June 8, 2004: The Transit of VenusBy: John E. Westfall, Coordinator, Mercury/Venus Transit Section, Association of Lunar and Planetary Observers P.O. Box 2447, Antioch, CA 94531-2447, USA. email: email@example.com
At the time of writing (November, 2003), the planet Venus has appeared in the southwestern sky after sunset. It will gradually pull away from the Sun until next spring, when it reaches greatest elongation on March 29th. Then in April and May it accelerates toward the Sun, and passes through inferior conjunction on June 8th, 2004.
This in itself is nothing special--Venus does this every 19 months and usually passes well north or south of the Sun as it does so. But one inferior conjunction in about 40 is exceptional, with Venus crossing the face of the Sun itself, creating a transit of Venus.
Although one of the rarest astronomical phenomena, transits of Venus are very predictable and come in cycles of 243 years. (e.g., 1761/2004, 1769/2012) They also come in series, each series being all the transits in a cycle. Currently, when one transit takes place, another happens eight years later. Each member of an eight-year pair occurs in the same month, which is either June or December.
Our century two transits are in 2004 and 2012, both in June, which favors observers in the Northern Hemisphere. The last pair was in 1874 and 1882, both in December.
Thus our most recent observing experience of a transit of Venus is 121 years old. We have no digital images or photoelectric photometry of a transit of Venus. We do, however, have observers notes, drawings, and film photographs.
The 2004 transit of Venus lasts lasts slightly over six hours, and of course can be seen only from those areas where the Sun is shining. This means that about a quarter of the world can see the entire transit; another quarter sees only the first part of the transit, and a third quarter can watch only the last part.
In 2004 the favored longitudes that will see the entire event span most of the Old World, and thus most of mankind. As is always the case with June transits, anyone in the Arctic, weather permitting, can watch the transit from beginning to end. However, the Sun will set before the transit ends in Australia and easternmost Asia. On the other hand, the Sun will rise with the transit already in progress for most of the Americas. The eastern Pacific and western part of North America fall in the zone where the entire transit occurs at night.
For the imaginary geocentric observer, the transit begins at 05:14 Universal Time and ends at 11:26. The times of transit phenomena throughout the portion of the world where they can be seen will differ by at most 7 minutes from the geocentric times.
Choosing an Observing Location
The A.L.P.O. is not organizing tours or observing sites for the transit, but advertisements for such tours are starting to appear in astronomy magazines and on the World Wide Web. Also, several astronomical clubs have announced their observing plans.
For those living there, or willing to travel so far, the Eastern Hemisphere (except for western Africa, eastern Asia, and Australasia) is a desirable destination because the transit will be visible there from beginning to end. Those dwelling in North and South America should plan to be in the eastern part of their continents in order to see at least the egress phase of the event.
Unlike the case with the narrow track of totality for a solar eclipse, transit travelers can choose an observing place from an area of millions of square kilometers. In selecting a site, human issues such as cost, convenience, and local politics are obviously important. In terms of nature, an area that has a relatively high mean daily hours (or percentage) of sunshine in June is desirable. This criterion favors the southeastern United States for North America. In the Old World, the Mediterranean Basin, northern Africa, and the Middle East are similarly favored. The real question is what the weather will be like on June 8, 2004. Observers who can relocate in response to short-term weather forecasts and satellite cloudiness images will have a better chance for clear skies than one at a fixed location. Finally, the higher the Sun is in the sky during the transit, the better the chance of avoiding clouds (which, due to perspective, tend to bunch near the horizon), and of having better seeing (i.e., less atmospheric turbulence due to a shorter path length through our atmosphere).
The chief interest in transits of Venus in the 18th and 19th centuries was timing limb contacts in order to determine the value of the solar parallax, and thus the Earth-Sun distance. However, even by the time of the 1874/1882 transit pair other methods for finding the scale of the Solar System had become competitive, and by now we know these values to far greater accuracy than can be had by transit observations.
However, a number of other scientifically significant phenomena are associated with the transits of Venus.
First, within a few days of the transit, Venus will be nearer the Sun in our sky than in most other apparitions. The two bodies will appear within 10 degrees of each other from June 2nd-14th, and within 5 degrees from June 5th-11th. Under these conditions, Venus appears as a very narrow sunlit crescent, with its horns faintly extended, sometimes forming a complete circle. This phenomenon is caused by scattered light in the planet atmosphere above its cloudtops, and was the first evidence for an atmosphere of Venus. Observing the horn extension will be difficult, requiring that either one observe Venus near the horizon in bright evening or morning twilight; or, while taking extreme care to avoid accidentally getting the unfiltered Sun in one field of view, using tube extensions or sunshades to observe Venus near the Sun in complete daylight.
