Tomorrow, Venus will pass across the face of the sun, producing a silhouette that no one alive today will likely see again, says NASA.

Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it.

The nearly 7-hour transit begins at 6:09 pm Eastern Daylight Time. The timing favors observers in the mid-Pacific where the sun is high overhead during the crossing. In the USA, the transit will at its best around sunset. That's good, too. Creative photographers will have a field day imaging the swollen red sun "punctured" by the circular disk of Venus.

Observing tip: Do not stare at the sun. Venus covers too little of the solar disk to block the blinding glare. Instead, use some type of projection technique or a solar filter.

A No. 14 welder's glass is a good choice. Many astronomy clubs will have solar telescopes set up to observe the event; contact your local club for details.

Transits of Venus first gained worldwide attention in the 18th century. In those days, the size of the solar system was one of the biggest mysteries of science.

The relative spacing of planets was known, but not their absolute distances. How many miles would you have to travel to reach another world? The answer was as mysterious then as the nature of dark energy is now.

Venus was the key, according to astronomer Edmund Halley. He realized that by observing transits from widely-spaced locations on Earth it should be possible to triangulate the distance to Venus using the principles of parallax.