Consider it the first interplanetary photobomb. On July 19th, NASA's Cassini spacecraft will photograph Earth through the rings of Saturn--and NASA wants you to jump into the shot.

"Cassini has photographed Earth before, but this will be the first time Earthlings know in advance their picture will be taken from a billion miles away," says Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, CA. "We hope that people around the world will go outside to wave at Saturn while the photo-shoot is underway."

Cassini's cameras will be trained on Earth during a 15 minute interval that begins at 2:27 p.m. PDT (5:27 p.m. EDT or 21:27 UTC).

"I am excited about this rare opportunity to send photons of all of us waving at Saturn," adds Spilker. "I am encouraging my family and friends to wave at Saturn on that day also."

The circumstances of this photo-op are extraordinary. From Cassini's point of view, the body of Saturn will eclipse the sun, so that the rings are magnificently backlit. Earth will appear as a tiny blue speck just outside the E ring.

Opportunities to image Earth from the outer solar system are rare. Since the Space Age began, there have been only two images of Earth from the outer solar system. The first and most distant was taken 23 years ago by NASA's Voyager 1 spacecraft beyond Neptune. The second was Cassini's image from the Saturn system in 2006.

Cassini's image of Earth in 2006 inspired Carolyn Porco, the Cassini imaging team lead at the Space Science Institute in Boulder, Colorado. "Ever since we caught sight of Earth among the rings of Saturn in September 2006, I have wanted to do it all over again, only better," she says. "This time, I wanted to turn the event into an opportunity for people all over the globe to celebrate together the extraordinary achievements that have made such interplanetary photo sessions possible."

This photo-shoot will improve upon Cassini's previous effort in two ways: The July 19, 2013, image will be the first to capture the Saturn system with Earth in natural color, as human eyes would see it. It also will be the first to capture Earth and its moon with Cassini's highest-resolution camera.

The Americas will be facing Saturn at the time of the image. For North Americans, the event happens in broad daylight, so the best way to participate is to go outside, face east, and wave at the blue sky. You won't be able to see Saturn, but it is there.

Go outside again after sunset. By that time, Saturn will have moved into the southwestern sky. It pops out of the twilight, a slightly-golden pinprick about twice as bright as a first magnitude star. Saturn is in the constellation Virgo, not far from the bright star Spica.

If you have a telescope, point it at Saturn. Even inexpensive backyard optics will show you Saturn's rings and its biggest moon Titan. Observers who see Saturn for the first time through the eyepiece of a telescope often gasp. The view is Hubble-esque, but the experience is much more personal. You're seeing Saturn with your own eyes, a celestial wonder right out of the pages of an astronomy magazine.

From Cassini's point of view, Saturn's rings are too wide to capture in a single image, so the spacecraft will take a series of exposures. These will be combined on Earth to produce a breathtaking mosaic.

"Seeing the whole mosaic of the backlit rings when it is put together will be incredible," says Spilker. "We will be looking for changes in Saturn's faint rings, especially the E ring, from the mosaic we took back in 2006."

The highlight of the day, however, will likely be our own planet. Says Porco, "It will be a day to celebrate life on the Pale Blue Dot."

Friday, July 19th @ 2:27 p.m. PDT: Go outside and add your photons to Cassini's portrait.