When the transit itself takes place, the beginning of the planet entry onto the Sun disk is called First Contact, the start of Ingress. Second Contact takes place when Venus completes Ingress. Third Contact occurs when Venus starts to leave the Sun disk, the beginning of Egress, which ends at Fourth Contact, when the planet completely leaves the Sun, ending the transit. Venus crosses the Sun southeast limb during Ingress (First Contact will be at position angle 116, 26 degrees south of celestial east), and then travels southwestward to the Sun southwest limb (Fourth Contact will be at position angle 216, 36 degrees west of celestial south).
The Universal Times on June 8, 2004 that the four contacts are predicted to occur are listed below for some locations worldwide ("---" means that Venus will be below the place horizon at the time):
Geocentric 05:13.5 05:32.8 11:06.6 11:25.9 Chicago, IL, USA --- --- 11:05.3 11:25.2 Fairbanks, AK, USA 05:13.9 05:33.7 --- --- Houston, TX, USA --- --- --- 11:26.3 New York, NY, USA --- --- 11:05.9 11:25.7 Halifax, NS, Canada --- --- 11:05.7 11:25.4 Mrida, Mexico --- --- --- 11:27.6 So Paulo, Brazil --- --- 11:13.2 11:32.2 Athens, Greece 05:19.6 05:39.4 11:04.3 11:23.4 London, United Kingdom 05:19.9 05:39.6 11:04.1 11:23.5 Paris, France 05:20.0 05:39.8 11:04.3 11:23.6 Berlin, Germany 05:19.7 05:39.4 11:03.5 11:22.8 Moscow, Russia 05:18.8 05:38.3 11:02.1 11:21.4 Cairo, Egypt 05:19.7 05:39.0 11:04.6 11:23.6 Nairobi, Kenya 05:18.6 05:37.5 11:07.2 11:25.9 Pretoria, Republic of South Africa 05:17.4 05:36.3 11:10.0 11:28.6 Tehran, Iran 05:18.5 05:37.6 11:02.8 11:21.8 New Delhi, India 05:16.2 05:35.1 11:01.6 11:20.7 Bejing, People's Republic of China 05:13.2 05:32.3 10:59.4 11:18.9 Manila, Philippines 05:11.1 05:29.9 --- --- Tokyo, Japan 05:11.3 05:30.4 --- --- Alice Springs, Australia 05:08.5 05:27.0 --- ---
The notorious black drop occurs near Second and Third Contacts. when the limbs of Venus and the Sun gradually separate (Second Contact) or merge (Third Contact). Thus, the contact timings made by observers even at the same site can differ by tens of seconds.
The literature sometimes blames the black-drop effect on Venus atmosphere. However, the same phenomenon occurs during transits of airless Mercury. Actually, the black drop is simply due to solar limb darkening and to the inevitable blurring of any telescope image due to diffraction and atmospheric seeing.
The one real phenomenon that is unique to transits of Venus is the ring of light or aureole that appears on the limb of Venus. silhouetted against the sky during ingress and egress. The effect is due to refraction of sunlight in the planet upper atmosphere. The aureole is far brighter than the horn extension mentioned earlier, but has never been photographed; all we know of it comes from written descriptions and drawings. Thus, obtaining photographs and electronic images of the aureole should be one of the highest priorities for the 2004 transit.
Some historical observers of transits of Venus have reported anomalous phenomena, such as deformations of the planet limb, areas or points of light within the dark hemisphere of Venus, or a halo of light, much wider than the aureole, around the planet when fully on the disk of the Sun. Such phenomena are almost certainly due to contrast effects and light scattering within our atmosphere, or intrinsic to one telescope, eyepiece, filter, or eye itself.
Observing the Transit of Venus
Observing a transit of Venus involves looking at the Sun, so every safety precaution that one would use for a solar eclipse or sunspot observation should be taken when watching or imaging the transit. The two safe methods are either:
There has been some debate whether Venus can be seen against the Sun simply with one eyes--safely filtered of course. Actually, there are numerous reports from the last transit, in 1882, that many people succeeded in this. The writer has confirmed this by viewing the sun image reflected in a mirror with a black spot the same apparent size as Venus during transit (about 1 arc-minute).
However, you may want to use some additional optical equipment, particularly if you make a special trip to see the event.
