Transit of Venus Story
A transit of Venus occurs when Venus passes directly between the sun and earth. This alignment is rare, coming in pairs that are eight years apart but separated by over a century. The most recent transits of Venus were a thrilling sight in June 2004 and 2012, with the next transit of Venus pair occurring in December 2117 and 2125.
Observers from two locations on earth see two distinct paths (red and blue) of Venus across the sun. The slight difference in times Venus takes, moving from edge to edge, can mathematically unlock the distance from earth to the sun, and thus the size of our solar system. For 17th & 18th century transits, intrepid explorers set out to answer a leading question of mankind. Not all of them made the voyage back home.
Mystery of "Black Drop"Just before or after the circular black dot of Venus seems to touch the edge of the sun, a peculiar "black drop effect" sometimes occurs between the contact points. A ligament of darkness smears the juncture of Venus and the sun. You can see a similar anomaly if you almost pinch your thumb and forefinger together. Just before you sense contact, a black feature spans your two digits.
Transits Lead the Hunt
Once again, transits are on the leading edge of new discoveries. The NASA Kepler mission and others are using the transit method to find habitable planets around distant stars. The Kepler spacecraft monitors over 150,000 stars, looking for periodic dips in their light curves which reveal the presence of companion planets. You, too, can join this quest for new worlds.
Midwest Treasure: TROVE
Art exhibits, family activities, a bus tour, historic artifacts, lectures, webcasts, telescope viewing, and more complemented the visual spectacle near the Michigan-Indiana border. This hub of 2012 transit of Venus activity in Michiana celebrated the math, science, history, and art of the celestial phenomenon.
- Poster: Transit of Venus Time Keg
- Community Celebrates
- Closure for Transit of Venus
- Vision For Future
- Video Follows Michiana Experience
- Transit of Venus Time Keg
- Viewing Great, Timing Difficult
- Time to Set Sail
- What if it's cloudy?
- You Can Learn a Lot From a Dot
- Can I Use Welding Glass to View the Sun?
1 As suggested in the safety guidelines above, the use of eclipse shades or of #14 shade welding glass will permit a large number of people who do not have specialized equipment to observe this event. However, as the planet approaches the limb of the sun, subtleties like the "black drop" effect will not be discernible. At one minute of arc in size, Venus is near the visual limit of most people's eyes. It's tiny compared to the sun, which is about 32 arcminutes in diameter.
Eclipse Shades or Solar Shades appear similar to sunglasses, but they have a special filter that permits safe viewing if the filter is in new condition. Eclipse/solar shades are available through Rainbow Symphony and other retailers listed at http://www.mreclipse.com/Totality/TotalityApC.html under "Solar Filters." Before looking at the sun, inspect the material to make sure the lenses are not scratched or compromised in any way. If so, discard the shades.
2 Pinhole projectors are a safe, indirect viewing technique for observing an image of the sun. While popular for viewing solar eclipses, pinhole projectors suffer from the same shortcomings as unmagnified views when Venus approaches the edges of the sun. Small features like the 'black drop' effect will not be discernible.
Dr. Hugh Hunt demonstrates a successful pinhole projection (right) of the 2004 transit of Venus at http://www2.eng.cam.ac.uk/~hemh/transit.htm. Additional instructions for pinhole projectors are at http://www.exploratorium.edu/eclipse/how.html; from the Exploratorium.
3 You may project a magnified view of the sun through a reflector telescope or binoculars onto a white surface, which conveniently allows a larger number of people to watch concurrently. See http://casa.colorado.edu/~dduncan/wp/?page_id=261 for video instructions for projecting the sun, by Dr. Doug Duncan.
The projection technique sometimes has its own limitations. Because magnified projections usually have an exposed focal point beyond the eyepiece, a bystander can inadvertently put her eye or body in the sight line of the sun. Hence, a projecting telescope must not be left unattended. (See Unattended Equipment Hazards, left column.) Large reflector telescopes can generate too much heat by concentrating a lot of the sun's energy on the secondary mirror and eyepiece, so the incoming light must be attenuated first. "Stop down" the aperture. Likewise, SCT or Schmidt-Cassegrain telescopes can experience too much heat build-up as the light bounces internally.
Hubert van Hecke provides the design and instructions for making his sunspotter. Additional pages at his Ask Mr. Science web page indicate how to take sunspot data and analyze them.
The Exploratorium demonstrates how to view a planet in transit safely by projecting the image of the sun with binoculars. Important: Do not look at the sun through binoculars without solar filters on the large ends of both the barrels. Do not leave this rig unattended.
4 A method for allowing a large crowd to witness the transit of Venus concurrently is to project a magnified image through a closed-loop device.
A popular projection device used during the 2004 transit of Venus was the now-improved Sun Funnel. Made from simple materials (a plastic funnel, a clamp, an eyepiece, and some projection fabric), the device fits in your telescope like an eyepiece with an appendage. A clear image of Venus transiting the sun appears on the screen. Because the entire light path is enclosed, observers are not at risk. A larger version of the screen uses a bucket to yield a larger image. Download simple instructions and supplies list written by AAS Press Officer Richard Tresch Fienberg.
