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About the Blue Marble


While it looks a lot like the photo taken by Apollo astronauts, you may be surprised to know that this popular image of Earth is not actually a snapshot. Robert Simmon, who worked on this for NASA's Earth Observatory, was kind enough to answer a few quick questions about this image from the Blue Marble project.

KH: What's different from the picture that the astronauts did take?

RS: Instead of being a snapshot, this picture is a composite: one month of images taken from a satellite were combined together into a single map.

KH: How did you create that image?

RS: I used a 3D program (Electric Image) to render out separate "layers" (surface, clouds, fake atmosphere, and a fake specular highlight (the shiny spot)). I then combined the separate layers in Photoshop, which gave me individual control over opacity, brightness, contrast, color, and saturation. Source imagery and the high-resolution final results are here: http://earthobservatory.nasa.gov/Newsroom/BlueMarble/BlueMarble_2002.html

KH: How much science goes on before you do your part?

RS: Lots! The raw data--collected as individual pixels from 440 miles above the Earth--need to be processed to account for everything from the curvature of the Earth to influence of the atmosphere and optical properties of the instrument. These are then stored in a data archive as thousands of individual images, called scenes. Literally hundreds of NASA workers are involved in this part of the process, which is automated and goes on continuously, day and night (about a terabyte a day of data are processed for a single instrument). A colleague of mine, Reto Stckli, then processed one season's worth of these scenes to remove clouds, snow, and other things blocking the satellite's view of the surface, and built a single composite image of the surface. He also made an image of the clouds from two days of data. We've subsequently developed a series of even more detailed maps for each month of 2004: http://earthobservatory.nasa.gov/Newsroom/BlueMarble/BlueMarble_monthlies.html

KH: Why are clouds hard to get right?

RS: The hardest part (for me) of creating the image was getting the cloud to look realistic. The color and brightness of a real cloud is due to a combination of its thickness and the angle of the surface to the sun. Unfortunately our cloud map is only a two-dimensional approximation of the cloud's opacity. It took many hours of experimenting with brightness curves and layer transfer modes in Photoshop to get clouds that looked right, and I'm still not happy with the final result.

KH: Why didn't you get credit for it?

RS: I work (through a contracting company) for NASA--part of the federal government, so the image is in the public domain.

KH: What does it mean that the image is in the public domain?

RS: Anyone can use it however they like, with the exception of using an image form NASA to imply that NASA endorses your product or service.

KH: What other images have you worked on?

RS: The most widely-seen is probably an image of the Earth's city lights at night. I also create images on a daily basis for the NASA Earth Observatory.


For more information on the Blue Marble, visit the History of Blue Marble: BlueMarble_2002 and BlueMarble_history.


Considering that the photo's a fake, I wonder if our assumption that the world isn't flat is up for debate.