Download The

The
Earth
our study of objects in the Solar System with the Earth because it
is interesting in its own right, and it provides a test of many
observing techniques that we wish to use for other objects in the
Solar System.
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The Earth is, at least by human standards, a beautiful planet, as the following
images indicate.
Apollo 11 shot of
Earth (Ref)
Galileo shot of the Earth
and Moon
Galileo image of S.
America (Ref)
Here is an interactive viewer that displays either a map of the Earth showing the
day and night regions at this moment, or a view of the Earth from the Sun, the
Moon, the night side of the Earth, above any location on the planet specified by
latitude, longitude and altitude, from a satellite in Earth orbit, or above various
cities around the globe (Credit: John Walker). Here is the NASA Earth from
Space image gallery, where you can find many images of the Earth taken from
space.
The Earth is the third planet from the Sun in our Solar System. It is the
planet we evolved on and the only planet in our Solar System that is known
to support life.
SIZE
The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the
fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and
Neptune).
Eratosthenes (276-194 BC) was a Greek scholar who was the first person to
determine the circumference of the Earth. He compared the midsummer's
noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and
Alexandria. He properly assumed that the Sun's rays are virtually parallel
(since the Sun is so far away). Knowing the distance between the two
locations, he calculated the circumference of the Earth to be 250,000 stadia.
Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he
was very close. He also accurately measured the tilt of the Earth's axis and
the distance to the sun and moon.
THE MOON
The Earth has one moon. The diameter of the
moon is about one quarter of the diameter of the
Earth.
The moon may have once been a part of the
Earth; it may have been broken off the Earth
during a catastrophic collision of a huge body
with the Earth billions of years ago.
The Earth and the moon.
Photo taken by NASA's
Galileo mission in 1990.
MASS, DENSITY, AND ESCAPE VELOCITY
The Earth's mass is about 5.98 x 1024 kg.
The Earth has an average density of 5520 kg/m3 (water has a density of 1027
kg/m3). Earth is the densest planet in our Solar System.
To escape the Earth's gravitational pull, an object must reach a velocity of
24,840 miles per hour (11,180 m/sec).
LENGTH OF A DAY AND YEAR ON EARTH
Each day on Earth takes 23.93 hours (that is, it
takes the Earth 23.93 hours to rotate around its
axis once - this is a sidereal day). Each year on
Earth takes 365.26 Earth days (that is, it takes
the Earth 365.26 days to orbit the Sun once).
The Earth's rotation is slowing down very
slightly over time, about one second every 10
Earth rising over the moon.
Photo taken by NASA's
Apollo 8 mission.
years.
THE EARTH'S ORBIT
The Earth orbits, on average, 93 million miles
(149,600,000 km) from the Sun. This distance is
defined as one Astronomical Unit (AU). The Earth is
closest to the Sun (this is called perihelion) around
January 2 each year (91.4 million miles = 147.1
million km); it is farthest away from the Sun (this is
called aphelion) around July 2 each year (94.8 million
miles = 152.6 million km).
Planet-Sun
Orbital
Diagram
Label the aphelion
(farthest point in
orbit) and perihelion
(closest point in
orbit) of a planet in
orbit.
Answers
Orbital Eccentricity
The Earth' orbital eccentricity is 0.017; it has an orbit that is close to being
circular.
THE EARTH'S AXIS TILT AND THE SEASONS
The Earth's axis is tilted from perpendicular to the plane of the ecliptic by
23.45°. This tilting is what gives us the four seasons of the year:
Summer, Spring, Winter and Autumn. Since the axis is tilted, different
parts of the globe are oriented towards the Sun at different times of the
year. This affects the amount of sunlight each receives. For more
information on the seasons, click here.
SPEED
At the equator, the Earth's surface moves 40,000 kilometers in 24 hours.
That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by
dividing the circumference of the Earth at the equator (about 24,900 miles or
40,070 km) by the number of hours in a day (24). As you move toward
either pole, this speed decreases to almost zero (since the circumference at
the extreme latitudes approaches zero).
The Earth revolves around the Sun at a speed of about 30 km/sec. This
compares with the Earth's rotational speed of approximately 0.5 km/sec (at
middle latitudes - near the equator).
For more information on the speed of
the Earth, click here.
TEMPERATURE ON EARTH
The temperature on Earth ranges
from between -127°F to 136°F (88°C to 58°C; 185 K to 311 K). The
coldest recorded temperature was on
the continent of Antarctica (Vostok
in July, 1983). The hottest recorded
temperature was on the continent of
Africa (Libya in September, 1922).
The size of the atmosphere in this
The greenhouse effect traps heat in illustration is greatly exaggerated in order
our atmosphere. The atmosphere lets to show the greenhouse effect. The Earth's
some infrared radiation escape into atmosphere is about 300 miles (480 km)
space; some is reflected back to the thick, but most of the Earth's atmosphere
planet. is within 10 miles (16 km) of the Earth's
surface.
For more information on the greenhouse effect, click here.
ATMOSPHERE
The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is
composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon
dioxide, and trace amounts of other gases.