Bruce Hegerberg's design for a Sun Gun is online at www.sunguntelescope.com.
Another viewing tool is Gene Zajac’s modified version of a Sun Gun (see 1999 GLPA Proceedings). The device safely allows a crowd of spectators to view a large projection of the sun, the transiting planet, and sunspots.
TIP: To avoid excessive heat build-up on your eyepiece, do not aim the telescope continuously at the sun for an extended time. For large scopes, stop down or attenuate the incoming light, for the telescope's purpose is to magnify the image of the sun, not to gather a lot of sunlight.
The Sunspotter is commercially available from Science First. It provides a surface on which you can safely trace the sun's outline and sunspots onto a piece of paper.
TheVenuscope and solar shades are commercially available from SODAP-SOBOMEX- Department Sky & Space.
A Solarscope is commercially available from [Light Tec Optical Instruments]
The transit of Venus is perhaps best when viewed directly when magnified, which demands an appropriate solar filter over the large end of the telescope. Often made of glass or Mylar, these "white light" filters block about 99.99% of the incoming sunlight, which allows the eyepiece then to magnify the image. A filtered, magnified view will show the sun (either blue or orange), the planet Venus, the "black drop" effect, and sunspots. See Solar Filters or http://skyandtelescope.com/observing/objects/sun/article_101_1.asp for a list of retailers.
Note #1: The sun's immense energy must be drastically reduced before it enters the telescope. Do not use small filters that fit over the eyepiece (as found in some older, cheaper telescopes), for the concentrated sunlight can shatter them.
Note #2: Remove unfiltered finder scopes so they are not inadvertently accessed. Do not rely on a lens cap--even if it is taped on--to keep the eyes of a prying person at bay. (See Unattended Equipment Hazards in left column.)
Special telescopes with built-in hydrogen-alpha filters show additional solar features, such as the sun's surface granulation and prominences extending outward into space. Though more expensive than traditional telescopes, they offer wonderful views of the magnified sun not seen by astronomers in previous centuries.
6 Transit not visible from your location, or clouds interfering? Watch the live webcast from atop Mauna Kea in Hawaii, with expert commentary brought to you by the fun team at NASA EDGE. Don't miss the 2012 transit of Venus!
B. Ralph Chou, MSc, OD
Associate Professor, School of Optometry, University of Waterloo
[See also http://youtu.be/4RGr9FcBrSM for talk at 2012 Symposium at University of Toronto.]
Watching the profile of Venus as it passes across the Sun during a transit is a wonderful demonstration of the way the solar system works. Over the course of several hours, Venus traces a path across the disk of the Sun, then leaves, in what can be thought of as an extreme example of an annular eclipse of the Sun.
Observing the Sun, however, can be dangerous if the proper precautions are not taken. The solar radiation that reaches the surface of Earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290 nm, to radio waves in the metre range. The tissues in the eye transmit a substantial part of the radiation between 380–400 nm to the light-sensitive retina at the back of the eye. While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the primary concern over improper viewing of the Sun during the transit is the development of “solar retinopathy” or retinal burns.
Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function, which may be either temporary or permanent depending on the severity of the damage. When a person looks repeatedly, or for a long time, at the Sun without proper eye protection, this photochemical retinal damage may be accompanied by a thermal injury—the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (the retina has no pain receptors), and the visual effects do not become apparent for at least several hours after the damage is done (Pitts 1993). Viewing the Sun through binoculars, a telescope, or other optical devices without proper protective filters can result in immediate thermal retinal injury because of the high irradiance level in the magnified image.
Because the apparent diameter of Venus is only 1/30 that of the Sun, there is never a time during the transit when it is safe to look at it without proper eye protection. Failure to use proper observing methods may result in permanent eye damage and severe visual loss. This can have important adverse effects on career choices and earning potential, because it has been shown that most individuals who sustain solar retinopathy eye injuries are children and young adults (Penner and McNair 1966, Chou and Krailo 1981, and Michaelides et al. 2001).
The same techniques for observing the Sun outside of eclipses are used to view and photograph the transit (Sherrod 1981, Pasachoff 2000, Pasachoff and Covington 1993, and Reynolds and Sweetsir 1995). The safest and most inexpensive method is by projection. A pinhole or small opening is used to form an image of the Sun on a screen placed about a metre behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. All of these methods can be used to provide a safe view of the transit to a group of observers, but care must be taken to ensure that no one looks through the device. The main advantage of the projection methods is that nobody is looking directly at the Sun. The disadvantage of the pinhole method is that the screen must be placed at least a metre behind the opening to get a solar image with a silhouetted disk of Venus that is large enough to be easily seen.
