Astronomy

Photographic Earth



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Welcome to Planet Earth, the third planet from a star named the Sun.

[Because of the limitations of Helium, this article links to images of the astronomical wonders of our universe. Please click on the links to feast your eyes on the images being described.]

The Earth is shaped like a slightly squashed sphere and is composed mostly of rock. Over 70 percent of the Earth's surface is water. The planet has a relatively thin atmosphere composed mostly of nitrogen and oxygen. The above-linked picture of Earth, dubbed Blue Marble, was taken from Apollo 17 in 1972 and features Africa and Antarctica. Earth has a single large moon that is about 1/4 of its diameter and, from the planet's surface, is seen to have almost exactly the same angular size as the Sun. With its abundance of liquid water and moderate temperatures, Earth supports a large variety of life forms, including such potentially intelligent species as dolphins and humans.

The linked photograph shows what the Earth looks like at night. Can you find your favorite country or city? Surprisingly, city lights make this task quite possible. Human-made lights highlight particularly developed or populated areas of the Earth's surface, including the seaboards of Europe, the eastern United States, and Japan. Many large cities are located near rivers or oceans so that they can exchange goods cheaply by boat. Particularly dark areas include the central parts of South America, Africa, Asia, and Australia. This image is actually a composite of hundreds of pictures made by the Defense Meteorological Program Satellites.

Earthrise can be seen from the right locations on the moon, owing to Lunar limb libration. It can also be seen from circumlunar space. In December of 1968, the Apollo 8 crew flew from the Earth to the moon. As the Apollo 8 command module rounded the farside of the moon, the crew could look toward the lunar horizon and see the Earth appear to rise, due to their spacecraft's orbital motion. The famous picture that resulted, of a distant blue Earth above the moon's limb, was a marvelous gift to the world.

Higher than the highest mountain, higher than the highest airplane, lies the realm of the aurora. Aurora rarely reach below 60 kilometers, and can appear at altitudes as large as 1000 kilometers. Auroral light results from solar wind and eruptions which cause energetic electrons and protons to strike molecules in the Earth's atmosphere. This link takes you to a spectacular aurora borealis which formed above the frozen landscape of Bear Lake, Alaska, USA in January 2005.

Auroras can also be seen from space. The International Space Station (ISS) orbits at nearly the same height as many auroras, sometimes passing over them, and sometimes right through them. In 2003, ISS Science Officer Don Pettit captured the green aurora, pictured here in a digitally sharpened image. From orbit, Pettit reported that the auroral light appeared to crawl around like giant green amoebas. Over 300 kilometers below, the Manicouagan Impact Crater can be seen in northern Canada, planet Earth.

Manicouagan Crater in northern Canada is one of the oldest impact craters known. Formed about 210 million years ago during the Triassic period by the impact of an asteroid roughly 5 kilometers in diameter, the present day terrain supports a 70-kilometer diameter hydroelectric reservoir in the telltale form of an annular lake. By means of comparison, the Chicxulub crater on the Yucatan peninsula, which is often associated with the extinction of the dinosaurs, is thought to have been caused by the impact of an asteroid roughly twice this size. The crater itself has been worn away by the passing of glaciers and other erosional processes. Still, the hard rock at the impact site has preserved much of the complex impact structure and so aids scientists in understanding large impact features on Earth and other Solar System bodies.

Earth supports a wide variety of atmospheric weather phenomena. A recent example was the Category 5 Hurricane Katrina. A Category 5 Hurricane is perhaps the most powerful storm on Earth, with sustained winds greater than 250 kilometers per hour. Pictured in the link is a digitally processed image from the orbiting GOES-12 weather satellite that shows the massive storm system in the Gulf of Mexico. Starting as a slight pressure difference, hurricanes grow into large spiraling storm systems of low pressure, complete with high winds and driving rain. A hurricane is powered by evaporating ocean water, and so typically gains strength over warm water and loses strength over land. Much remains unknown about hurricanes and cyclones, including how they are formed and the exact path they will take.

The first person to note that the Aleutian Cleveland Volcano was spewing ash was astronaut Jeffrey N. Williams aboard the International Space Station. Looking down on the Alaskan Aleutian Islands, Williams noted, photographed, and reported a spectacular ash plume emanating from the Cleveland Volcano. Starting just before the linked image was taken, the Cleveland Volcano underwent a short eruption lasting only about two hours. The Cleveland stratovolcano is one of the most active in the Aleutian Island chain. The volcano is fueled by magma displaced by the subduction of the northwest-moving tectonic Pacific Plate under the tectonic North America Plate.

In Iceland in 1991, the volcano Hekla erupted at the same time that auroras were visible overhead. Hekla, one of the most famous volcanoes in the world, has erupted at least 20 times over the past millennium, sometimes causing great destruction. The last eruption occurred only six years ago but caused only minor damage. The green auroral band seen in the linked photograph occurred fortuitously about 100 kilometers above the erupting lava. Is Earth the Solar System's only planet with both auroras and volcanoes?

The Antikythera Calculator is an enigmatic device found at the bottom of the sea aboard an ancient Greek ship. Its seeming complexity has prompted decades of study, although many of its functions remained unknown. Recent X-rays of the device show that the mechanism is a mechanical computer of an accuracy thought impossible in 80 BC, when the ship that carried it sunk. Its wheels and gears create a portable orrery of the sky that predicted star and planet locations as well as lunar and solar eclipses. The Antikythera mechanism, shown above, is 33 centimeters high and similar in size to a large book.

At the summer solstice, the Sun reaches its northernmost point in Earth's sky. The date traditionally marks a change of seasons - from spring to summer in Earth's Northern Hemisphere. The linked photo shows the rising sun over Stonehenge at the summer solstice in 2005. Thousands of people gathered at sunrise to see the sun rise through the 4,000 year old solar monument. The celestial alignment of Stonehenge results in the sun rising at the point on the horizon defined by its pillars.

Like a modern version of Stonehenge, twice each year streets in Manhattan will flood dramatically with sunlight just as the Sun sets. Usually, the buildings of New York City's tallest borough hide the setting Sun. The linked picture shows the Sun setting down 34th Street as viewed from Park Avenue, as happens in late May and mid July.

Twice a year, at the Spring and Fall equinox, the Sun rises due east. The linked photograph records the Sun rising exactly aligned with the Western Canal, in Tempe, Arizona. The cultural significance of this alignment is not known.

This mosaic photo caught the Moon in several stages as it moved between the Earth and the Sun. During the center frame, a total solar eclipse is visible, when the Moon completely blocks the Sun. In the darkened skies, the magnificent corona of the Sun becomes visible.

Most photographs don't adequately portray the magnificence of the Sun's corona. The human eye can adapt to see features and extent that photographic film usually cannot. As a result, seeing the corona first-hand during a total solar eclipse is best. However, the linked picture is a combination of twenty-two photographs that were digitally processed to highlight faint features of a total eclipse that occurred in August of 1999. It is a fairly representative image of what an observer with binoculars would see at a total solar eclipse.

As a total solar eclipse is caused by the shadow cast on the Earth by the moon, what would the surface of the Earth look like during a total solar eclipse? The linked photograph shows the Earth during a 1999 solar eclipse. The shadow of the moon can be seen darkening part of Earth. The shadow moves across the Earth at nearly 2000 kilometers per hour. Only observers near the center of the dark circle see a total solar eclipse - others see a partial eclipse where only part of the Sun appears blocked by the moon.

Future articles in this series will illustrate the wonders of the rest of the Solar System, our Milky Way galaxy, and the Universe itself.




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