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Science Sunday: The Transit of Venus

Do you remember what you were doing eight years ago on June 8th, 2004?  No?  Neither do I, but on that early day in the Northern Hemisphere’s summer, celestial bodies in our solar system were aligning.  The planet Venus’ orbit brought it directly between the Earth and the Sun and for six hours, Venus appeared to observers on Earth as a black silhouette against the solar disk.  It was an itty, bitty eclipse that dimmed the amount of incident sunlight touching the Earth.  You know, typical Tuesday stuff.

The transit of Venus is a rare astronomical event that occurs only twice every 243 years.  Venus has an average orbital velocity of 35.02 kilometers per second and overtakes the Earth, traveling at an average speed of 29.78 kilometers per second, in its orbital race around the Sun once every 1.6 years.  The elliptical path that Venus takes is also at a slight incline to the Earth’s, as can be seen below in Figure 1, so most of the time Venus passes by Earth above or below the Sun relative to Earth’s perspective.  If you want to work out the transit frequency for yourself, there’s actually a really nifty diagram that you can make out of a paper plate, designed by Kathryn Williamson of Montana State University.  The pattern goes like this: 121.5 years pass with no alignment, a transit occurs, 8 years pass, the second transit occurs, and then another 105.5 years pass without incident.  Wash, rinse, and repeat.  Even though the last transit happened only eight years ago, people will be making a bigger fuss this time around because the position of Venus at sunset on June 5th through sunrise on June 6th will be this century’s last transit.  That’s in two days, people, so mark your calendars.  The next transits of Venus will occur in December 2117 and December 2125.  The weeks leading up to the transits are also the optimal time for viewing Venus at night since the planet comes out from behind the Sun so close to Earth during this time.


Diagram of the orbits Venus (shown in blue) and Earth (shown in green) take around the Sun (center, yellow) with Venus orbiting at an incline relative to Earth

Figure 1: Orbits of Venus (blue) and Earth (green) around the Sun (center, yellow). Note that the positions of Venus and Earth in the bottom diagram, that shows the alignment necessary for a transit, are different than their positions in the top diagram, that shows a missed transit.

Source: Wikipedia


To see the transit of Venus, first you need to figure out where in the world you’re going to be when the transit begins and ends.  Some areas of the world won’t be able to see the event at all because the entire transit will occur at night when observers standing on Earth will be pointed away from the Sun and Venus.  The map below in Figure 2 shows what portions of Earth will get to see the transit and how much.  The best areas for viewing are in eastern Australia, in eastern Asia, in the northwestern portion of North America, and over the western half of the Pacific Ocean.  If you live in the southeastern part of South America, the western half of Africa, or in a boat in the western part of the Atlantic ocean, you’re out of luck.  The Transit of Venus blog also has a Google map setup that can calculate your local transit times.


Figure 2: Map of the world transposed with the visibility of Venus’ 2012 solar transit on June 5th and June 6th.  Areas in the brightest portions of the map will have the opportunity to view the entire transit and areas in the darkest portions won’t be able to see the transit at all.  Sorry, Antarcticans.

Source: Sky & Telescope Magazine (with bigger map)


There are a number of ways to safely observe the transit, depending on your resources and budget.  Staring into the Sun to watch the transit is a big no-no unless you own a pair of glasses that have been fitted with special solar filters.  The lenses look dark but they are not sunglasses.  Wearing ordinary sunglasses to shield your eyes while looking directly at the Sun is like wrapping yourself with toilet paper to keep yourself warm on vacation in Antarctica.  The Sun, seen below in Figure 3, emits a ton of radiation across a broad spectrum of wavelengths, some of which are filtered out by the Earth’s atmosphere but the rest hits the surface in the form of infrared, visible range, and ultraviolet waves.  This is more than enough heat to bake your retinas before you can realize that you suddenly can’t see.


Photograph of the Sun taken by the Solar Dynamics Observatory (SOD) at 304 angstroms, in the extreme ultraviolet.  The Sun appears as a bright orange sphere against black space.

Figure 3: AAAHH OWWW @#!*& MY EYES!

