sun) or new (directly towards the Sun). At these positions and phases, an eclipse will occur, when the Moon or Sun “fails to appear”, which is the Greek meaning of the word (this is also the origin of the name ecliptic).

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4.3 Lunar Eclipses (Discovering the Universe, 5th ed., §1-10) A lunar eclipse occurs when the Moon is full and it is close enough to the ecliptic that it passes partially or completely through the Earth’s shadow. A lunar eclipse can be observed from anywhere on the night side of the Earth. The darkest part of the Earth’s shadow is called the umbra; it is where the Sun’s light is completely blocked out. The umbra is not totally dark, however, because the Earth’s atmosphere scatters red light into it. The Earth’s partial shadow is called the penumbra; it is where the Sun is only partially blocked by the Earth. Photo information When the Moon completely enters the umbra, a total lunar eclipse occurs as the Moon almost disappears from view. The long-exposure photograph at the right shows the entire span of a total eclipse. Because of the scattered light in the umbra, the Moon does not completely disappear but takes on a dull red hue which brightens toward the edge of the umbra. Photo information A total lunar eclipse can last as long as 1 h 42 min, depending on how close to the center of the umbra the Moon passes and its distance from the Earth. The total lunar eclipse shown above right lasted 1 h 18 min. A partial lunar eclipse occurs when the Moon only passes partway through the umbra. The eclipse at the right was 92% total (more details). A penumbral lunar eclipse occurs when the Moon only passes through the penumbra.

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4.4 Solar Eclipses (Discovering the Universe, 5th ed., §1-11) A different kind of eclipse, a solar eclipse, occurs when the Moon is new and it is close enough to the ecliptic that its shadow partially or completely reaches the Earth. The Moon’s umbra forms a circular region on the surface of the Earth, and if you are in that shadow the Sun is blocked out. This is a total solar eclipse. Photo information The Moon’s umbra has a maximum diameter on the surface of the Earth of 270 Km. The umbra of the eclipse of August 11, 1999 can be seen at the right. Photo information A total solar eclipse is dark enough that animals will actually begin their nocturnal habits, e.g. birds will stop chirping. Once again, however, a total solar eclipse is not completely dark, because the dim glow of the Sun’s atmosphere can be observed around the edge of the Moon. More details As the Moon moves in its orbit, we see the Moon pass across the face of the Sun: The shadow moves rapidly across the surface of the Earth, sweeping out a narrow path as it speeds by at about 0.5 Km per second. As a result, the maximum time that a total solar eclipse can last is 7.5 min, depending on the shadow’s size and speed. If you are located in the Moon’s penumbra, which is much larger than the umbra, the Sun is only partially blocked out. This incomplete covering of the Sun is called a partial solar eclipse. Photo information When the Moon is farthest from us, the tip of the umbra doesn’t quite reach the Earth. From our point of view here on the Earth, the Moon does not quite cover the Sun, so a ring of sunlight will surround it. This type of partial eclipse is called an annular eclipse. The eclipse at the right is just barely annular.