Using a minimum of equipment, you should be able to comfortably watch the event with binoculars--if both lenses are securely filtered. Likewise, you should be able to photograph the transit with a filtered telephoto lens--say 200 mm or greater. Exposures will be short, so an equatorial mounting is unnecessary.
However, if you want a chance of seeing or recording the black drop or the aureole you will need a telescope. Here, our most recent observational experience comes from the transits of 1874 and 1882! Most of the telescopes used by the 19th-century expeditions were in the aperture range of 3 to 8 inches, with 5- and 6-inch clock-driven refractors the most popular. Observers also used Newtonian reflectors, particularly if they lived in the visibility zone and ob served from home or an existing observatory.
(1) Observing Venus off the Photosphere. The possibility of observing Venus horn extensions during the days before and after the transit has already been described. In addition, on the day of the transit itself, Venus may be seen off the Sun photosphere (surface visible in white light) both before First Contact and after Fourth Contact. Actually, if one has access to a coronograph (a rare form of telescope used to observe the Sun corona outside eclipses) one might spot the planet hours before and after the official time of the transit. More probably, using one of the much-more-common Hydrogen-alpha filters, Venus might be seen perhaps as much as a half-hour before First Contact or after Fourth Contact, silhouetted against a prominence or the Sun's chromosphere.
(2) Visual Timing of Transit Contacts. This was the most popular form of observation during the historical transits because contact time differences among the widely separated observing stations could be deduced to calculate the value of the solar parallax and thus the distance of the Earth from the Sun. Today we know this value from other methods to a very high degree of precision, so that contact timings no longer serve their original purpose. Nonetheless, observers may wish to conduct such timings as an educational exercise, in particular combining their timing with those of others to see how closely their results compare with predicted times and with the modern value of the solar parallax. Second and Third Contacts are the best to time, with Second Contact considered as the moment during ingress when the filament between Venus and the Sun limb breaks, when the two limbs are first clearly separated. Likewise, Third Contact is considered the moment when the filament forms again during egress. These moments should be timed to one second precision in Universal Time (UT), with the time standard either a shortwave time signal (e.g., WWV) or a GPS signal. Note that such a project is being coordinated by the European Southern Observatory (see their website: http://www.eso.org/outreach/eduoff/vt-2004/).
(3) Recording the Appearance of Venus During the Transit. There are several means of recording the appearance of Venus during the transit (or even before or after it--see above), especially for the critical ingress and egress phases.
Basic Documentation.--What will be particularly useful will be records of transit phenomena--visibility off the photosphere, the aureole, the black drop, and any anomalous appearances (see above). Whatever the form of one observational record, its scientific value depends on adequate documentation. What is needed for all forms of observation is: observer name and postal address (and email address if applicable); latitude and longitude or observing site (to 0.01 degrees or one arc-minute); description of instrument (eye; or aperture of telescope or aperture and magnification of binoculars); description of filter used (or of projection method); atmospheric transparency (on a 0-5 scale, with 5 best and 0 worst); and Universal Time of each remark, drawing, or image accurate to one second if possible.
Visual Observation.--Visual observations can take the form of remarks (including contact timings) or drawings. Refer to celestial directions either using the words north, south, preceding (the direction of drift with one drive turned off), or following (the opposite of preceding); or as celestial position angle (measured from 0 degrees for celestial north counterclockwise 360 degrees through celestial east, south, and west). In referring to directions, correct for the reversal that occurs if you used a right-angle viewer for direct visual observation. Note that with projection the Sun's disk will normally be reversed, but will be correct with the use of a right-angle viewer. The orientation of drawings should be clearly indicated. Besides the basic documentation above, give the magnification used (as well as the projected disk diameter if you used the projection method), and the atmospheric seeing on the 0-10 scale (0 is worst and 10 is perfect).
Still Photography.--Film or digital still photographs taken with a suitably-filtered telephoto lens should show Venus while in transit. The camera can be mounted on an ordinary tripod because the exposure will be very short. However, to capture any of the transit phenomena mentioned earlier, a telescope will be necessary, along with a suitable full-aperture filter. The duration of the exposure can be judged at the time of the transit with a digital camera, but for a film camera should be found previously by experimentation with the Sun, and with the same optical arrangement. Remember that the delicate aureole will probably require a longer exposure than for the Sun's photosphere.
