From our vantage point on Earth, it may look like merely a tiny black dot crossing the surface of the sun.
But in reality, the Transit of Venus that begins about 5:03 p.m. today is an extremely rare event that has occurred just a handful of times since it was discovered in 1639 and won't occur again until 2117.
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The black dot is the planet Venus, passing between Earth and the front of the fiery sun.
Transits of Venus are so rare because the planet's orbit is tilted relative to the Earth's. The two planets line up with the sun only four times every 243 years. (The timing between transits is odd: 121½ years, then eight years, then 105½ years, then eight years again.)
To get a look at the transit, you could buy special viewing glasses to gaze safely and directly at the sun. A live NASA webcast from Hawaii also will stream the event online.
But for a spectacular show, A.J. Poelarends, a professor of astronomy and physics at Wheaton College, suggests peering through the two Hydrogen-alpha telescopes in the school's observatory.
The Wheaton College Astronomical Observatory, which usually does not offer visiting hours during the summer, is opening at 4:30 p.m. Tuesday to provide free viewings. The observatory is in the college's Science Building at the corner of University Place and Howard Street, Wheaton.
The telescopes will show the sun's finer details, from swirling flames to arches of gas. So when Venus begins to approach the sun, it will look like a bubble is surrounding the planet.
"To see Venus moving in front of those features on the sun is so beautiful," Poelarends said. "I hope people will look at that with their own eyes (through a telescope) and have that sense of awe and wonder."
Amateur astronomers and others interested also can view the rare event at the Harper College Observatory in Palatine, which is open to the public from 4 p.m. to sunset; the Elgin School District U-46 Observatory, 312 Watch St., in Elgin, is open for a viewing beginning at 5 p.m.; Triton College's Cernan Earth & Space Center in River Grove also will be open for public viewing of the transit; and the Adler Planetarium in Chicago will be open from 4 to 9 p.m.
At Wheaton, white-light telescopes also will be available for the viewing. Another kind of telescope will capture real-time video recordings of the transit that will be broadcast in the school's Science Center.
The transit, Poelarends said, is one of the rarest planetary alignments, an event that once triggered scientific expeditions in the 18th century. At the time, scientists were seized with questions about the distance from the Earth to the sun, and hoped the transit would explain them.
Modern scientists will be studying Tuesday's transit to understand exoplanets that move in front of stars in a similar phenomenon, Poelarends said.
He recommends two prime times for the transit: at 5:04 p.m. Venus will appear to touch the sun and at 5:21 p.m. Venus will seem to move inside, almost merging with the sun.
While the event depends on the weather, Poelarends says the forecast looks promising.
Eight years ago, Poelarends was studying in his native Netherlands when he was captivated by the last viewable transit.
"What I hope that people learn is just amazement, how beautiful this universe is and how beautiful some of the alignments are," he said.
• Daily Herald wire services contributed to this report.