Source: NASA Solar Dynamics Observatory (SDO) Chanel AIA 304


If you own a refractor telescope or know someone who does, you can make or buy a closed-loop device for the eyepiece that will project a beautiful, magnified image of the Sun on a piece of fabric.  Chuck Bueter has compiled a complete list of viewing options, that include tutorials and safety tips, on the Transit of Venus blog.  If you don’t have access to any of these things and live in an area that has a local astronomy club, check to see if they’re hosting a viewing party.  If you’re someone who’s planning on hosting a party, you can make a submission to the transit of Venus event registration page, set up by NASA and Sun-Earth Day, so that other locals in your area can find it.  It is important to never, ever, ever, ever look through the eyepiece of an optical device pointed at the Sun unless you have checked to make sure it is correctly fitted with a filter that will block out most of the solar radiation.  Remember that the lenses of these devices are designed to bring visible light and other forms of radiation into acute focus at a single, concentrated point of energy.  Don’t leave any optical setups unattended or pointed at the Sun for an extended period of time because you could set something on fire.  Solar viewing is cool and all but it would be kind of a bummer if you lost your house or destroyed your lawn.   A clear sky will probably help out the most with viewing the transit but that’s something more or less out of your control.

If you miss the transit of Venus and never get to see the black spot move across the surface of the Sun in real time, honestly, that’s okay.  Don’t beat yourself up over it.  The transit of Venus was a major event in early eighteenth century astronomy because it was the only known way of securing accurate measurements for the distances between Earth, Venus, and the Sun at the time.  You might have heard about a math technique called triangulation.  Let me explain it to you: say you want to measure the distance between yourself and a distant, faraway object, but you have a physical limitation that prevents you from taking out a measuring stick and crossing the distance from where you stand at point A to where the second object is at point B.  Even though you can’t reach the object at point B, you have some mobility around your standing point A.  Due to a perceptive quirk known as parallax, when you move from point A to a third position, point C, the object that you’re observing at point B will appear to have shifted.  In astronomy, the distance between the observer at point A and the object at point B is often so great that angles can be approximated when using trigonometry to solve for the calculation with only two numbers: the distance between the observer’s standing positions at point A and point C and the distance of the parallax shift from point B to point D.  In 1716, Edmond Halley figured out how to use the timing of the transit of Venus, as seen at different points on Earth, to calculate the distance of the parallax and thus the distance between Venus and the Sun.  Since Halley would be dead before the next transit in 1761, he had to be content with leaving the task up to future astronomers.

Since the concept is a bit difficult to wrap your brain around, here’s a brilliant video that explains how parallax and other measurements are used today in astronomy:

The numbers collected during the transits of Venus in 1761 and 1769 were used to draw the first ever semi-realistic portrait of the solar system.  Keep in mind that up until this point, previous estimates had placed the Sun at distances less that twenty times the distance between Earth and the Moon.  To give you an idea of how outrageous this underestimate is:  According to NASA, the Moon is 378,000 km from the Earth’s equator and that distance multiplied by twenty is 7,560,000 km.  This estimate places Earth inside the orbit of Mercury, since the actual average distance between Mercury and the Sun is 57,910,000 km.  Just in case you don’t do things like go outside or drink water and wouldn’t otherwise know: we are not inside the orbit of Mercury.  We would be fried.

If you’re interested in reading more about the historic significance of the transit, there are a number of new books that have come out to celebrate this year’s event, including Andrea Wulf’s Chasing Venus, Nick Lombe’s Transit of Venus: 1631 to the Present, and Mark Anderson’s The Day the World Discovered the Sun.  Jennifer Ouellette has written an amazing piece for her blog about the heroic lengths astronomers took to secure triangulation measurements for the 1761 and 1769 transit of Venus.  The perils of global travel back then meant that numerous people had to risk their lives to ensure that these measurements would be taken in time.  Now there’s an iPhone app that anyone in the world can download and use to record and share measurements of when the 2012 transit starts and ends.  It’s funny how things work like that.

This year, observing the transit of Venus is more of a public outreach service for astronomy than anything that will shatter the face of astrophysics as we know it.  The Hubble Space Telescope will be aiming cameras at Earth’s moon in hopes of detecting the minute dip in reflected sunlight during the transit and thus more information about Venus’ atmosphere that can lend itself to exoplanet research.  It’s good to give a nod to the event’s history, given that science is a cumulative effort built on a foundation of hard work done by prior generations.  If the weather’s favorable, make yourself an ice cold drink of tea or lemonade on June 5th or 6th and sit outside to bask in the ever-so-slightly-diminished glow of Venus and the Sun.


Further Reading:

Women scientists and the transit by Randall Rosenfeld

Sic Transit Venus by Jennifer Ouellette

Six ways to see the transit by Chuck Bueter

Viewing the Transit & Eye Safety by B. Ralph Chou (note: scroll down the page a little to see the article)


Article’s header image is a photograph of a transit phenomenon called the aureole, from from the Transit of Venus blog.

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Alyssa is a recent liberal arts graduate with a B.A. in physics and a background in wrangling high-vacuum equipment. She is an aspiring science writer who writes when she's not busy hunting for full time work in nanomaterials and renewable energy. She likes to get her energy by drinking coffee and absorbing electromagnetic radiation from the Sun.

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