Minnesota Starwatch

University of Minnesota

Department of Astronomy

Minnesota Starwatch for June 2004
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Starwatch Newsletter

Minnesota Starwatch is a newsletter describing the night sky in the Midwest.

It is updated monthly, and is produced by the
Department of Astronomy, University of Minnesota
116 Church Street SE, Minneapolis, MN 55455

Minnesota Starwatch for June 2004

Hello, this is Minnesota Starwatch for June, 2004. The sun reaches its highest point in the northern sky on June 20, marking the Summer Solstice. In our culture we consider that the first official day of summer, although some traditions count the solstice as the middle of the summer season. Although solar heating is greatest near the summer solstice, usually our warmest weather lags by several weeks into mid-July, since the earth continues to absorb more heat than it loses for a while, and the atmosphere and oceans transport heat northward, adding to local heating.

The past several months brought us an unusual showing of bright planets. That continues into June, although the pattern exhibits some notable changes now. Mars, Jupiter and Saturn remain prominent in the evening sky. Jupiter will be the brightest planet after sunset, well up in the southwest in the constellation Leo, around midnight. On June 23 the slightly crescent Moon will pass just above Jupiter. Saturn and Mars begin to fade into the twilight this month, with Saturn becoming difficult to find by month's end.

Venus undergoes the most striking changes during June. Recently it has been a dazzling bright light in the western evening sky. But, it has been dropping lower and lower towards twilight in recent weeks as it moves into a position between the Earth and Sun. On June 8 Venus transits directly in front of the Sun. Such a transit is a very rare event, despite the fact that Venus passes between the Earth and Sun approximately every year and a half. On most such occasions Venus passes below or above the Sun, since its orbit around the sun is tilted by about three degrees with respect to the earth's orbit. The last transit of Venus took place in 1882, and before that in 1874.

Today a transit of Venus is mostly a dramatic spectacle to watch. In the 19th century, however, there was great excitement from the astronomical community about the events because of their unique scientific potential at the time. In particular, the exact timing of the events promised to give the best determination available on the size of the solar system. By then it was well-known that the orbits of the planets, including the earth, followed a precise mathematical rule call Kepler's third law of orbital motion that connected the sizes of the orbits to their orbital periods. The timing of a transit of Venus depends on this. But, in addition, the exact times that Venus lines up with the edges of the Sun's disk varies from place to place on the earth. That depends on the geometry of the arrangement between the Earth, the Sun and Venus. That is, they form a triangle, whose angles are known exactly at those moments. This information, coupled with knowledge of the size of the earth is enough to determine the distance between the Earth and the Sun. Kepler's Law already gives us the sizes of the other planet orbits relative to the Earth-Sun distance.

Today we have more precise ways of measuring these distances directly. For example, radar signals have been bounced off Venus. The time of the round trip, divided by the speed of light measures the distance directly. Still, despite its reduced role as a scientific tool, transits of Venus draw a lot of attention, because of their rarity.

In Minnesota the June 8 transit will be well underway at sunrise. The last contact with the Sun will occur around 6:24 AM CDT, when the sun is only about 8 degrees above the horizon. Even so, observers should be extremely careful to avoid looking directly at the unfiltered sun, if they try to see the event with binoculars or a telescope. Permanent damage may occur.

Friday night telescope viewing at the U's Astronomy Department begins at 9pm. Please call (612) 626-0034 for more information.

For fun Astronomy Outreach programs check out our Public Outreach link, or if you're interested in how you can help build the new Minnesota Planetarium, please call 612-630-6151 or visit http://www.mplanetarium.org.

Como Planetarium is offering fun family shows about astronomy on a limited basis; please call 651-293-5398 for more info!

This has been a Minnesota Starwatch produced by the University of Minnesota Astronomy Department.