The Seasons

The Earth

In space, the Earth's axis is tilted.

The Earth rotates on its axis.
It rotates once every 24 hours or 1 day.
The Earth's rotation creates day and night.

The Earth revolves around the Sun.
It takes 1 year for it to revolve all the way around.
One year also equals 12 months or 365 days.

The tilt of the Earth causes the seasons.
As the Earth orbits the Sun,
the tilt of the Earth's axis does not change.
The Earth always leans in the same direction.

When the North Pole is titled toward the Sun
during Summer, the North Hemisphere gets direct light.
The air in the atmosphere then becomes hot.

When the North Pole is titled away from the Sun
during Winter, the North Hemisphere gets indirect light.
The air in the atmosphere then becomes cold.

The Autumnal Equinox arrives on September 22 or 23.
On the first day of Autumn,
the sun's light is directly over the Equator.
This causes us to have an equal number
of hours of light and darkness.

The Winter Solstice arrives on December 21 or 22.
The North Pole is tilting far away from the Sun.
The first day of Winter is the shortest day of the year.
Because the Sun is lower in the sky,
the days are shorter and colder.

The Vernal Equinox arrives on March 20 or 21.
On the first day of Spring,
the Sun's light is directly over the Equator.
There are an equal number
of hours of daylight and darkness.

The Summer Solstice arrives on June 20 or 21.
The North Pole is as close to the Sun as it will get.
The first day of Summer is the longest day of the year.
Because the Sun is higher in the sky,
the days are longer and hotter.

◘◘◘..EarTh r0tatEs ar0unD an !maginaRy axIs...◘◘◘

Axis of the Earth: An imaginary straight line running through the center of the earth from the North Pole to the South Pole. The earth is said to rotate (spin) on this axis, which is "tilted" in relation to the sun. This causes our seasons.
Daylength (also known as photoperiod): The length of time between sunrise and sunset.
Equinox: The word means "equal night." It refers to the two moments each year when the sun appears to cross the equator. The spring (vernal) equinox is around March 21 and the fall (autumnal) equinox is around September 23. Everywhere on earth has about 12 hours of daylight and 12 hours of night on the equinoxes. The sun doesn't really move, but it looks like it does to us. That's because our Earth is tilted as it revolves around the sun once a year.	Spring or fall equinox
Greenwich Mean Time, or GMT (also called Universal Time, or UT): An international time-keeping standard based on the local time in Greenwich, England (0 degrees longitude). GMT (UT) enables us to refer to things that happen at the same moment in different time zones. (It would be confusing to use "local time.") At any moment, GMT is the same everywhere on earth. (That's why it's also called universal time.) What time is it now? Check the Greenwich Mean Time clock.	This map shows how many hours you need to add to or subtract from your local time to get the GMT. (Click to enlarge.)
Hemisphere: A mapping division meaning half of the earth (sphere). First, picture cutting the earth in half at the equator (0 degrees latitude). This divides the earth into the Northern Hemisphere (north of the equator) and the Southern Hemisphere (south of the equator). These are labeled in yellow on the illustration. Next picture cutting the earth in half at the Prime Meridian (0 degrees longitude). This divides the earth into the Western Hemisphere (west of the Prime Meridian) and the Eastern Hemisphere (east of the Prime Meridian). These are labeled in red on the illustration.	Click to enlarge this image of the earth's hemispheres.
Latitude: Imaginary horizontal mapping lines on the Earth. They are known as "parallels" of latitude because they run parallel to the Equator. The number of degrees of latitude shows how far north or south of the Equator a specific location is.
Longitude: Imaginary vertical mapping lines on Earth known as "meridians" of longitude. The number of degrees of longitude shows how far east or west of the Prime Meridian a specific location is.
Meridian: An imaginary line that runs vertically, north and south, from the North Pole to the South Pole. All points on a meridian have the same longitude.
Photoperiod (also called daylength) The length of time between sunrise and sunset.
Prime Meridian: The Prime Meridian serves as the starting point for longitude measurement, so is indicated as 0 degrees longitude. The Prime Meridian passes directly over the British Royal Observatory in Greenwich, England. (See illustration, above.)
Revolution of the Earth: The yearly 365 1/4 days trip the earth takes around the sun. We experience different seasons as the tilted earth revolves around the sun.
Rotation of the Earth: The spinning or turning of the earth on its axis. The earth makes one complete rotation every twenty-four hours.	We see sunrise and sunset each day because the earth rotates.
Solstice: The word means "sun stop." It refers to the two moments each year when the sun appears to be farthest from the equator. On the winter solstice in our hemisphere (around December 22), the sun seems to reach its most southerly point. It is our shortest day of the year. From there, it seems to head north until it reaches its most northerly point. That is our summer solstice (around June 21): the longest day of the year. When the Northern Hemisphere has its summer solstice, the Southern Hemisphere has its winter solstice, and vice versa. The sun doesn't really move, but it looks like it does to us. That's because our Earth is tilted as it revolves around the sun once a year.	Winter Solstice in the Northern Hemisphere Summer Solstice in the Northern Hemisphere
Twenty-Four Hour Clock: Instead of keeping time on the basis of 12 a.m. hours and 12 p.m. hours, the 24-hour clock runs straight through. It begins at 00:00 (midnight) and runs to 23:59 (11:59 PM). This is also known as "military time."

