NEWTOWN -- Mercury makes about 13 passages across the sun each century.
The transits of Venus are far rarer.
For the first time since 2004, and the last until 2117, the second planet moved directly between the Earth and the center of the solar system. While their telescope was set in the perfect position Tuesday night, the cloud cover forced the members of the Marple Newtown High School Astronomical Society to view the historic placement through a live feed from Hawaii.
"A solar eclipse is a '10' for most people and for an astronomer, the transit of Venus is a 'nine,'" said Robert Speca, who co-sponsors the club with Don Sloat. "We worked hard to make sure we were ready."
First recorded in 1396, the transit of Venus appears as a small dot gliding across the face of the sun. The infrequent alignment was used to gain the initial estimate of the size of the solar system, as observations of the 1639 transit provided scientists with the most accurate approximation of the distance between the sun and the Earth.
Viewings in various parts of the world combined with scientific theory subsequently allowed astronomers to calculate the distances among the remaining planets. The sighting will provide a number of additional research opportunities, particularly in the refinement of techniques for the search of exoplanets, bodies which lie beyond the current solar system.
The complete transit requires six hours and the telescope and newly purchased camera were in place long before the beginning of the expected 5:55 p.m. sighting. The students had planned to photograph the image in increments to make a time lapse recreation for future viewing.
As a result, they watched the phenomenon on www.nasa.gov, which was stationed at the Keck Observatory atop the summit of Mauna Kea. Venus, which appeared about the size of sun spots they had observed earlier in the afternoon, was easy to see against the orange background.
"Since it is on top of a mountain, there is usually no cloud interruption," said Speca. "It allows for a clear image."
For some students, the curiosity with astronomy began as young children, while others noted their interest was peaked during the study of planets and galaxies in the freshman earth science course.
"I've always looked at the stars, then began to think of them not just as specks of light, but as balls of gas," said sophomore Nawaal Amer. "Everything I believed became so much cooler."
Despite watching the event on a screen in the classroom, the transit of Venus still captured the imagination of the budding astronomers.
"This is a once-in-a-lifetime event," said society vice president Michael Mayer. "It is fascinating."