The first photo of Earth was taken in 1990 from 3.7 billion miles away. The second was shot in 2006 from a distance of 926 million miles. And everything's coming together for the third on July 19 from 898 million miles away.

When Saturn eclipses the sun on Friday, the planet will block the sun's light and allow the Cassini spacecraft to reach just the right position to snap a photo of Saturn and its entire ring system - with Earth's blue speck included in the frame. Earth will appear as a tiny, pale blue pixel between Saturn's rings.

The first photo of Earth, taken from a vast distance, was the famous Pale Blue Dot image by Voyager 1 in 1990 as it was leaving the solar system. Carl Sagan requested that NASA turn the spacecraft's camera around and snap Earth from 3.7 billion miles away.

In his book, "Pale Blue Dot: A Vision of the Human Future In Space," which was inspired by the photograph and published in 1994, Sagan writes, "From this distant vantage point, the Earth might not seem of particular interest. But for us, it's different. Consider again 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 second photo was shot from 926 million miles away, also from a point near Saturn like Friday's photo will be. While the first, most distant photo shows Earth a fraction of a pixel in size suspended in a sunbeam like a mote of dust, the second shows a dot floating among Saturn's rings. Both images are astounding and bring a whole new dimension to the meaning of "perspective."

Like the one in 2006, Friday's photo shoot is intended to allow scientists to study Saturn's rings and check for changes seven years and one Saturnian season later. The part of the photo mosaic imaging of Earth will be shot from 5:27 to 5:42 p.m. Eastern time. Look low on the southeastern horizon and wave. Saturn won't be visible to us while the photo is being taken (it'll be too light out), but step outside after sunset and find the nearly full moon rising in the east. You'll see two bright stars spanning the southeastern sky. They are Saturn and Venus. Saturn is the middle "star" between the moon and Venus.

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SKY CALENDAR

July 22: Full moon at 4:15 p.m. This full moon was known by early Native-American tribes as the Full Buck Moon because the male buck deer would begin to grow their new antlers at this time of year.

July 27, 28: Delta Aquarids meteor shower peaks, producing up to 20 meteors an hour made of debris left behind by comets Marsden and Kracht. The second quarter moon will block out most of the faint meteors, but you should still be able to catch quite a few good ones if you are patient. Best viewing will be from a dark location after midnight. Meteors will radiate from the constellation Aquarius, but can appear anywhere in the sky.