By BRUCE McCLURE

Be sure to circle June 5 on your calendar. This day will present the last chance in your lifetime to witness the planet Venus swinging directly in front of the Sun.

From the North Country, you can watch Earth's sister world appear in silhouette as a small, dark spot in front of the solar disk from about 6 p.m. until sunset (8:30 p.m.) on Tuesday, June 5.

This is really a special kind of eclipse, whereby a smaller celestial object, like a planet, passes directly in front of a larger celestial object, such as the Sun. Astronomers refer to this type of solar eclipse as a transit of Venus.

Proper eye protection is absolutely essential for watching the transit of Venus, else you risk blindness or permanent eye damage. Fortunately, public programs at St Lawrence University in Canton and the Adirondack Public Observatory at Tupper Lake will enable you to view this grand astronomical attraction in complete safety. Bring your family and friends.

Professors Aileen O'Donoghue and Jeff Miller will host the event, starting at 5:45 p.m. at the southwest corner of the SLU practice fields (near where Park Street crosses the river). In case the weather doesn't cooperate, the activities will move into Bewkes Hall 232 at 7 p.m. Contact info at 229-5470 or aodonoghue@stlawu.edu.

The Adirondack Public Observatory will set up on Little Wolf Beach in Tupper Lake. If the weather doesn't allow for direct viewing, a live video from NASA will be shown at Flammer Theatre in the Wild Center. Directions and more information can be obtained at http://www.apobservatory.org/pages/events.html or 518-359-3538.

Only planets that orbit the Sun inside of Earth's orbit can ever transit the Sun. These two worlds are Mercury, the innermost planet, and Venus, the second planet outward from the Sun. If the orbits of the Mercury, Venus and Earth were aligned on the same plane, a transit of Mercury would take place 41 times every 13 years, and a transit of Venus would happen 5 times every 8 years.

As the geometry of the solar system would have it, though, Mercury's orbital plane is inclined 7 degrees to Earth's orbital plane and Venus orbital plane at 3.4 degrees to the plane of our orbit. Whenever Mercury and Venus in their orbits pass in between the Earth and Sun, Mercury and Venus more often than not swing above or below the Sun -- meaning no transit is visible from Earth. In the 21st century, there are only 14 transits of Mercury and two transits of Venus (June 8, 2004, and June 5-6, 2012).

Yet, the orbit of any solar system planet crosses the Earth's orbital plane at two points called nodes. If the planet is traveling from south to north, it's an ascending node. Or if the planet is going from north to south, it's a descending node.

Luckily for us, Venus will be near its descending node as its passes between the Earth and Sun on June 5, 2012, to stage the last transit of Venus until December 11, 2117!

Astronomer Bruce McClure of Norwood writes for EarthSky, an astronomy information and entertainment service at EarthSky.org.