The atmosphere was formed by planetary degassing, a process in which
gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were
released from the interior of the Earth from volcanoes and other processes.
Life forms on Earth have modified the composition of the atmosphere since
their evolution.
For more information on the atmosphere, click here.
Earth Activities


Earth coloring page
 The Earth and the Moon coloring page
 Find It!, a quiz on the Earth.
 Label the Planet-Sun Orbital Diagram. Answers.
Continents Printout: Label the continents, the poles, and the equator.
 Ocean Map: Label the Oceans
 Interactive Earth Puzzle
 Make a globe from papier-mâché
 A simple-to-make earth pendant
 Earth day crafts
 Earth Cloze Printout: A fill-in-the-blanks activity on Earth.
Answers
 How to write a report on a planet - plus a rubric.
Equator
The equator is an imaginary line on the Earth's surface equidistant from the North
Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a
Southern Hemisphere. The equators of other planets and astronomical bodies are
defined analogously
The latitude of the equator is, by definition, 0°. The length of Earth's equator is about
40,075.0 km, or 24,901.5 miles.
The equator is one of the five main circles of latitude that are based on the
relationship between the Earth's axis of rotation and the plane of the Earth's orbit
around the sun. It is the only line of latitude which is also a great circle. The
imaginary circle obtained when the Earth's equator is projected onto the heavens is
called the celestial equator.
The Sun, in its seasonal movement through the sky, passes directly over the equator
twice each year, on the March and September equinoxes. At the equator, the rays of
the sun are perpendicular to the surface of the earth on these dates.
Places on the equator experience the quickest rates of sunrise and sunset in the world.
Such places also have a constant 12 hours of day and night throughout the year, while
north or south of the equator day length increasingly varies with the seasons.
The Earth bulges slightly at the equator. It has an average diameter of 12,750 km, but
at the equator the diameter is approximately 43 km greater.
Locations near the equator are good sites for spaceports (e.g., Guiana Space Centre in
Kourou, French Guiana), as they are already moving faster than any other point on the
Earth due to the Earth's rotation, and the added velocity reduces the amount of fuel
needed to launch spacecraftTemperatures near the equator are high all year round
(except at altitude). In many tropical regions people identify two seasons: wet and
dry. However, most places close to the equator are wet throughout the year, and
seasons can vary depending on a variety of factors including elevation and proximity
to an ocean.
The surface of the Earth at the equator is mainly ocean. The highest point on the
equator is 4,690 m (15,387'), at 00°00′00″S, 77°59′31″W on the south slopes of
Volcán Cayambe (summit 5,790 m, 18,996') in Ecuador. This is a short distance
above the snow line, and is the only point on the equator where snow lies on the
ground.
Equatorial countries and territories
, these are:
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São Tomé and Príncipe – passing through Ilhéu
das Rolas, an islet in this archipelago
Gabon
Republic of the Congo
Democratic Republic of the Congo
Uganda – including some islets in Lake Victoria
Kenya
Somalia
Maldives – misses every island, passing between
Gaafu Dhaalu Atoll and Gnaviyani Atoll
Indonesia – crosses many islands, most notably
Sumatra, Borneo, Sulawesi, and Halmahera
Kiribati – misses every island, passing between
Aranuka and Nonouti Atolls in the Gilbert
Islands
Baker Island (unincorporated territory of the
United States) – passes through territorial
waters (NB the equator also passes through the
exclusive economic zones around Howland
Island and Jarvis Island, but not through their
territorial waters)
Ecuador (literal translation of its official name is
"Republic of the Equator") – including Isabela
Island in the Galápagos Islands
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Colombia
Brazil – including some islands in the mouth of
the Amazon River and passing through Macapá,
capital of Amapá state
Contrary to its name, no part of Equatorial Guinea's territory lies on the equator.
However, its island of Annobón is about 100 miles (200 km) south of the equator, and
the rest of the country lies to the north
Exact length of the equator
The equator is modeled exactly in two widely used standards as a circle of radius an
integer number of meters. In 1976 the IAU standardized this radius as 6,378,140 m,
subsequently refined by the IUGG to 6,378,137 m and adopted in WGS-84, though
the yet more recent IAU-2000 has retained the old IAU-1976 value. In either case the
length of the equator is by definition exactly 2π times the given standard, which to the
nearest millimeter is 40,075,016.686 m in WGS-84 and 40,075,035.535 m in IAU1976 and IAU-2000.
(Although millimeter precision can be important up to the scale of a mile, it has
negligible physical significance at the scale of a geographic feature such as the
equator. From a computational standpoint however millimeter precision or better can
be valuable for maintaining consistent results when used in programs for surveying
etc. As an overly simple example, if a program were to convert back and forth
between the radius and the circumference of the earth sufficiently often while
maintaining precision only to a meter each time, errors might accumulate until they
became noticeable.)
The geographical mile is defined as one arc minute of the equator, and therefore has
different values depending on which standard equator is used, namely 1855.3248 m or
1855.3257 m for respectively WGS-84 and IAU-2000, a difference of nearly a
millimeter.
