Transit of Venus: A Rare Astronomical Event

Eight years ago in 2004, astronomers and lay people around the globe were anxiously waiting for a very unique astronomical phenomenon to occur. In fact, this event was so rare that no one then alive had observed it before. The last time it had occurred was back in 1882. This rare and fascinating event was a transit of our planetary neighbor Venus across the face of the Sun.

Transits of Venus occur in pairs and can be seen over a large area of the Earth’s surface. In 2004, residents in this area of the country were able to view the transit already in progress as the Sun rose. Despite some initial clouds, we finally did get some great views. (See my 2004 observing report on the Skyscrapers website, plus also find an expanded version of this article with more historical overviews there as well.)

Well, the second Venus transit of this current pair happens on June 5-6. In this hemisphere it occurs on the 5th. To prepare everyone for this rare event I will first present a brief historical overview on Venus transits in general, and I will provide the local observing circumstances with some tips on safe viewing.

Let’s first explore what a transit is. Simply stated, an astronomical transit occurs when one celestial object, such as a planet or a moon, passes in front of another celestial object. If you think about it, technically a solar eclipse is really a transit of the Moon across the face of the Sun! We just have another name for it!

But, let’s extend that analogy a little further. The Moon passes between the Earth and Sun and occasionally we observe a solar eclipse. Sometimes it’s a partial, sometimes it’s a total, and most times it doesn’t occur at all. It all depends upon the alignment of these three bodies! But why don’t we get a solar eclipse every New Moon? We would if the Moon’s orbit wasn’t tilted or inclined at an angle to the Earth’s orbit around the Sun. If all things were perfect, we would see a total solar eclipse and a total lunar eclipse every lunar month. It would become so routine that we probably wouldn’t pay much attention to it.

When Mercury and Venus pass inferior conjunction (between us and the Sun), most of the time they pass above or below the solar disk as seen from the Earth. This concept is simply stated here, but it took the greatest astronomical minds of the past to solve this great mystery. The process took much observation, dedication and deduction to organize the solar system design we have all come to know.

What effect does this fact have on the frequency of transits for both of these planets? With Mercury closer to the Sun than Venus, its passages across the solar disk are therefore more likely to occur! Between 1907 and our current day there have been 16 such transits. Not very rare. In fact, on May 7, 2003, we were well positioned here in New England to watch a portion of a Mercury transit. Unfortunately we were clouded out. Though a first-hand look is much better than watching an event on a television or computer screen, I did watch the event on the Web from several locations, including the Canary Islands, South Africa, Hong Kong, and Australia. Another Mercury transit was similarly clouded out locally on November 8, 2006.

However, when we talk about a transit of Venus, we are talking about a very rare astronomical event indeed (only 81 will occur between 2000 B.C. and 4000 A.D.). This rarity is due to Venus’ nearly circular orbit and its inclination. When the calculations are performed, Venus transits usually come in pairs (like Mercury), but the interval between the second transit of the pair and the next one is generally 122 years.

On June 8, 2004, Venus transited the Sun for the first time since 1882. Though I believe there is no new knowledge we can obtain from such an event these days, professional and amateur astronomers alike had been anxiously awaiting their chance to experience this rare occurrence. Like the old days when scientific expeditions were sent worldwide to observe these transits, some folks will still travel to far away lands to be ideally situated to watch the event in its entirety.

Like we did for the 2004 Venus transit, I and many fellow Skyscrapers members will be content to observe the second of this pair of transits on June 5 from right here in Rhode Island. (A quick note: Seagrave Observatory will not be open to the public for this year’s transit. Since the transit occurs with the Sun very low in the western sky, our tree-studded horizon will prevent any observations from Seagrave.) At the time of this writing the other local observatories have not yet finalized any observing plans.

Before we get to the local circumstances for this event and how to safely observe it, we first should examine why transits of Mercury and Venus were thought to be so important. Astronomers were trying to determine the mean distance from the Earth to the Sun, known as the astronomical unit (AU).

You can read up on more details at the Skyscrapers website about the early expeditions sent around the globe to position observers to make precise measurements and timings to calculate the solar parallax. (This measurement is an apparent shift in the position of Venus’ transit across the disk of the Sun due to its being observed from different locations on the Earth’s surface.)

Unfortunately bad weather plagued many an expedition (some things never change). Also, a phenomenon called the “black drop” effect made getting precise timings nearly impossible.

The black drop effect was something I was anxious to observe for the first time back in 2004. It looks like a drip about to detach itself from a faucet, or like the shape of a teardrop. As the black disk of Venus begins to cross the limb or edge of the Sun, it is a perfectly round dark spot –- blacker and more round than any sunspot. Just as Venus is about to enter fully (second contact) or begins to exit (third contact) the solar disk, a portion of the planet seems to elongate outward toward the blackness of space along the Sun’s limb (see accompanying graphic detail).

The effect can last for several seconds, depending upon atmospheric conditions, thereby preventing astronomers from obtaining precise timings of the beginning (ingress) and ending (egress) of the transit. Observations differed greatly, thereby throwing calculations off by millions of miles.

Here in the colonies during 1769, some of the first notable astronomical observations were made during the transit of Venus. Joseph Brown, a prominent citizen in the Providence community, learned of the upcoming event and sent for a telescope and other equipment from England. It arrived about one month before the transit. (This same instrument can be seen today in the John Hay Library at Brown University.) Much preparation ensued to determine accurately the precise latitude and longitude of the site from which they would observe the transit. This fact was critical if the solar parallax was to be determined and hence the calculation of the astronomical unit. As reported in Florence Parker Simister’s book, Streets of the City: An Anecdotal History of Providence, Brown set up a temporary observatory “on the crest of a hill one hundred feet to the east of Benefit Street between what is now Governor and South Water Streets, at the southern end of Thayer.”

