Star Watch column: Transit of Venus

June’s best star watching happens in the daytime, as Venus makes a rare transit across the face of the sun. Try not to miss it; this is the last transit of Venus until December 2117.

The show starts at 5:04 p.m. June 5, when the planet first makes contact with the northeastern edge of the sun. Venus then spends some six hours tracing a line across the northern part of the sun.

From our Midwestern vantage point, the sun will set with Venus still a dark dot on its face. To see the spectacle, take the same precautions as with a solar eclipse: Only watch it live under the guidance of a trusted professional. If in doubt, watch it on TV or online. For more information, see the NASA website: sunearthday.nasa.gov/2012/transit/viewing.php.

It may seem as though Venus should cross the face of the sun every time it passes between Earth and the sun, which it does once every 584 days. That would happen if Venus’ orbit were in the same plane as ours. But it’s not; our orbits are inclined 3.4 degrees from each other, and the two orbital planes cross at junctions called nodes. Venus can only transit the sun’s disc when it is both between our planet and the sun, a position called inferior conjunction, and crossing or very near to crossing a node.

No wonder transits of Venus happen only 16-18 times every thousand years. The nodes shift position over the centuries, but now they are in line with the sun (as seen from Earth) in early June and early December.

If you’ll be in the Twin Cities area June 5, the University of Minnesota will host a lecture and viewing of the transit, beginning at 4 p.m. in Room 166 Tate Laboratory of Physics. If the weather is cloudy, the transit will still be viewable via a live NASA feed from Hawaii. For more information, see www.astro.umn. edu/venustransit/.

But Venus isn’t the only show in town. Mars, now in the west after sunset, moves steadily eastward toward Saturn. The ringed planet is low in the south to southwest, just above bright Spica, in Virgo.

Above Saturn is brilliant Arcturus, the jewel of kite-shaped Bootes, the herdsman. Just east of Bootes hangs Corona Borealis, the Northern Crown, and again to the east we see the upside-down form of Hercules, identifiable by six stars that form an hourglass.

June’s full “strawberry” moon undergoes a partial eclipse in the predawn hours of the 4th, when it dips its southernmost part in the Earth’s inner, dark umbral shadow. The moon enters Earth’s penumbra at 3:48 a.m. and makes contact with the umbra at 5 a.m. The moon sets with the umbra shading part of its southern face.

Summer arrives with the solstice at 6:09 p.m. on the 20th, when the sun reaches its highest point in the northern sky, directly above the Tropic of Cancer.

The University of Minnesota Duluth offers public viewings of the night sky at the Marshall W. Alworth Planetarium: www.d.umn.edu/planet.

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