The earth is standardly modeled as a sphere flattened about .336% along its axis. This
results in the equator being about .16% longer than a meridian (as a great circle
passing through the two poles). The IUGG standard meridian is to the nearest
millimeter 40,007,862.917 m, one arc minute of which is 1852.216 m, explaining the
SI standardization of the nautical mile as 1852 m, more than 3 meters short of the
geographical mile.
Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in
our galaxy.
diameter:
1,390,000 km.
mass:
1.989e30 kg
temperature: 5800 K (surface)
15,600,000 K (core)
The Sun is by far the largest object in the solar system. It contains more than 99.8%
of the total mass of the Solar System (Jupiter contains most of the rest).
It is often said that the Sun is an "ordinary" star. That's true in the sense that there
are many others similar to it. But there are many more smaller stars than larger ones;
the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably
less than half the mass of the Sun.
The Sun is personified in many mythologies: the Greeks called it Helios and the
Romans called it Sol.
The Sun is, at present, about 70% hydrogen and 28% helium by mass everything
else ("metals") amounts to less than 2%. This changes slowly over time as the Sun
converts hydrogen to helium in its core.
The outer layers of the Sun exhibit differential rotation: at the equator the surface
rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior
is due to the fact that the Sun is not a solid body like the Earth. Similar effects are
seen in the gas planets. The differential rotation extends considerably down into the
interior of the Sun but the core of the Sun rotates as a solid body.
Conditions at the Sun's core (approximately the inner 25% of its radius) are
extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion
atmospheres. At the center of the core the Sun's density is more than 150 times that of
water.
The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is
produced by nuclear fusion reactions. Each second about 700,000,000 tons of
hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons
(=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the
surface, the energy is continuously absorbed and re-emitted at lower and lower
temperatures so that by the time it reaches the surface, it is primarily visible light. For
the last 20% of the way to the surface the energy is carried more by convection than
by radiation.
The surface of the Sun, called the photosphere, is at a
temperature of about 5800 K. Sunspots are "cool" regions,
only 3800 K (they look dark only by comparison with the
surrounding regions). Sunspots can be very large, as much as
50,000 km in diameter. Sunspots are caused by complicated
and not very well understood interactions with the Sun's
magnetic field.
A small region known as the chromosphere lies above the photosphere.
The highly rarefied region above the chromosphere, called
the corona, extends millions of kilometers into space but is
visible only during a total solar eclipse (left). Temperatures in
the corona are over 1,000,000 K.
It just happens that the Moon and the Sun appear the same
size in the sky as viewed from the Earth. And since the Moon
orbits the Earth in approximately the same plane as the Earth's
orbit around the Sun sometimes the Moon comes directly
between the Earth and the Sun. This is called a solar eclipse; if the alignment is
slighly imperfect then the Moon covers only part of the Sun's disk and the event is
called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and
it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the
Earth but the region from which a total eclipse is visible, called the path of totality, is
very narrow, just a few kilometers (though it is usually thousands of kilometers long).
Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see
a partial eclipse several times per decade. But since the path of totality is so small it is
very unlikely that it will cross you home. So people often travel half way around the
world just to see a total solar eclipse. To stand in the shadow of the Moon is an
awesome experience. For a few precious minutes it gets dark in the middle of the day.
The stars come out. The animals and birds think it's time to sleep. And you can see the
solar corona. It is well worth a major journey.
The Sun's magnetic field is very strong (by terrestrial standards) and very
complicated. Its magnetosphere (also known as the heliosphere) extends well beyond
Pluto.
In addition to heat and light, the Sun also emits a low density
stream of charged particles (mostly electrons and protons) known as
the solar wind which propagates throughout the solar system at about
450 km/sec. The solar wind and the much higher energy particles
ejected by solar flares can have dramatic effects on the Earth ranging
from power line surges to radio interference to the beautiful aurora
borealis.
Recent data from the spacecraft Ulysses show that during the minimum of the solar
cycle the solar wind emanating from the polar regions flows at nearly double the rate,
750 kilometers per second, than it does at lower latitudes. The composition of the
solar wind also appears to differ in the polar regions. During the solar maximum,
however, the solar wind moves at an intermediate speed.
Further study of the solar wind will be done by the recently launched Wind, ACE
and SOHO spacecraft from the dynamically stable vantage point directly between the
Earth and the Sun about 1.6 million km from Earth.
The solar wind has large effects on the tails of comets and even has measurable
effects on the trajectories of spacecraft.
Spectacular loops and prominences are often visible on the Sun's
limb (left).
The Sun's output is not entirely constant. Nor is the amount of
sunspot activity. There was a period of very low sunspot activity in the latter half of
the 17th century called the Maunder Minimum. It coincides with an abnormally cold
period in northern Europe sometimes known as the Little Ice Age. Since the formation
of the solar system the Sun's output has increased by about 40%.
The Sun is about 4.5 billion years old. Since its birth it has used up about half of the
hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years
or so (although its luminosity will approximately double in that time). But eventually
it will run out of hydrogen fuel. It will then be forced into radical changes which,
though commonplace by stellar standards, will result in the total destruction of the
Earth (and probably the creation of a planetary nebula).