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Full-circle Earth

Q: I enjoyed your question about having to be infinitely far away to see an entire hemisphere of the Earth. But how high do you have to be to see the entire circular shape of the Earth (a full circle containing, obviously, less than a hemisphere)-Barry S., Los Angeles, California

A: About 12,000 miles high says Richard W. Underwood, retired NASA photogrammetrist and expert extractor of information from photographs.

Right: [JSC/NASA] About 28,000 miles from Earth

I had to hunt to find the answer to this one. I started off hunting for an image: thinking that might lead to how far out you can see a full-circle Earth.

I found the NASA photograph of Earth shown in the figure. Its caption says that the Apollo 17 crew took the picture while "traveling toward the moon". But it doesn't say how far out the crew was when they snapped the photo or what kind of lens they used.

Dave Ransom, retired aeronautical engineer figures the crew took the picture sometime after trans-lunar injection, which occurred about 3 hours 18 minutes after launch and after the separation from the Saturn S-IVB at 4 hours 45 minutes. Then, the Apollo spacecraft was about 15,400 miles out. Ransom notes the image clarity and thinks the crew took the picture close to Earth.

"Plus that was quite a sight," he says. "the first time for an Apollo image of the South Pole, and I suspect the astronauts might have taken the image as soon as it was possible. Were I a betting man, I might put my money on about 20,000 nautical miles [23,000 statute miles]."

Left: [JSC/NASA] About 28,000 miles from Earth

Pretty close to the 28,000 statute miles I learn from Mike Gentry at Houston NASA, who called the septuagenarian Underwood for the answer.

Then NASA sends an even closer image-taken the 10^th of November 1967 at 22,300 miles high-and shown in the second figure. You see four continents (parts of North and South America, Africa, and Europe with the Greenland ice cap kicked in), major weather moving across the US, and a tropical storm (bottom center). This is probably the nearest full-circle photo of Earth.

Underwood says that you'd have to go out about 12,000 miles to see Earth as a circle, assuming your helmet or the spacecraft window doesn't restrict your view.

(Answered by April Holladay, science correspondent, August 27, 2001)

Further Surfing:

JSC/NASA: Earth images

Dave Ransom: Current orbital data

Heavens-Above: Info for observing satellites