May 30, 2020

Astro-Event: Venus in Transit.

The 2004 transit of Venus as seen by NASA’s TRACE spacecraft.

(Credit: NASA/LMSAL).

It’s almost upon us. If skies are clear next Tuesday June 5th-6th, you’ll be able to witness one of the rarest events in astronomy; the transit of Venus across the disk of the Sun. This event lasts almost seven hours and spans over three quarters of the world. A good swath of humanity (the largest ever) will see at least a portion of the transit… and unlike the annular eclipse a few weeks ago, you don’t have to be along a narrow track to see the action. If you can see the Sun from 22:09:41 UT June 5th to 04:49:31 UT on June 6th, you can see at least a portion of the transit. This stands not only as the top astronomical event of 2012, but one of the great astronomical events of the 21st century.

How rare is a transit of Venus? Rarer than an eclipse at a Korn concert; the last transit occurred in 2004, but the last one before that occurred in 1882. And the next one doesn’t occur until 2117, so it’s pretty safe to say that no one alive today will see that far off future transit, unless they perfect the technique of preserving heads in jars like in Futurama. In the current epoch, transits of Venus occur in pairs eight years apart that are separated by alternating spans of 105.5 and 121.5 years. This stems from the fact that 13 Venusian orbits (which has a sidereal year of almost 225 Earth days) very nearly equals 8 Earth years; in 2020, you can expect Venus to very nearly mimic its spring 2012 celestial path, but instead miss the edge of Sun by 24’. In the 20th century, no transits of Venus where seen.

Maps of the 2012 transit courtesy of Eclipse-Maps. (Click to enlarge).

This is will only be the 7th transit of Venus to be observed since Kepler realized they occurred in the 17th century. If Venus orbited squarely across the ecliptic, a transit of Venus would be a much more frequent affair, occurring at every inferior conjunction about 19.5 months apart. But since Venus’s orbit is tilted in respect to our own to the tune of 3.4°, transits are much more infrequent as it “misses” the solar disk. In fact, roughly every 4 years before or after a transit, the opposite occurs and Venus passes its maximum separation of about +5° degrees above or below the Sun and can actually be tracked through inferior conjunction, as occurred in 1998, 2009 and will be possible again in 2015 & 2017.

Planning ahead; an 1883 map of the 2012 transit by R. A. Proctor.

(Public Domain image courtesy of Eclipse-Maps).

Alas, Kepler never lived to observe the 1631 transit, and the first successful observation of the transit of Venus goes to Jeremiah Horrocks in 1639. Since that time, the race was on as expeditions fanned out all over the globe to accurately measure the ingress and egress of the planet and measure the scale of the solar system; such adventures are recounted in the fascinating books The Transit of Venus by William Sheehan & John Westfall and The Day the World Discovered the Sun by Mark Anderson. These were some of the first truly international scientific collaborative efforts and they make for a fascinating snapshot of the history of astronomy.

Historical sketches depicting the aureole and black drop effect. (Public Domain).

So, what can you expect to see next week? Well, observers in North America will catch the transit as the Sun is setting; this is a Pacific-based event with the maximum point (where the Sun will be at the zenith during the transit) a few hundred miles south east of Japan. Continuing westward, the Sun will appear ever lower in the morning sky on the 6th as the transit is occurring; Europe, the Middle East and north eastern Africa will witness the transit as the Sun is rising.

The crucial contact times of the transit (numbers corresponding to the map above) are;

June 5th (all times are worldwide and in Universal Time)

1- 22:09 ingress (exterior contact)

2- 22:27 ingress (interior contact)

June 6th:

01:29 Mid-transit

3- 04:31 egress (interior contact)

4- 04:49 egress (exterior contact)

As you can guess, the most “action packed” times will be when Venus first slides onto the solar disk and later slides off. Watch for the “aureole effect” or a ghostly halo encircling Venus as it enters the solar disk. Long reported by observers, it was only recently photographed for the first time during the 2004 transit. Venus has a substantial atmosphere, and this phenomena is caused by sunlight being refracted towards us through it. No aureole is seen, for example, when airless Mercury transits in front of the Sun. Incidentally, Mercury last transited the Sun on November 8th, 2006 (the only transit we’ve seen thus far)  and will next transit the Sun on May 9th, 2016. It will also just miss the Sun by <8’ later this year (more on that in November!)

It’s interesting how the transit pairs also serve as a “snapshot” of the technology of the day; for example, photography was barely in its infancy in 1882. Recently, the plates of David Peck Todd were stitched together into a charming “animation” of the 1882 transit. Even in modern times, off-the-shelf tech has grown immeasurably since the 2004 transit and I don’t doubt that some amazing photos will be flooding the ‘Net next week. It’s amazing to think that prior to 2004, very few images of the transit of Venus existed!

Another elusive phenomenon to watch out for is known as the Black Drop Effect. As Venus enters and exits the disk of the Sun around the time of interior contacts, a slight “elongation” or extension between the disk of Venus and the limb may be seen. More of an optical illusion, this effect has vexed astronomers of yore in their attempts to get ultra-precise timings of ingress and egress. What compounds the problem is the effect of limb darkening on the edge of the Sun. Can you do better than these olden-time observers? Sketching the black drop may be a fun project as well.

This transit will pass about 9’ from the center of the Sun’s disk, and the transit cord is 25’ long. Venus will appear about 1’ arc minute in size (the largest any other planet can appear from the Earth) and be moving at a rate of 1’ per every fifteen minutes and appear 1/32nd the angular diameter of the Sun. Venus passes aphelion on July 11th, 2012.

A peek at our “transit rig”… (Photo by Author).