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4.5 The Frequency of Eclipses (Discovering the Universe, 5th ed., §1-9) As mentioned above, eclipses can only occur when the Moon is close to a node and it is also either full or new. For this alignment to happen, the line of nodes must point near the Sun. Just like the Earth’s axis, the line of nodes is relatively fixed in space. With no other forces acting, the line of nodes would therefore be in line with the Sun every six months. However, also like the Earth’s axis, the gravitational force of the Sun on the Moon causes the line of nodes to precess. This precession is a much more noticeable effect than the Earth’s precession, with one every 19 years. As a result, the time between alignments is decreased to about 5.4 months. Because of the finite size of the Earth, Moon, and their shadows, multiple eclipses can occur whenever the line of nodes points near the Sun. So, eclipses are actually very common! During a one-year period, there can be between two and five eclipses of each kind (solar and lunar), with a total of between four and seven. This includes partial and penumbral lunar eclipses, and partial and annular solar eclipses. Lunar eclipses are much more likely to be observed, since anyone on the night side of the Earth can see them. Solar eclipses, on the other hand, cover only a small fraction of the Earth, and often occur over unpopulated locations such as the polar regions or the oceans. The table below lists upcoming eclipses for the next several years: Date (Peak) Time (Peak) Type Fraction of Totality Duration of Totality Where Visible 1997 Mar 8 8:24 PM Solar, Total 100% 2 min 50 s East Asia, Alaska 1997 Mar 23 11:39 PM Lunar, Partial 92% — Americas 1997 Sep 1 8:04 PM Solar, Partial 90% — Australia, Antarctica 1997 Sep 16 2:47 PM Lunar, Total 100% 1 h 2 min Europe, Africa, Asia, Australia 1998 Feb 26 12:28 PM Solar, Total 100% 4 min 9 s Americas 1998 Mar 12 11:20 PM Lunar, Penumbral 0% — Americas 1998 Aug 7 10:25 PM Lunar, Penumbral 0% — Americas, Europe, Africa 1998 Aug 21 10:06 PM Solar, Annular 97% — Southeast Asia, Australia 1998 Sep 6 7:10 PM Lunar, Penumbral 0% — East Asia, Australia, Americas 1999 Jan 31 11:17 AM Lunar, Penumbral 0% — Asia, Australia, Hawaii, Alaska 1999 Feb 16 1:34 AM Solar, Annular 99% — South Africa, Antarctica, Australia 1999 Jul 28 7:34 AM Lunar, Partial 40% — Australia, Hawaii, North America 1999 Aug 11 7:03 AM Solar, Total 100% 2 min 23 s Europe, North Africa, Middle East 2000 Jan 20 11:43 PM Lunar, Total 100% 1 h 18 min Americas 2000 Feb 5 7:49 AM Solar, Partial 58% — Antarctica 2000 Jul 1 2:32 PM Solar, Partial 48% — South Pacific 2000 Jul 16 9:56 AM Lunar, Total 100% 1 h 48 min Asia, Australia, Hawaii, Alaska 2000 Jul 30 10:13 PM Solar, Partial 60% — Siberia, Alaska 2000 Dec 25 12:35 PM Solar, Partial 72% — North America 2001 Jan 9 3:20 PM Lunar, Total 100% 1 h 2 min Eastern Americas, Eurasia, Africa, Australia 2001 Jun 21 8:04 AM Solar, Total 100% 4 min 57 s Southern Africa 2001 Jul 5 10:55 AM Lunar, Partial 50% — Eastern Africa, Asia, Australia 2001 Dec 14 3:52 PM Solar, Annular 97% 3 min 53 s Central America 2001 Dec 30 5:29 A.M. Lunar, Penumbral 0% — Asia, Australia, Americas 2002 May 26 8:03 AM Lunar, Penumbral 0% — Eastern Asia, Australia, Western Americas 2002 Jun 10 7:44 PM Solar, Annular 99.6% 23 s Pacific 2002 Jun 24 5:27 PM Lunar, Penumbral 0% — South America, Africa, Europe, Asia, Australia 2002 Nov 19 8:46 PM Lunar, Penumbral 0% — Americas, Africa, Eurasia 2002 Dec 4 2:31 AM Solar, Total 100% 2 min 4 s Southern Africa, Australia The information in this table is derived from NASA’s The Eclipse Home Page, where you can find lots more information about eclipss..