♥♥♥ . . .GALAXY. . .♥♥♥

The Universe within 50000 Light Years
The Milky Way Galaxy

About the Map

This map shows the full extent of the Milky Way galaxy - a spiral galaxy of at least two hundred billion stars. Our Sun is buried deep within the Orion Arm about 26 000 light years from the centre. Towards the centre of the Galaxy the stars are packed together much closer than they are where we live. Notice also the presence of small globular clusters of stars which lie well outside the plane of the Galaxy, and notice too the presence of a nearby dwarf galaxy - the Sagittarius dwarf - which is slowly being swallowed up by our own galaxy.

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Additional Maps
	A Map of the Milky Way Here is another map of the Milky Way viewed from above. This page also explains what scientific data there is for the spiral structure of our galaxy.
	A Galactic Sky Chart This is an all-sky plot of the 9000 brightest stars, plotted in galactic coordinates, and showing all of the constellations in the sky.

Data and Catalogs
	Globular Clusters Large galaxies are surrounded by a halo of tight spherical clusters of stars known as globular clusters. There are roughly 150 known globular clusters around our galaxy, and here is a list of them.

The Sagittarius Dwarf Galaxy

This dwarf galaxy is the nearest galaxy to our own. However, it was only discovered as recently as 1994. It lies on the far side of the galaxy from us and is heavily obscured by the intervening gas, dust and stars. It is approximately 78000 light years away and about 10000 light years in diameter. It is orbiting our galaxy in a period of about 1 billion years but it cannot be expected to last much longer, in a few hundred million years it will be ripped apart by our own galaxy. It contains about one hundred million stars. It also lies in roughly the same position as the globular cluster M54 but whether this globular cluster is actually part of the dwarf galaxy is unclear.

Galactic Cannibalism

The Sagittarius Dwarf Galaxy will probably not be the first galaxy that has been 'eaten' by our galaxy. The Sloan Digital Sky Survey for instance report that outside of the Galaxy there are huge clumps of stars that appear to be the remains of smaller galaxies that were ripped apart by the Milky Way more than a billion years ago. The distribution of these stars shows at least two clumps that are several thousand light years in size and more than 100 000 light years from the center of the Galaxy.

The Galactic Plane

Above - An all-sky plot of the 25000 brightest, whitest stars (B-V<0)>

Below - An infra-red view towards the centre of our Galaxy from the Two-Micron All Sky Survey. Our view of the Milky Way is much better in infra-red light. Visible in this image are the huge clouds of dust which block our view of the Galaxy in visible light. The Sagittarius Dwarf galaxy is also very dimly visible in this picture extending downwards from the left side of the bulge.

The earth's layers.

Image courtesy of
Lawrence Liverm

In 1970 Russian geologists started drilling into the Kola Peninsula, near Finland, hoping to learn more about Earth’s enigmatic insides. After 22 years of digging, work had to stop when the crust turned gooey under the drill bit; at 356 degrees Fahrenheit, the underground rock was much hotter than expected at that depth. The result of the scientists’ grand effort: a tunnel as wide as a cantaloupe extending all of 7.6 miles down.

The Kola borehole is by far the deepest one ever dug, yet it reaches a mere 0.2 percent of the way to the core. The rest of Earth’s interior remains as frustratingly out of reach as it was three centuries ago, when astronomer Edmond Halley suggested that our planet was hollow and filled with life. His ideas seem laughable today, but the truth is, when it comes to the inner Earth, no one knows anything for sure. Might a massive crystal sit at the center? What about a natural nuclear reactor? Are we so sure that the textbook diagram of the Earth sliced open, with nested layers of yellow, orange, and red, reflects reality?