All of the safety rules that were laid out during the May annular eclipse still hold true for this transit. Do not look at the Sun with the unaided eye or with a filter that’s not approved for solar viewing; projection or a filter that fits snuggly over the aperture end of the telescope are the best bet. We have made several homemade solar filters over the years using Baader filter paper; ten years after our initial 30$ purchase and six filters later, and we still have half a sheet left! In fact, this is how we’ll be observing, along with our trusty hydrogen-alpha PST scope;

Solar viewing glasses (NOT sunglasses!) and #14 welder’s glass are also approved for safe use. One caveat is in order; unlike an eclipse, viewing the transit of the Sun via a pin-hole projection will be difficult; a pinhole camera effectively has a focal ratio of about 100 to 1, and the projected image when focused will be extremely tiny; building a pin-hole mirror is your best bet.

Eclipse (or Transit) viewing… safety first!

(Image courtesy of Chou Hui-Chi @SumiKeiKi).

Transits of Venus also occur in seasons that, in the current epoch, fall either in June or December. The 2004 & 2012 pairing happens in June, while they are bracketed with December transits in 1874/1882 and 2117/2025. The path of this transit is most similar to the June 3rd, 1769 transit witnessed by Captian Cook’s expedition from Point Venus, Tahiti and will next have similar circumstances on June 9th, 2255. Incidentally, single (i.e. non-paired) transits can occur, as happened in 1396 and will occur again on December 18th, 3089. Eclipses also occur in lettered series similar to eclipse saros cycles, with the current actives series being “E” (2004) and “F” (this year’s). This current cycle wraps up with a brief polar transit on December 14th, 2854 AD.

As with the annular eclipse, several satellites will be able to see this historic event; although ESA’s SOHO satellite will miss the transit, NASA’s SDO, JAXA’s Hinode, and ESA’s PROBA-2 will all be watching as Venus transits the Sun. Though most of these will be doing their observations for pure “gee whiz” value, Hubble will be testing a technique for “teasing out” the spectra of sunlight streaming through the Venusian atmosphere by staring at Tycho crater on the waning gibbous Moon during the transit. Refining said technique may go a long way towards identifying exoplanet atmospheres.

And speaking of SOHO, the ESA satellite’s LASCO C/3 camera will be a great place to watch Venus “take the plunge…” it also passes just 0.26° degrees from the planet Mercury in the closest planet-planet conjunction of the year just days before the transit on June 1st. When is the last evening prior to the transit that you’ll be able to see Venus?

It’s also always interesting to attempt to nab a transit of the International Space Station in front of the Sun while Venus transits. 1st accomplished in 2004, your best bets for this transit look to be around the 23:42 UT pass over South East Asia and Australia;

A first look at the ISS path during the the transit of Venus ~23:42UT.

(Created by the author using Orbitron).

CALSky should have accurate path tracks for those who plan to chase the twin shadows of the ISS and Venus up by the end of this coming weekend.

Weather is always a crapshoot for most of the month of June. Although this year’s transit favors the sunward-tipped northern hemisphere, hurricane/typhoon season begins on June 1st and got off to an early start this year. Drier climes such as northern Australia and the United States south west have the best viewing opportunities, while areas such as the Asian Far East and our own home state of Florida offer chancier prospects.  The good news is, you only have to be able to see the Sun to see the transit; pristine skies aren’t required. Areas that have the transit directly overhead have the best chances of a clear view, while regions that have the transit rising or setting risk low cloud cover. Of course, a transiting Venus on the horizon with an interesting foreground is much more photogenic!

As of this writing, weather prospects for Astroguyz HQ in Florida look to be at about a 75% chance of cloud cover, which is typical for Florida skies in summer afternoons. Doubtless, the weather forecast will flip-flop all the way up to show time the evening of June 5th. Some good weather sites to watch for accurate cloud cover predictions and Skippy Sky and Clear Sky Chart.

Solar activity as of May 29th (Photo by author).

Solar activity is also another big question mark; while we’re rolling towards solar max for cycle #24 next year, the Sun as of late has only exhibited minor activity (see image). A dynamic, sunspot-speckled Sun would make for excellent photos, and most of what’s on the Sun’s disk currently will still be present during the transit about a week from now… the equatorial regions of the Sun rotate once every 22 days. Still, it’s always possible that a huge active sunspot region will make its presence known just before show time!

Interested in replicating those observations of yore? In 2012, the transit of Venus meets social media as an Australian-based group seeks to replicate the determination of the astronomical unit via the Delislean method at Transit 2012. The method is simple; follow @venusobs, register at their site, and note in internal/external ingress/egress contact times that are visible from your location and tweet ‘em to #venusobs. If enough measurements are gained worldwide, a fun and historical experiment will be replicated… I plan on using WWV radio for accurate time signals. Just imagine if Cook had had Twitter!

Stranded on the wrong side of the world (or socked in with clouds) and want to watch the transit live? The solar observing Global Oscillation Networking Group (GONG) will have the transit in its worldwide sights. Many live broadcasting sights will crop up towards the weekend, but this is your best bet!

Want to see something truly weird? If you head to Mars in 2086, you can see a transit of Earth, the Moon… and Phobos!:

Finally, don’t forget to simply observe this uber-rare spectacle and have fun. The transits of Venus serve as a collective snapshot of where science and society is and what we hope to gain from such a rare celestial dance. As we noted in our July 2012 Sky & Telescope focal point article, it’s sobering to wonder what the world might be like in 2117. Watch this space for our after-action report on the final transit of Venus for this century, and follow us on @Astroguyz at Twitter for all the latest updates!