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Contents

Top §4

Index

Next: §5

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Star charts are produced on a Macintosh with the Voyager II program, and are ©1988-93 Carina Software, 830 Williams St., San Leandro, CA 94577, (510) 352-7328. Used under license. ©1996-2001 Scott R. Anderson Last Update: 2001 June 18 Please send questions, comments, suggestions, or corrections to srca@mindspring.com. The material on this website may be reused as described under the Open Course License. The Gateway to Educational Materials (GEM) is the key to one-stop, any-stop access to thousands of high quality lesson plans, curriculum units and other education resources on the Internet! GEM is a project of the U.S. Department of Education. The Introduction to Astronomy Webbook is catalogued in the Gateway, and Scott R. Anderson is a member of the GEM Consortium.

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About Climate, Greenhouse Effect and Global Warming

The greenhouse effect is the primary determinant of Earth’s average temperature and therefore of climate. But there are lesser, but still significant factors;

The Sun’s brightness varies enough to effect climate.

The Sun’s 11 and 22 year sunspot cycle is well known. Sunspots have been observed for thousands of years. Apparently there were no sunspots between 1645 and 1715, a cool period known as the Maunder Minimum. It is known that the Sun emits slightly less energy when there are no sunspots.

The Earth’s albedo can change and effect climate.

Definition of Albedo Albedo is the fraction of light (or heat) that is reflected by a body or surface. A white body has high albedo, a black body has low albedo.

Albedo is a measure of “whiteness” or reflectivity. Albedo, albino and albumen derive from the same Latin word, albus, meaning white. Albedo values range from 0 to 1. Virgin snow has an albedo of 0.95, a lump of coal has an albedo of 0.1. At 0.84, the planet Venus shines so brightly because it is as white as snow. At 0.113, the moon’s albedo is similar to a lump of coal. The Moon would be much brighter if it were covered with snow.

Agricultural land has a higher albedo than the forest that existed in the past. Ice covered water has much higher albedo than open water. Soot from coal burning and diesel engines has lowered the albedo of the whole Earth.

Aerosols effect climate.

Natural examples of aerosols are pollen and salt crystals from windblown seawater. Soot and sulfates from coal burning and diesel engines are anthropogenic examples of aerosols. Soot can cause heating when it lands on snow. Aerosols block some sunlight and thus cause surface cooling. Sulfate aerosols are highly reflective which leads to cooling. Aerosols can have a strong effect on both heating and cooling.

Aerosols also effect cloud formation.

Climate and weather are chaotic.

Weather prediction beyond a few days very difficult. Meteorologists say that weather is subject to the “butterfly effect.” What they mean is that small things seem to have large effects.

This snapshot of the behavior of a double pendulum illustrates chaotic behavior. The plot would look very different if the initial conditions were only slightly different. An explanation and an animated version of this plot may be found here . Real weather is much more complicated than a double pendulum.

Climate exhibits metastability.

Stable State 1 (drought)	Tipping Point	Stable State 2 (wet period)

Climate may seem stable but a seemingly small event can tip climate to a new regime. A wet period lasting many years might be followed by a dry period also lasting many years. The transition between the two states may be sudden. There are many examples of metastability. The El Niño Southern Oscillation is one; the monsoon cycle of India is another.

Climate exhibits negative and positive feedback

Here is an example of negative feedback. Carbon dioxide concentration increases and stimulates plant growth enough to remove some carbon dioxide from the atmosphere. Negative feedback reduces variations in either direction.

Here is an example of positive feedback. The ice covering the Arctic Ocean melts a little during the summer and refreezes during the winter. An unusually warm summer results in more ice free water. This decreases the albedo and increases the amount of heat absorbed by the water. This increases melting. The effect increases next year because the winter ice is thinner and melts sooner next summer. Positive feedback amplifies variations in either direction.

Here is a hypothetical example of extreme positive feedback also called “runaway.” Large areas of northern Eurasia and northern Canada are covered by permafrost. Large amounts of organic matter are buried in the permafrost. This organic matter would release large quantities of the greenhouse gasses methane and carbon dioxide if the permafrost thawed. This would initiate a “positive feedback loop” that would go something like this:

1) Some permafrost melts.
2) Bacterial action releases methane and carbon dioxide.
3) Some methane is lost in upper atmosphere.
4) Some carbon dioxide is taken up by plants.
5) But the warmer ocean absorbs less carbon dioxide.
6) Enough methane and carbon dioxide remains to cause more global warming.
7) Go to step 1.

