"We came all this way to explore the moon, and the most important thing is that we discovered the Earth." -Bill Anders, Apollo 8 Astronaut

This Earth Day, I think -- for anyone interested in space, astronomy, or the Universe -- gives us perhaps the best opportunity to look back on our planet as we understand it now, having traveled so far away from it.

It was only, believe it or not, back in the 1940s that we first photographed our planet from high enough up to directly observe that, in fact, the Earth is curved!

The above image, from 1948 in space over New Mexico, was the first panorama ever taken from space. Compare that to a recent experiment done by a group of students from MIT for $150, where they managed to capture this photo.

Of course, technology has advanced quite a bit over the last 60 years, so that's no surprise. But since we've first started our voyages into space, looking back at our world helps remind us how small and yet how fortunate we are to have our world.

Let's use this opportunity to take a look at some of the greatest views of Earth ever taken.

Bill Anders, the author of the quote atop the page, uttered those famous words as his Apollo 8 spacecraft orbited the Moon: the first manned craft to do so. As they emerged from the night side of the Moon for the third time, they were greeted by the sight of the Earth rising over the limb of the Moon. The above photo, Earthrise, has inspired the world ever since.

But we've gone a lot farther than just the Moon; we've sent things to the farthest reaches of the Solar System! It was the Voyager spacecrafts that were the first to travel to those outermost reaches, but every once in a while, they remembered to look back. In 1977, Voyager 1 became the first spacecraft to turn around and photograph the Earth and Moon together, showing the same illuminated face as one another, above.

And below, you might think, is a much blurrier version of the same thing.

Believe it or not, this is the first photo of Earth -- shot in 2003 -- taken from Mars! NASA's Mars Global Surveyor looked back a hundred-million kilometers, and photographed the Earth and Moon together, above.

But we don't just go out. Recently, in fact, we've gone in, to the innermost part of our Solar System. And when the Messenger Mission, from the same orbit as the planet Mercury, looked back at Earth, what did it see?

The Earth and Moon, full and bright, as outer planets always appear when viewed from one so much closer to the Sun. Absolutely breathtaking, how small and insignificant we appear even from within our Solar System.

And that brings us to my favorite photo of Earth...

Captured by Cassini, here are the great rings of Saturn. Take a look at a faint, distant dot just outside the main rings, on the right side of the photograph. It's blown-up in the upper-left-hand corner. That's the Earth, as photographed from Saturn! Looking less significant than even one of Saturn's minor moons, the "fuzzball" off to one side is actually our Moon, the cause of our tides and our greatest light in the night sky.

Take a moment, today, to appreciate the fragility of our world, and to think about -- despite its cosmic insignificance -- its great significance to us. Take care of it. It's the only one we've got.

And happy Earth day to one and all!