The questions are so compelling that they inspired one geophysicist to draw up blueprints for a journey to the center of Earth. Nobody is doing it just yet; it would require cracking open the ground and pouring in thousands of tons of liquid metal. But that and other far-fetched ideas may inspire the ambitious projects necessary to catch a glimpse of the core—a place just 3,950 miles below our feet and yet, in many ways, less accessible than the edge of the visible universe, 13.8 billion light-years away.

ore National Labs

♥♥♥...layers of the lithosphere...♥♥♥

Another Diagram of the Inside Parts of Earth

If you could slice the Earth in half, you would see four layers: the crust, the mantle, the inner core, and the outer core. Each layer is made of different materials, has a different density, and has a different thickness. The Crust The crust is the top layer. Compared to the other layers, it is very thin. The crust varies from 5 to 70 kilometers in thickness. The crust includes rocks, minerals, and soil. There are two kinds of crust: continental and oceanic. The crust is made of many types of rocks and hundreds of minerals. These rocks and minerals are made from just 8 elements: Oxygen (46.6%), Silicon (27.72%), Aluminum (8.13%), Iron (5.00%), Calcium (3.63%), Sodium (2.83%), Potassium (2.70%) and Magnesium (2.09%). The oceanic crust has more Silicon, Oxygen, and Magnesium. The continental crust has more Silicon and Aluminum. The Mantle Directly below the crust is the mantle. The mantle makes up the largest volume of the Earth's interior. It is almost 2900 kilometers thick and comprises about 83 % of the Earth's volume. It has two parts, an upper layer and a lower layer. The crust and upper mantle form the brittle upper layers of the Earth's interior called the Lithosphere. The upper mantle is about 670 kilometers in depth. It is brittle and less dense. It is thought to be made of peridotite, a rock made from the minerals olivine and pyroxene. The rocks in the upper mantle are more rigid and brittle because of cooler temperatures and lower pressures. The Lower Mantle is much thicker and more dense. It is 670 to 2900 kilometers below the Earth's surface. This layer is hot and plastic. The higher pressure in this layer causes the formation of minerals that are different from those of the upper mantle. brittle - breaks easily plastic - can change shape without breaking The Outer and Inner Core The region beneath the mantle is called the core, and is made of two parts, a liquid outer core that is about 2250 km thick and a solid inner core which is 1220 km thick. The core is mostly made of iron, with a little bit of nickel. The outer core is at 1,800 - 3,200 miles (2,890-5,150 km) below the earth's surface. The temperature in the outer core is about 7200 - 9032 ºF (4000-5000ºC). The molten, liquid iron in the outer core is important because it helps create Earth's magnetic field. The inner core is 3,200 - 3,960 miles (5,150-6,370 km) below the earth's surface and mainly consists of iron, nickel and some lighter elements (probably sulphur, carbon, oxygen, silicon and potassium). The temperature in the inner core is about 9032 - 10832 ºF (5000-6000 ºC). Because of the high pressure, the inner core is solid. The outer core and the inner core together cause the earth's magnetism. Because the earth rotates, the molten outer core spins. The inner core does not spin because it's solid. This is what causes the earth's magnetism. Earth's magnetic north and magnetic south are NOT at the poles. Earth's "magnetic north" is in northern Canada and "magnetic south" is north of Antarctica and south of Australia. Another strange fact about Earth's magnetic poles is that they reverse every few million years. (North becomes south and south becomes north. This is called a "geomagnetic reversal." Scientists still do not fully understand why geomagnetic reversals happen.

Metamorphic Rocks

Pictures of Foliated and Non-Foliated Rock Types

Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are provided below.

Metamorphic Rock Types Menu
Amphibolite	Gneiss	Hornfels	Marble
Phyllite	Quartzite	Schist	Slate

Sedimentary Rocks

Picture Gallery of the Most Common Rock Types

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1) clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3) organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris. Pictures and brief descriptions of some common sedimentary rock types are shown below.

Sedimentary Rock Types Menu
Breccia	Chert	Coal	Conglomerate	Iron Ore
Limestone	Rock Salt	Sandstone	Shale	Siltstone