Ocean currents profoundly effect climate.

The Gulf Stream carries warm water from the tropics to the North Atlantic. The climate of Europe, Scandinavia and Iceland would be much colder without the Gulf Stream. This fact is the “scientific” basis to the movie, The Day After Tomorrow. Yet the Gulf Stream is only a segment of the Great Ocean Conveyor Belt. It is known that in the past the Great Ocean Conveyor Belt has taken different routes and has even stopped.

Volcanic gas and dust effects climate. 

The Icelandic volcano Laki exploded in 1783. The following winter was the coldest ever recorded in the United States–about 4.8°C below a 225 year average. Europe also experienced an abnormally severe winter. Benjamin Franklin correctly guessed that gas and dust from Laki were responsible.

The Indonesian volcano Tamboro exploded in 1815 and launched 150 cubic kilometers of material into the atmosphere. A similar thing happened in 1883 when the volcano Krakatoa exploded. Both years were known as “years without summers.” Mount Pinatubo in the Philippines blew its top in 1991. The gas and dust lowered the average global temperature by about 0.5°C. The effects of volcanic events last only a few years.

The largest volcanic event of the past million years occurred 73,000 years ago. That is when another Indonesian volcano known as Toba erupted with enough force to send more than 2500 cubic kilometers of volcanic material into the atmosphere.

The gas emitted by volcanoes is mainly sulfur dioxide, which accounts for most of the global cooling associated with volcanoes. Volcanic dust plays a smaller role. Coal burning also produces sulfur dioxide and dust. The study of volcanoes provides insights into the impact of coal burning.

Sulfur dioxide combines with water to produce small drops of sulfuric acid. These small drops produce a haze, also called an aerosol, which has a high albedo. The aerosol reflects sunlight, resulting in cooling. Sulfur dioxide also produces acid rain.

Astronomy 

The Earth spins on an axis which is pointed at the North Star. The Earth moves around the Sun in a nearly circular path that lies in the plane of the ecliptic. The Earth’s spin axis is tilted by 23.5 degrees. This is what causes seasons. The Northern Hemisphere tilted toward the Sun during Summer. The Northern Hemisphere is tilted away from the Sun in Winter. The effect is very pronounced near the poles.

Imagine what it would be like if the Earth’s spin axis were tilted 90 degrees. The Earth’s seasons would become violent. At the North Pole, the Sun would be almost directly overhead for months and it would get hot enough to boil the ocean. At the South Pole, there would be no Sun and the oceans would freeze. The poles would switch places 6 months later. The huge temperature differences would cause storms dwarfing anything on today’s Earth.

The Earth’s distance to the Sun varies over the course of a year because the Earth’s orbit is not circular but elliptical. Earth’s orbit is said to be eccentric. The Earth receives more energy on January 2 when the Earth is closest to the Sun and less energy 6 months later when the Earth is furthest from the Sun. The difference is 7.0%. The effect is to moderate Northern Hemisphere seasons and to accentuate Southern Hemisphere seasons.

The tilt of Earth’s axis and the eccentricity of Earth’s orbit around the Sun have varied in the past. These variations are called Milankovitch cycles and they can be accurately calculated. For example, in 10,500 years the Earth will be furthest from the Sun on January 2. Milankovitch cycles are not well understood, but it is clear they have profound effects on the Earth’s climate.

Heat storage

The Earth’s surface varies markedly in the ability to store heat. A desert is very cold at night and very hot during the day because only a thin layer of sand is available to store heat. A deep ocean does not have large daily temperature changes because a deep ocean stores a lot of heat. Places near oceans have little seasonal temperature changes. Places with large seasonal temperature changes are usually very far from oceans.

