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Frequently Asked Questions (FAQ) |
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on question to jump to answers below.)
A transit of Venus is the observed passage of the planet
across the disk of the sun. The
planet Venus, orbiting the sun “on the inside track,” catches up to and
passes the slower earth. Venus,
appearing as a small dot in the foreground, will move from left to right across the
sun.
The word “transit” means passage or movement—in this case, across
the face of the sun.
http://sunearth.gsfc.nasa.gov/eclipse/transit/venus/Sun2004+2012-1.GIF
Path of Venus across the sun's disk; from Fred Espenak.
When
is the transit of Venus?
The next transit of Venus is June 5-6, 2012, as timed from the
prime meridian at Greenwich, England. From that part of the world, the
transit begins on the latter part of June 5 and continues into June 6. The exact
time is dependent on your latitude and longitude because of parallax (the
feature which gives the transit its scientific importance). The transit
begins on June 5, 2012, around 22:09 Universal Time (U.T.) and ends June 6 around
04:49 U.T. Note that 04:49 p.m. on June
6 in Greenwich is
actually late at night on June 5 for most North American time zones.
For much of North America, the transit begins in the evening on June 5, 2012, but the sun sets
before the transit is over.
http://home.hetnet.nl/~smvanroode/index.html
Find the times of all four contacts (that is, the four times when Venus
appears to touch the inside and outside edges of the sun) for your
location. If you know your latitude and longitude, local circumstances are
calculated courtesy of Steven van Roode.
http://sunearth.gsfc.nasa.gov/eclipse/transit/venus/city12-1.html
Contact times (Universal Time) and corresponding altitudes of the Sun for 121
international cities; from Fred Espenak.
http://sunearth.gsfc.nasa.gov/eclipse/transit/venus/city12-2.html
Contact times (Universal Time) and corresponding altitudes of the Sun for 60
cities throughout the USA; from Fred Espenak.
Is
the 2012 transit of Venus visible from my location?
Fred Espenak writes:
The global visibility of the 2012 transit is illustrated with the world map ( Low Res or High Res ). The entire transit (all four contacts) is visible from northwestern North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, Philippines, eastern Australia, and New Zealand. The Sun sets while the transit is still in progress from most of North America, the Caribbean, and northwest South America. Similarly, the transit is already in progress at sunrise for observers in central Asia, the Middle East, Europe, and eastern Africa,. No portion of the transit will be visible from Portugal or southern Spain, western Africa, and the southeastern 2/3 of South America.
http://sunearth.gsfc.nasa.gov/eclipse/transit/venus/Map2012-1.GIF
World visibility map of 2012 transit of Venus; from Fred Espenak.
Is
it safe to view the transit of Venus?
You may view the sun—and thus the planet Venus passing in front of
the sun—only if you use proper solar filters or indirect viewing
techniques. See http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html
for details, including links for purchasing solar viewing aids.
For an overview of the techniques listed below, see our Safety
page.
The preferred method
is to view a magnified image of the sun through a telescope that has a solar
filter on the large end. Do
not look through a telescope without a solar filter on the large end.
And never use small solar
filters that attach to the eyepiece (as found in some older, cheaper
telescopes).
You can project
the image with binoculars, but do
not look through binoculars unless you have proper solar
filters on the large ends. You can
also make a clever device that will project
onto a viewing screen the image of the sun.
Because the dot of Venus will be small
relative to the sun, pinhole projection techniques—suitable for tracking
eclipses—are ineffective for witnessing and timing the transit
of Venus.
While it has been suggested that you can use #14
welder’s glasses or darker, do not view through welding glasses
that transmit more light. Do not
consider the numbers to be additive; for example, do not use two #7 welding glasses on top
of each other to make a #14.
For many observers in the North America, the transit begins
near sunset on June 6, 2012. Do
not be lulled into thinking the sun’s intensity is significantly diminished
when it is low on the horizon. Severe
eye damage or blindness can result if you view the sun without proper eye
protection.
What
can I expect to see?
A magnified view (which demands a proper solar filter) will feature
the black dot of Venus slowly moving across the face of the sun from east to
west. The planet Venus appears to traverse the northern
half of the sun, moving from upper left to right.
The most important moment is when Venus touches the inner edge of the sun. Look for
the “black
drop” effect, when Venus sometimes appears to elongate near internal contact.
It is a phenomena that has frustrated astronomers and explorers since
1761. Because the edge of Venus
sometimes smears against the edge of the sun at this moment, observers have had a tough time
estimating the exact second when contact is actually made.
When looking through solar filters, also look
for sunspots scattered along the mid-section of the sun.
Notice how these solar blemishes often have a dark central portion
(umbra) within a less intense sunspot outline (penumbra).
When
else have transits been seen?
The first
recorded observation of a transit was by young Jeremiah Horrocks in 1639,
about three decades after the invention of the telescope. Since
then, transits have been witnessed in 1761, 1769, 1874, 1882, and 2004.
Why
is a transit of Venus so rare?
Transits of Venus have a strange pattern
of frequency. A transit will
not have happened for about 121 ½
years (prior to 2004, the last one was 1882).
Then there will be one transit (such as the one in 2004) followed by
another transit of Venus eight years later (in the year 2012).
Then there will be a span of about 105 ½
years before the next pair of
transits occur, again separated by eight years.
Then the pattern repeats (121 ½
, 8, 105 ½
, 8).
If Venus and the earth orbited the sun in the same plane as
the sun, transits would happen frequently.
However, the orbit of Venus is inclined
to the orbit of earth, so when Venus passes between the sun and the earth
every 1.6 years, Venus usually is a little bit above or a little bit below the
sun, invisible in the sun’s glare.
