After passing behind the sun last August, Venus has swung out to the east of the sun as it comes around toward Earth. On its inner, faster, orbit, it will pass Earth on June 5, 2012 (EDT) and actually pass in front of the face of the sun in an event called a transit.
The transit of Venus after this one won't occur until Dec. 11, 2117, so it is worth anticipating and making plans to observe the transit in 2012.
The diagram at right shows the sky tonight with the waxing crescent moon just above Venus
(filled circle) and the bright stars of Enif (EEN-if), Altair (al-TAIR)and Vega (VEE-ga) emerging in the twilight. The circles show how Venus' position at sunset every Tuesday will change through the winter and spring but does not show how the stars will change.
Keep in mind that the sun is moving eastward against the background of stars at about 1 degree per day.
To rise in the sunset sky, Venus must move slightly faster relative to the stars. Jupiter, moving more slowly, gets carried to the west with the stars, coming into the western sky in February, nearly aligns with Venus on March 13, and is lost in the sun's glare by April 24.
Venus climbs away from the setting sun to its greatest eastern elongation on March 27 (on the diagram, it appears to be at its highest on April 10 but this is due to the
diagram's curved horizon). As it climbs, it will move through the constellations of Capricornus (in which it is currently found), Aquarius, and Pisces to meet Jupiter in Aries.
The waxing crescent moon will pass close to Venus on Jan. 26 and Feb. 25. On March 26, Jupiter will join them in a striking evening display. Venus will then move into Taurus and pass through the southern extent of the Pleiades over the course of April 2, 3, and 4. This should provide a delightful sight through binoculars or a telescope as Venus will be a thick crescent amidst the stars like the moon a few days before first quarter. After this, the Hyades and Aldebaran (al-DEB-uh-ron), forming the bull's face, will pass beneath Venus, closely followed by glittering Orion.
On April 24, the crescent moon will again join Venus as though to bid adieu to Jupiter as it disappears into the glare of dusk. Venus' motion on its orbit will then carry it back toward the sun from our perspective and it will quickly plunge toward the horizon. One last time, on May 22, a very thin waxing crescent will join Venus low on the horizon, both setting well before the sky is dark.
On June 5, for the second time in our lifetimes, we will be able to see a transit of Venus. Venus' orbit is tilted 3.4 degrees with respect to Earth's. The planets align with the sun only when Venus crosses the plane of Earth's orbit at inferior conjunction, when it's
between the Earth and sun. Even then, the alignment only lasts for about 8 hours.
Venus comes to inferior conjunction every 584 days, but is usually too far above or below Earth's orbital plane to be seen against the sun. Thus transits of Venus are rare
events, occurring in a regular cycle with intervals of 8, 105.5, 8 and 121.5 years.
On June 8, 2004, the transit of Venus ended soon after the sun rose over the Adirondacks. Fortunately, I was on a boat cruising up the Yangtze River where I projected the event through my binoculars onto the deck for my fellow passengers! This time, we will be able to see almost half of the transit as it will begin just after 6 pm and continue until the sun sets at 8:42 (EDT). The entire transit will again be visible from most of the length of the Yangtze, all of east Asia, Alaska and eastern Australia. The best source
of information about it is NASA's Eclipse page at eclipse.gsfc.nasa.gov/eclipse.htm.
The first known observation of the transit of Venus took place in 1032 by the Persian astronomer Avicenna, giving evidence that it was closer to Earth than the sun.
Galileo was aware of the transits and Johannes Kepler, with his study of solar system orbits, was the first to predict the year and day of a transit, 1631, but could not
predict the time. Unfortunately, it was unobservable from Europe. Jeremiah Horrocks improved on Kepler's prediction of the transit of 1639 and observed it from his home north of Liverpool, England. He used his observation to estimate the size of Venus and the distance to the sun, about two-thirds of today's accepted value of 92 million miles.
Edmond Halley, of comet fame, realized that observers at widely spaced latitudes carefully observing the transit of Venus could use trigonometry to determine the distance to the sun. Astronomers fanned out across the world to observe the transits of Venus in 1761 and 1769, several of them dying on the journey, and succeeded in measuring a distance only 2.3 percent greater than today's accepted value.
Watching Venus arc across the sunset sky through the winter and making an effort to observe the transit in June can connect us all to the long history of human fascination with astronomical events. We benefit from all the work of previous generations in knowing more than they could imagine about Venus and all the planets of our solar system. But we have the added advantage of the Kepler spacecraft
(kepler.nasa.gov) with instruments sensitive enough to detect planets transiting other stars.
So far, Kepler has identified 2,326 planet candidates and confirmed 33 planets orbiting other stars, including a few Earth-sized planets in orbits in the "habitable zone" where liquid water could be found on their surfaces. Perhaps one of these will be the first other world where evidence for the presence of life is confirmed. So the chasing after transits humanity has been engaged in for almost four centuries may end up teaching us that we are not alone in this universe. Quite an amazing accomplishment from watching a spot cross the sun. I invite you all to anticipate and savor June's opportunity.
If you have any questions about Venus, transits or any other astronomy, please visit the Adirondack Public Observatory web site at apobservatory.org or email firstname.lastname@example.org.