Some parts of the ocean circulate very slowly which means that some parts of the ocean responds slowly to global warming.

Global air circulation 

Low altitude air movements carry heat from one place to another. The Santa Anna winds are an example of this. People in Southern California usually enjoy a pleasant climate because of the westerly winds that blow across the Pacific Ocean. But once in a while the wind blows from the East bringing very hot and dry air to the region. Low altitude air movements can also carry water from one place to another.

High altitude air movements are usually called jet streams. They occur at altitudes that jet aircraft fly. Jet streams strongly influence the movements of weather systems.

Conclusion

It must be obvious by now. Weather prediction and climate prediction is complicated. The Earth is a complex thing.

source:http://www.planetforlife.com

Weather ,Climate and Global Warming

The current temperature, wind speed and direction, precipitation, humidity, and cloud cover constitutes a weather report. The study of weather is meteorology. Weather as it varies over the course of years constitutes climate. The study of climate is called climatology. The study of climate over thousands of years is called paleoclimatology.

The greenhouse effect

The greenhouse effect is the primary determinant of Earth’s average temperature and therefore of climate. An understanding of climate depends on an understanding of the greenhouse effect. [1]

Temperature Conversion -18°C = 0° Farhrenheit 15°C = 59° Farhrenheit

If an astronaut digs a hole on the Moon and measures the temperature at the bottom of the hole, the reading will be -18°C. During the day (which lasts 14 earth days) the surface will be much hotter and at night it will be much colder. The temperature in the hole reflects the average temperature. It is the distance to the sun that determines the average temperature. Any object in the vicinity of the earth will have an average temperature of -18°C. The object might be black or white or big or small. Only the distance to the sun matters. Any object in space gets very cold unless warmed by the sun.

The significance of the greenhouse effect

The average temperature of the Earth is not a cold -18°C. It is a pleasant 15°C. The Earth is 33°C warmer because of the greenhouse effect. Without the greenhouse effect, Earth would be an ice world and there could be no life. It is important to understand the greenhouse effect and to do that you need to know some physics.

Electromagnetic radiation

Electromagnetic radiation is ubiquitous and part of every day life. Rainbows, radio waves, and x-rays may seem very different, but they are not. They differ only in wavelength. They are all completely described by Maxwell’s equations. The elegance and terseness of the equations are apparent, but the mathematics is subtle and advanced.

The wavelength of electromagnetic radiation can vary enormously. Gamma rays have wavelengths smaller than the width of an atom. The radio waves used to communicate with submarines are many kilometers long.

Wavelength	Common Name	Uses	Atmosphere
1 to 10 nanometers	gamma rays	cancer therapy	opaque
10 to 100 nanometers	X-rays	cancer therapy	opaque
100 to 1000 nanometers	Sunlight
100 to 400 nanometers	Ultraviolet	suntans, sunburns	partly opaque due to ozone
400 to 700 nanometers	Visible light		mostly transparent
1 to 10 microns	Near infrared	night vision goggles	partly opaque due to GHG
10 to 100 microns	Far infrared		opaque
100 to 1000 microns			opaque
1 to 10 millimeters			opaque
1 to 10 centimeters	microwaves	radar, microwave ovens	mostly transparent
10 to 100 centimeters	microwaves	cell phones	transparent
1 to 10 meters	ultra high frequency (UHF)	television,	transparent
10 to 100 meters	very high frequency (VHF)	FM radio, television	transparent
100 to 1000 meters	short wave	AM radio, long distance radio	transparent
1 to 10 kilometers	long wave	“atomic” clocks	transparent
10 to 100 kilometers	extra long wave (ELF)	submarine communication	transparent

Examine the above table and especially note the following:

Human eyes can sense only a tiny fraction of the electromagnetic spectrum. (Some insects can see ultraviolet. Some snakes have a special organ that can “see” infrared.)
Wavelengths shorter than 400 nanometers can break chemical bonds and kill cells. (This is good for cancer therapy, killing pathogens, and preserving food. It is bad when it causes sunburns and skin cancer.)
The atmosphere is partly opaque to ultraviolet and infrared while it is transparent to visible light. (However, clouds, fog and haze block visible light at times.)

