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Monthly Archives: July 2009

Due to atmospheric refraction, the Earth's terminator is a hazy line

Due to atmospheric refraction, the Earth's terminator is a hazy line

No sudden, sharp boundary marks the passage of day into night on planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness that we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet’s atmosphere. A clear high altitude layer, visible along the dayside’s upper edge, scatters blue sunlight and fades into the blackness of space. This picture actually is a single digital photograph taken in June of 2001 from the International Space Station, orbiting at an altitude of 211 nautical miles.

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Seen from 6 billion kilometres (3.7 billion miles), Earth appears as a tiny dot.

Seen from 6 billion kilometres (3.7 billion miles), Earth appears as a tiny dot.

“Pale Blue Dot” is a photograph of planet Earth taken in 1990 by the Voyager 1 space probe from a record distance (6 billion km), showing it against the vastness of space. Both the idea for taking the distant photo and the title came from scientist and astronomer Carl Sagan, who also wrote the 1994 book of the same name.

In a commencement address delivered May 11, 1996, Sagan related his thoughts on the deeper meaning of the photograph:

Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.

Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.

"Earthrise", taken on December 24, 1968.

"Earthrise", taken on December 24, 1968.

“Earthrise” is the name given to NASA image AS8-14-2383, taken by astronaut William Anders during the historic Apollo 8 mission, the first manned voyage to orbit the Moon. The photograph was taken from lunar orbit on December 24, 1968 with a Hasselblad camera.

The famous "Blue Marble" photograph

The famous "Blue Marble" photograph

The famous “Blue Marble” shot represents the first photograph in which Earth is in full view. The picture was taken on December 7, 1972, as the Apollo 17 crew left Earth’s orbit for the moon.