A similar thing happens with our moon. Every month the moon passes between the sun and the earth, yet we do not see a solar eclipse every month. That’s because the moon’s orbit is also slightly inclined to earth’s orbit, so the new moon is usually a little above or a little below the sun. The transit of Venus is essentially an annular eclipse of the sun by Venus.
What
is the significance of a transit?
For starters, transits help us to determine our place in
the cosmos.
Consider this brief history: In 1716 astronomer Edmond Halley calculated that you can
quantify the distance from the sun to the earth by
having observers across the globe time the passage of Venus across the sun.
Knowing he would not live to see the next transit, Halley predicted global sites
that would suitable for viewing a transit and called upon future generations to
pursue his plan. For the 1761 transit and every transit opportunity since then, explorers
sailed to distant lands to time the transit.
The quest
to time the transit of Venus in 1761, during the Seven Years War, marked the
first time the international community cooperated to answer one of the leading
scientific questions of the day.
By quantifying the distance from the sun to the earth, a
simple application of Kepler’s Third Law gives you the distances of all the
planets from the sun, and thus the scale of the solar system.
Today, the transit of Venus as a means to measure the sun-earth distance
is largely of historical interest, for tools such as radar have measured distances
to planets much more accurately.
In modern times, however, astronomers seek transits of
planets around distant stars. The NASA Kepler mission will look at 100,000 stars and try to detect
earth-class planets orbiting them. Once
again transits will help us to find out where we fit in the big picture of our
universe.
When you look at the planet Venus passing in front of the sun,
realize that you have a front row seat to the same phenomenon that the Kepler
spacecraft is trying to witness around stars that are light years away.
Realize also (especially if you have the misfortune of being on the night
side of the earth opposite the direction of Venus and the sun during the
transit) that somewhere out in distant space someone may be looking toward our
star and could detect two planets--Venus and Earth--passing in front of the sun.
What
can students do?
To recreate the historic endeavors to time transits from across the
globe, several institutions have established Internet observing projects.
Observers can time the transit from their location, submit the results to
a collective pool of data, and calculate the distance from the sun to the earth,
or the Astronomical Unit (A.U.). For
a simple activity, observers may track
the path of Venus to recreate the experience of Jeremiah Horrocks, who first
witnessed a transit of Venus in 1639.
See the Education
Resources for supporting activities and educational information.
Where
do I get more information?
Glad you asked. First,
this website was initially developed as a clearinghouse of information and images
related to the 2004 transit of Venus. An
abundant array of topics are linked from the Site Map at sitemap.htm.
A few key pages, listed below, have thumbnail images and brief descriptions of what
you can expect to find at the respective linked sites. Some of the material is basic, some of it is detailed or
technical.
Home
Page
2012 Transit of Venus
Education Resources
Science and Math of
Transits
Historical Observations
And Global Expeditions
Spacecraft and the
Search for Extra-Solar Planets
Images from 2004 Transit of Venus
Miscellaneous Transit Items
Site Map
As noted on the home page, "this website will guide you to instructions for safe viewing; interactive education and hands-on activities; global observing programs for students; background information and tutorials; insights into historical endeavors and the adventures of explorers; the role of spacecraft and the search for extra-solar planets; and miscellaneous items--some bizarre--relating to the transit of Venus."
As with anything posted on the Internet, not all sites
(including here) are guaranteed to have accurate information, as our
caveat suggests. We do ask that
you please respect the copyrighted
ownership of material where it applies. If
you find any resources or websites to which we should link, please forward
the address to us at transitofvenus.org.
We thank you in advance.
Three noteworthy books in print as of 2004 are The
Transit of Venus & the Quest for the Solar Parallax by David
Sellers;
The Transits of Venus by William Sheehan and
John Edward Westfall, and June 8,
2004--Venus in Transit, by Eli Maor.
What
have others said through the ages?
A collection of quotes hint at the rewards of witnessing a transit of Venus, as these samples suggest:
"Willingly would I burn to death like Phaeton, were this the price for
reaching the Sun and learning its shape, its size, and its
distance."
-Eudoxus
"Thy return Posterity shall witness. Years must roll away, but then at
length the splendid sight again shall greet our distant children's eyes."
Jeremiah Horrocks
"This sight...is by far the noblest astronomy affords..."
Edmond Halley
"That is the fate which often attends astronomers...exiling myself from
my motherland, only to be the spectator of a fatal cloud, which arrived in front
of the Sun at the precise moment of my observation, snatching from me the fruit
of my efforts and exertions."
Guillaume Joseph Hyacinthe Jean Baptiste Le Gentil
"Still, to have seen even a part of a transit of Venus is an event to
remember for a lifetime, and we felt more delight than can easily be expressed
at even this slight gleam of success."
Robert Ball
"No reader of this [Sky & Telescope] magazine will purposely
miss such a rare event--a chance to stand beside Edmond Halley and James Cook
and take a dip into the magic waters of astronomical
history."
David Levy
In his exhortation to future explorers/scientists, Halley wrote:
"There remains therefore Venus's transit over the sun's disk, whose parallax, being almost 4 times greater than that of the sun, will cause very sensible differences between the times in which Venus shall seem to pass over the sun's disk in different parts of our earth. From these differences, duly observed, the sun's parallax may be determined, even to a small part of a second of time; and that without any other instruments than telescopes and good common clocks, and without any other qualifications in the observer than fidelity and diligence, with a little skill in astronomy. For we need not be scrupulous in finding the latitude of the place, or in accurately determining the hours with respect to the meridian; it is sufficient, if the times be reckoned by clocks, truly corrected according to the revolutions of the heavens, from the total ingress of Venus on the sun's disk, to the beginning of her egress from it, when her opaque globe begins to touch the bright limb of the sun; which times, as I found by experience, may be observed even to a single second of time."