Blackbody radiation

Everyone is familiar with blackbody radiation although they would not use these words. Everything emits blackbody radiation unless its temperature is at absolute zero. The intensity and wavelength of the radiation depends on temperature. Hold your hand in front of your face and notice that you can sense the warmth of your hand. That is because your hand emits blackbody radiation. You can’t see it because the wavelength is too long for human eyes. Look at the heating element in the toaster the next time you make toast. It’s hotter and so you can see it radiating at 600 nanometers which is another way of saying it glows orange. The sun is hotter still; it is white hot. A major part of the blackbody radiation of the sun is visible light.

Greenhouse gasses (GHGs)

All gasses found in the atmosphere are transparent to visible light. Some gasses found in the atmosphere are opaque to certain wavelengths of infrared. These are the gasses that cause the greenhouse effect. The most important naturally occurring greenhouse gasses are water vapor, carbon dioxide and methane. Water vapor is sometimes not considered to be a GHG because water vapor can lead to heating and cooling. Condensed water vapor in the form of clouds reflect sunlight from their topsides. The reflected light is energy lost to space and therefore does not warm the Earth. The Sun’s energy does not heat the cloud and the Earth below is shaded. The net result of water vapor can be cooling.

The atmosphere also contains anthropogenic (human made) greenhouse gasses. All refrigerants and solvents made from fluorine and chlorine are potent greenhouse gasses. Almost all refrigerators and air conditioners made since 1928 use freon, the trademarked name of certain compounds of fluorine, chlorine, and carbon. One kind of freon, known as freon 12, has been outlawed. Scientists discovered freon 12 was destroying the ozone layer. Freon 12 has high chemical stability which means that it persists in the atmosphere for many years. Yet chemical stability is what makes freon 12 desirable as a refrigerant.

These compounds have a collective name. It is CFC (chloro-fluoro-carbon.) No one in 1928 had any idea that CFCs posed a threat to the entire Earth. The truth is that they are a threat in two unrelated ways. Certain CFCs destroy ozone in the upper atmosphere. All CFCs are potent GHGs. It is certainly fortunate that these dangers were recognized in time.

Another example of an anthropogenic GHG is sulfur hexafluoride. It is widely used in electrical equipment and metallurgy. Its global warming potential is 23,900 times more than carbon dioxide–more than any other gas. Any sulfur hexafluoride that escapes becomes a near permanent part of the atmosphere as it will persist for thousands of years. The amount of sulfur hexafluoride in the atmosphere is not contributing significantly to global warming at this time. But its continued use should be considered carefully.

An explanation of the greenhouse effect

Greenhouse effect misnomer A greenhouse gets hot when the sun shines but the reason has little to do with the greenhouse effect! The greenhouse gets hot because wind and convection currents are not allowed to carry away the hot air surrounding the sun warmed things in the greenhouse.

Enough physics has been presented to allow an explanation of the greenhouse effect.

Visible light from the sun travels through Earth’s atmosphere. The energy in the visible light is converted to heat on the surface of the Earth. The same energy is emitted from the surface of the Earth as black body infrared radiation. The incoming and outgoing energy balances when the Earth is at -18°C. However, some of the outgoing energy is reflected back to the Earth by greenhouse gasses in the atmosphere. This increases the balance point to 15°C.

Earth

Visible light (yellow arrows) from the sun warms the earth during the day. The warmed earth emits infrared (red arrows) in all directions. Greenhouse gasses in the atmosphere (blue) reflect part of the infrared back toward the Earth, resulting in global warming.

Sun

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