Cloudless skies prevailed that June 3, 1769. Simister writes, “Crowds of curious spectators, and those interested in scientific matters, gathered at the observatory while Governor Stephen Hopkins, Dr. Benjamin West, and Joseph Brown looked through their instruments and made their calculations.”

Though the observations from Providence fell in line with observations made elsewhere, the scatter due to the black drop effect still produced an unacceptable error. Regardless, the Providence scientists did produce valuable results which not only were published in the Transactions of the American Philosophical Society, as noted by Steven F. Crump, Jr. in a Rhode Island History Journal (Volume 27, January 1968) article titled, “Joseph Brown, Astronomer,” but also were well received by the Astronomer Royal of Greenwich, England.

Unfortunately, for all intents and purposes, the use of transits to determine the scale of the solar system proved fruitless. The expeditions to far away lands did provide valuable scientific discoveries in other disciplines, not to mention the exploration of our world. For example, if you want to follow up on just one of these expeditions, read about Captain Cook’s voyage and his involvement with the transit of 1769.

During the transits of 1874 and 1882, photography was the new method of acquiring data to determine the solar parallax and to make other discoveries. However, simpler methods had already revised the AU to unparalleled accuracy, and although scientific expeditions were still funded for the purpose of research, very little new information was forthcoming. The one thing that did arise from the 1882 event was an increased interest and excitement of the event by the general populace.

In fact, Skyscrapers’ own Frank E. Seagrave (whose 8-inch Clark refractor telescope we own and for whom our observatory in North Scituate, Rhode Island, is named) observed and photographed the 1882 transit from his original observatory in Providence. Visit the Skyscrapers website to see a letter Seagrave sent to Professor Asaph Hall of the US Naval Observatory in Washington, D.C. You also can read a more detailed account of the transit that appeared in the December 16, 1882, issue of Scientific American that was most likely compiled from Seagrave’s submitted report.

The more recent 2004 transit did pique some interest by the public, but the early hour of the event on a weekday more than likely kept observations at a minimum. Besides, without properly filtered telescopes or cameras, observing the transit can pose damage to one’s eyesight. (More about observing tips later.) June’s upcoming transit may pose similar challenges.

Let’s now look at the circumstances for 2012’s upcoming transit of Venus on June 5. Unfortunately we will not be able to view the entire event from here in southern New England. Folks in Hawaii have the best seats in the house. I understand a few spacecraft will be observing the transit as well.

The following times have been specifically calculated for Providence. Though these times will vary by geographic location, the difference for locations throughout the New England region will only be a matter of seconds. (See accompanying graphic of the path of Venus across the face of the Sun.)

The transit of Venus begins at 22:03:32 UT (universal time; also known as Greenwich Mean Time). We are four hours behind UT in June, so locally that is 18:03:32 or 6:03:32 pm EDT. At that time, called first contact, the disk of Venus will begin to “notch” the Sun. This event will begin along the Sun’s top left edge. Venus will continue to encroach upon the solar disk. At 22:21:12 UTC (18:21:12 or 6:21:22 EDT), Venus will be entirely in front of the solar disk. This event is called second contact.

The “black drop” effect I mentioned in my 2004 report occurs at this time, so be vigilant to observe it. A portion of Venus’ dark disk will seem like it is dripping towards the solar limb, or edge, like a teardrop. Within a few seconds the effect will vanish. The transit’s progress will continue locally through sunset at around 8:17 pm EDT. Choose an appropriate site with an unobstructed western view to maximize your observation.

Now, several words of caution are necessary to state here. Do not stare or even attempt to take occasional glimpses at the Sun without protection! Just because the sunlight may be dimmed by the dense atmosphere when the Sun is low on the horizon, do not stare at it. Number 14 welders’ glass is one safe method to use. DO NOT use exposed film of any kind. This method is not safe.

Only if you are an experienced solar observer should you attempt to observe the transit with a properly filtered telescope or use the solar projection method. If you have never observed the Sun before this event, don’t start now! Don’t risk your eyesight due to an oversight or an outright mistake. Even if you have one of those department store refractors that often come with small glass or plastic filters, do not be tempted to use them. They have been known to shatter when exposed to the Sun’s concentrated image.
An inexpensive alternative is to purchase some solar eclipse glasses from a reputable vendor through the Internet. These glasses will provide a safe view of the Sun, but be sure to place your order early, as the transit will be visible to millions of folks across the United States. Even when observing the Sun using these glasses, observe caution as well. Caution will be the keyword of the day. You will not wish to ruin your eyesight by looking at the event unfiltered, for there will be many more astronomical events for your eyes to experience and enjoy in the coming years.

In addition, I’m sure there will be some websites available from which you can safely watch the event. Surf the Web ahead of time to see what sites will be offering this great event. I agree that this method is not the same as experiencing it firsthand, but if you can’t observe it safely, don’t observe it at all. If all else fails, and I mean the weather, you may have no choice but to pull up a chair in front of your computer screen to watch the progress of the event. In fact, once the Sun sets for us here, I will continue to monitor the transit’s progress on the Web from those locations farther to the west. We can only hope we will have our opportunity to see the last Venus transit until the year 2117!

Good luck in whatever observing method you choose, and remember to keep your eyes safe.

And, just in case you’re wondering what the current value of the astronomical unit (mean Earth-Sun distance) is, it’s 92,955,807.3 miles, plus or minus about 10 feet!

David A. Huestis

David Huestis
David Huestis
Entry Date:
May 1, 2012
Published Under:
David Huestis's Columns
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