Even with a safe filter, the Sun's image will be so bright that a higher-quality medium-speed film or digital camera sensitivity setting (e.g., ISO 50-100) can be used. The exposure may still be so short that a clock drive will not be necessary to prevent blurring. Still, a clock-driven equatorial mounting will be a great asset in tracking Venus during the six hour-long transit, although the planet's motion will make it necessary to make frequent adjustments.
CCD Imaging.--Recording the transit, particularly ingress and egress, with a CCD camera will allow photometry of the aureole and the black drop. This is because taking flat frames and dark frames allows one to correct the image itself for background noise and variations of sensitivity among the camera's photosites. Then the CCD response is close to linear, and one could, for example, use the mean brightness of the center of the Sun's disk as a standard. For those traveling to a site, a possible disadvantage of using a CCD camera is that it must be connected to a computer. Another disadvantage is that most CCD cameras take monochrome (grey-scale) images, and the to color images one must take three images in succession through different color filters.
Video.--The disadvantage of taking still photographs or images is that there always is an interval between successive images, which may be a minute or more when making colored CCD images. Thus the observer may miss rapidly changing events during ingress or egress. Video allows continuous coverage at a typical frame rate of 30 frames per second. Admittedly, analog-video frames are noisy, and several must be stacked to obtain an acceptable image. Results are definitely better if one uses a digital video (dv) camera, or records onto a digital-format recorder from an analog camera.
Webcams.--Webcams provide a stream of digital images, thus also supplying continuous coverage. Excellent images of the May, 2003, transit of Mercury were obtained with webcams, so this medium has great potential for the 2004 transit of Venus. As with CCD cameras, Webcams need to be attached to computers.
Stacking Images.--This process uses a computer to register and combine anywhere from a few to thousands of individual digital images, with either manual or automatic selection of the best images to use. The stacking process can be carried out anytime after the images themselves are acquired. It is possible to stack digital still-camera or CCD images, but stacking is most effective when using the many images provided by video cameras or webcams. The final result is typically far better in resolution and contrast than even the best individual frames. However, the gain in spatial resolution is had at the expense of time resolution because one necessarily stacks images taken over a range of time. As Venus will be moving in relation to the Sun by about one arc-second every 20 seconds, it would be wise to stack no more than a few seconds worth of images at a time.
General Comments on Photography and Imaging.--Small-scale views that show Venus' position in relation to the Sun's limb or to sunspots or other solar features will provide an interesting and visually striking record of the transit. Multiple exposures, perhaps combined with computer image processing, will give a record of the planet's track across the face of the Sun.
Larger-scale views of Venus in relation to the Sun's limb, taken simultaneous at widely-separated stations, can be combined to give a three-dimensional view of the transit.
For transit photographs or images to be of scientific value in recording such phenomena as the aureole and the black drop, a large image scale is necessary, even to the extent that Venus covers a significant portion of the frame. With moderate-size telescopes this will require either afocal imaging at high magnification or imaging directly on the film or chip with eyepiece projection or Barlow-lens extension of the effective focal length.
Computer processing is possible with all forms of imaging. Photographs can be scanned and converted to digital images and analog videos converted to digital with an analog-to-video frame grabber. Digital video, digital still-camera, CCD, and webcam images are digital to start with. Common enhancement techniques include contrast stretching and sharpening by such methods as unsharp masking. Computer enhancement should be done cautiously because it can create artifacts, such as a false light ring around the planet, or a bright spot on its dark hemisphere. Certainly, the observer should always retain copies of all digital images in their unenhanced (raw form, and should document all enhanced images with comments on the types of enhancements used.
Besides the basic and specialized documentation already described, all still photographs and images, as well as video or webcam images, should be documented with the Universal Time of exposure, exposure time (shutter setting), and effective focal length of the optical system. Naturally, it is important to note if the photographs or images are reversed.
If you observe the 2004 Venus transit, your efforts will have permanent value only if they are communicated. All forms of observation--written notes, drawings, photographs, electronic images--along with the necessary documentation, should be sent to:
John Westfall A.L.P.O. Mercury/Venus Transit Coordinator P.O. Box 2447 Antioch, CA 94531-2447 (email: firstname.lastname@example.org )
More information on transits is available. There have been three books on the topic in the last four years, and at least one more is in press:
Eli Maor. June 8, 2004: Venus in Transit. (2000) Michael Maunder and Patrick Moore. Transit: When Planets Cross the Sun. (1999) David Sellers. The Transit of Venus. (2001) William Sheehan and John Westfall. The Transits of Venus (expected publication, March, 2004.)
There are many websites related to the coming transit. A small selection of them follows; note that most of the websites listed have links to other sites.