Transits

TRANSITS OF MERCURY AND VENUS

Monday, Nov. 11 - 7:30 a.m. to 1:05 p.m.

Safe solar-filtered telescopes will be set up outside Cline Observatory for viewing the 2019 November 11 Transit of Mercury (weather permitting).

What is a transit?

The planets of the solar system orbit the Sun on nearly the same plane, and so it is possible that they can come into direct alignment with us and the Sun. Since Venus and Mercury orbit between us and the Sun, such alignments will cause those planets to appear in silhouette against the face of the Sun. This phenomenon is called a transit. The inclinations of the orbits keep this from happening too frequently, but several times a century we see opportunities to see Mercury or Venus directly against the face of the Sun.

What happens during a transit?

A transit of Venus or Mercury is sort of like a solar eclipse, except that these planets appear much smaller than the moon, and thus cannot cover the whole sun. So, they appear as a black dot against the bright Sun, moving slowly for a few hours across the solar disk. In the case of Venus, which is about 33 times smaller than the apparent diameter of the Sun, a proper solar filter will allow an observer with good vision to see the planet during the transit. Mercury, however, appears too small, and thus requires properly filtered optical aid.

Venus transits are extremely rare, occurring in pairs separated by over a century. Only seven transits of Venus have been observed in recorded history (1639, 1761, 1769, 1874, 1882, 2004, and 2012). The next transit of Venus will occur on:

There have been two Venus transits during the history of Cline Observatory. The morning of Tuesday, 8 June 2004, observers in North Carolina could see the transit of Venus already in progress at sunrise. Cline Observatory staged an early-morning Venus transit observing session in the parking lot on the west side of GTCC’s Jamestown campus. The event was attended by about 50 people, and covered by two local TV stations, WXII and WGHP. Unfortunately, the beautifully clear skies at 4 a.m. that day gave way to fog and clouds by sunrise, so we missed the transit completely. Observatory Host Dennis Hands participated in an expedition to South Africa in 2004 and successfully viewed the transit from there.

On Tuesday, 5 June 2012, observers in North Carolina could see the first part of the transit of Venus during the hours before sunset. Cline Observatory held a Venus transit viewing session in the parking lot above Science Hall. Again, weather played a role. Partly cloudy skies allowed us to see the transit intermittently during its first half hour, but eventually the clouds thickened to spoil the view. Approximately 300 people attended the session, and we again had coverage from two local TV stations, this time WXII and WFMY.

Transits of Mercury are a bit more common. Mercury is closer to the Sun than Venus and thus completes its orbit more quickly, so opportunities for alignment happen more often. On average, there are about 13 or 14 Mercury transits visible from Earth each century.

Assuming current Standard Time/Daylight Time conventions hold, the Mercury transits visible during the twenty-first century are given below, with notes on visibility from North Carolina.

Source: NASA Eclipse Web Site, Seven Century Catalog of Mercury Transits: 1601 CE to 2300 CE https://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html

Transit Resources

For images of the 2004 Venus transit taken around the world, see this gallery of transit images at Spaceweather.com.

To find transit timings for a specific location, use the U.S. Naval Observatory Online Transit Computer.

Jay Pasachoff’s Transits page at Williams College. Professor Pasachoff is a noted eclipse and transit researcher who gave the 2014 Stellar Society Lecture at GTCC about Venus transits.

Viewing transits requires looking at the Sun – safety precautions must be taken!

You should NOT look directly at the sun without a proper solar filter. If you do not have such a filter, do not try to improvise one yourself. See this resource for a list of appropriate techniques and materials.

Safe solar viewers are available from several sources – just do a web search with the terms, eclipse shades, eclipse glasses, or solar viewers. Occasionally Cline Observatory makes these shades available through the GTCC bookstore in advance of significant eclipses.

A time-honored method for viewing the sun during eclipses or transits is the pinhole projection method, which produces a small image of the sun on a card. Typical pinhole projection methods show a partial eclipse very well but are not as effective for transits because Venus and Mercury appear so small., requiring the projection screen to be much farther away from the pinhole to produce a large enough image. This resource describes how to make an effective pinhole projector box.

Live streaming transit sessions are often available online.