The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most of these filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.003% (density ~4.5) of visible light and no more than 0.5% (density ~2.3) of the near-infrared radiation from 780–1400 nm. (In addition to the term transmittance [in percent], the energy transmission of a filter can also be described by the term density [unitless] where density, d, is the common logarithm of the reciprocal of transmittance, t, or d=log10[1/ t]. A density of ‘0’ corresponds to a transmittance of 100%; a density of ‘1’ corresponds to a transmittance of 10%; a density of ‘2’ corresponds to a transmittance of 1%, etc.). Figure 1 shows transmittance curves for a selection of safe solar filters (Chou 1981, 1998). The “safe” zones of the plot are transmittance levels less than 0.0001 between 200 and 780 nm (above the 1E-04 line in the graph) and transmittance levels less than 0.001between 780 and 1400 nm (above the 1E-03 line in the graph). The longer infrared between 1400 and 2500 nm does not get past the tears and front of the eyeball, so is not a problem.
An illustrated lecture delivered at the Smithsonian Institution
in Washington, D.C., on June 7, 2004
IntroductionThe beginning of modern astronomy in Hawaii can be dated to the arrival, in 1778, of the British explorer Captain James Cook (Fig. 1). In addition to being supplied with state-of-the-art navigational instruments – including the sextant and the chronometer – Cook also carried, on each of his two ships, the Resolution and the Discovery, a portable astronomical observatory (Fig. 2) designed by the astronomer William Bayly. Very much like the modern observatories that now populate the summit of Mauna Kea, Bayly’s observatory sheltered a telescope and a clock and was equipped with a revolving dome that could be opened and closed at will. With these astronomical aids at his command, Cook was able to do what no one prior to his time had ever done: He was able to assign the to islands a latitude and a longitude, and thus to place Hawaii on a map (Fig. 3). Observations of Jupiter’s satellites, for example, helped to fix the longitude of Kealakekua Bay for the first time in history; and as the distance between the Prime Meridian at Greenwich and a tropical island paradise became as firmly defined as the science of the day would allow, a British presence became firmly established in Hawaii.
Captain Cook was killed in 1779 at this very location, Kealakekua Bay – a place to which I shall return later in my story. Ten years earlier, his first Pacific voyage had taken him into the South Pacific where, in June of 1769, he had observed, from the island of Tahiti, a rare transit of the planet Venus across the sun. At Tahiti, a temporary garrison called Fort Venus (Fig. 4) provided a safe haven for Cook’s men and their instruments and the transit was observed under sunny skies. But, just as importantly, many of the elements of this pioneering eighteenth-century enterprise – the passing of a planet in front of the sun, the demonstration of British naval and navigational prowess, the transport of observers and equipment to a remote Pacific island, and the construction of a temporary fortress – proved a good rehearsal for what was to occur in Hawaii more than 100 years later.
When, in 1874, Venus again slithered across the sun, the British were once more active in the Pacific, and Hawaii, where British cultural influences were by then easily recognizable, was very much at the center of the enterprise. Cook’s three voyages of exploration, including his voyages to Tahiti and Hawaii – and, in particular, the astronomy that had informed those voyages – had set the stage for even more ambitious endeavors.
Not long after Cook’s arrival in Hawaii, the islands were united under a single ruler, Kamehameha the Great, and soon thereafter adopted for their government a European-styled monarchy. By the latter half of the nineteenth century, Hawaii had been ruled by a succession of hereditary chiefs, or ali‘i, and was still an independent kingdom when, on September 9, 1874, nearly a century after Captain Cook had appointed Hawaii a position on a map, a ship from England, HMS Scout, sailed into Honolulu Harbor carrying an expedition of seven astronomers (Fig. 5).
The mission of the expedition was – as Captain Cook’s had been in 1769 – to observe a rare transit of Venus across the sun for the purpose of better determining the value of the Astronomical Unit (Fig. 6) – that is, the Earth-sun distance – and thereby the absolute scale of the solar system. For although Copernicus had, by the 16th century, put the known planets in their correct order and had derived from his model of the solar system a set of relative distances among its members, their absolute distances remained hostage to the uncertain value of the Astronomical Unit (AU). Astronomers still needed a celestial yardstick of known length to measure distances among the planets and to link the planets to the stars beyond.
The circumstances of a transit of Venus and its relationship to the Astronomical Unit are, I shall assume, well known to people in this room and need not detain us here. They form the subject matter of a fine exhibition now on display here at the Smithsonian Institution, and the American participation in the transit observations of the nineteenth century is retold in the exhibit. But because Hawaii was still an independent kingdom at that time, and not an American state until 1959, it is my task today to tell you something about Hawaii’s role at the center of an international effort to solve what was once considered the most important problem in observational astronomy.
Read article: Seeking Goddess for Instant Astronomical Gratification (PDF)
- Solar Dynamics Observatory to Witness the 2012 Transit of Venus
- Crowd Gathers to Witness the 2004 Transit of Venus
- MichianaSTEM to Feature Imaging of Transits
- Simulation of Kepler Detecting Planets
- Night of the Transit
- Letters to Leaders
- 2004 Celebration in Mishawaka, IN
- 2004 Observing Report-Brian Davis
- 2004 Transit of Venus Images
- US Naval Observatory Expeditions
- Quilt: Transit Time, by Don Tuttle
- Monument Honors Jeremiah Horrocks