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Saturday, April 19, 2014

The Blue Planet

Posted by jlwile on August 21, 2013

The blue dot pointed out in this picture is the earth as seen from Saturn.
(Credit: NASA/JPL-Caltech/Space Science Institute) Click for a full-sized version.

Astronomers call earth “the blue planet,” because when you look at it from space, it often appears blue. That’s because most of the earth is covered with water, which reflects blue light better than the other colors of light. So when white light from the sun hits the earth, more blue light is reflected than any other color (as long as there isn’t an enormous amount of cloud cover).

The Cassini space probe that is currently in orbit around Saturn recently had a chance to photograph the earth. It appears as the tiny blue dot pointed out in the photo above. Please click on the photo to get the full-sized version. It really is magnificent. At the time the photo was taken, the earth was 898 million miles away from the Cassini space probe. Nevertheless, it appears as a vivid blue dot on a mostly dark background. In addition, if you “zoom” in close enough, you can actually see the moon orbiting the earth:

The earth (left) and moon as seen from 898 million miles away.
(Credit: NASA/JPL-Caltech/Space Science Institute) Click for a full-sized version.

Notice the differences between the earth and the moon. The moon is smaller, but more strikingly, it appears a stark white next to earth’s blue.

The earth has been intricately designed as a haven for life. Its blue color is a beacon proclaiming that, and the beacon can be seen from nearly 900 million miles away!


9 Responses to “The Blue Planet”
  1. Luke Lorenz says:

    That beacon even be seen from 900 million miles away! ;)

  2. Eduardo says:

    When I see this photo for the first time I remembered this Psalm: “When I see Your heavens, the work of Your fingers, the moon and stars that You have established,what is man that You should remember him, and the son of man that You should be mindful of him?, Yet You have made him slightly less than the angels, and You have crowned him with glory and majesty.”

  3. jlwile says:

    Thanks for the correction, Luke!

  4. Plato says:

    I have an off topic question: If galaxies and star are millions of light years away and the universe is only about ten thousand year old why can we see the stars and galaxies?

  5. jlwile says:

    Thanks for your question, Plato. It has an assumption in it that isn’t valid. Your question assumes that it takes a million earth years for light to travel a million light years across space. That’s just not true. Time passes differently depending on where you are in the universe. For example, time passes more quickly for the GPS satellites than it does here on the surface of the earth, due to the fact that they are exposed to a weaker gravitational field. If the GPS didn’t take this fact into account, it would not work. In the end, then, for light to take a million earth years to travel a million light years, the gravitational fields along the way would have to stay equal to the gravitational field at the surface of the earth, and we know that’s not true. Thus, a simple relationship between light travel time and distance doesn’t work on the scale of the universe.

    As a result, one has to rely on some assumptions about how the universe has expanded over time. If you assume the universe expanded the same in all directions with no geometry at all and you further assume that on a large enough scale, mass is spread roughly evenly throughout the universe (which is almost certainly not true), then you can say that it takes about a million earth years for light to travel a million light years across the universe. However, if you assume a spherical expansion (like the expansion we see in most expanding systems), you find something radically different. With that kind of expansion, time passes very slowly near the center of the expansion and very quickly near the edge. If earth is near the center of the expansion, earth years pass very slowly, while years farther out in the universe pass very quickly. As a result, light originating from the far reaches of the universe could travel billions of light years in just a few earth years.

    Creation Ministries International has a detailed discussion of this in a PDF file.

  6. plato says:

    Thank you, the PDF.file proved very informative,but how much faster does time progress outside a gravitational field?

  7. jlwile says:

    Plato, it depends a bit on how you do the surface integrals in the equations of general relativity. However, a commonly used metric yields the following equation for the increase in the rate at which time ticks:

    To/Tf = Square root[1 - 2GM/rc^2]

    Where “To” would be the time experienced at the surface of an object with mass “M,” “Tf” would be the time experienced by something far from the object, “G” is the gravitational constant, “c” is the speed of light, and “r” is the distance from the object. If you are considering only one object, then time can speed up by orders of magnitude, as long as “r” is large enough.

    If you are asking whether or not this effect is large enough to allow a thousands-of-years-old earth to see light from billions of light years away, the answer is yes. Two books discuss the details. The older one is Starlight and Time by Dr. Russell Humphreys, and the newer one is Starlight, Time and the The New Physics by Dr. John Hartnett.

  8. plato says:

    Thank you for the explanation and reading suggestions, I’ll look the up at the library.

  9. jlwile says:

    My pleasure, Plato. Thanks for asking the question!

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