**The Transit of Venus: Twice in a Lifetime**

**By Louis Mayo**

Where will you be on June 8^th^, 2004? If you don't know yet, you had
better decide quickly or you may miss one of the rarest events in
astronomy. "Where will you be?" is a question that astronomers have
asked for centuries on the rare occasion that the planet, Venus, passes
in front of the sun casting a dark silhouette against the bright solar
disk. Such a Venus Transit or Transit of Venus will happen again on the
8^th^ of June of this year, again on the 6th of June, 2012, but not
again until the year 2117. But besides just being a geometric rarity,
this celestial conjunction was hugely important for astronomers trying
to answer some of the most fundamental and central questions in
astronomy -- How far away is the sun? How big is the solar system? Is
the sun really at the center of the solar system? And How far away are
the stars? Upon these questions are built many of the pillars of modern
astronomy and it is with this humbling thought in mind that we begin
this story of discovery.

**A SHORT HISTORY**

For centuries, astronomers, philosophers, and theologians have sought to
place mankind and the Earth in their proper place in the universe. Early
attempts to do this were based more on philosophy and religion than
science with tested theories and observation. For much of written
history, astronomers lacked the tools needed to interpret their
observations of the sky. The magic of trigonometry coupled with
carefully made observations provided a powerful tool that could be used
to make the first scientific estimates of celestial distances.

The Greeks developed the first known model of the cosmos based on
systematic behavior of the stars and planets. Around 500 BC, Pythagoras
suggested that the Earth was round because a sphere is a perfect shape
and the gods would certainly not have made a celestial object less than
perfect. Around 400 BC, Plato suggested that the universe was Earth
centric and that all celestial objects moved in perfect spheres around
the Earth. Aristotle concluded that the Earth was a sphere because the
shadow cast on the moon during a lunar eclipse was curved and because as
he traveled southward, previously unseen stars emerged from the horizon.
He also believed that celestial motion was perfect and that the four
Earthly elements (fire, water, air, and Earth) were inherently flawed
and very different from the material and the physics that made up the
celestial world. It was Aristachus of Samos who first proposed a Sun
centered solar system. Around 300BC, he attempted perhaps the first
scientifically derived distance to the Sun (now known as the
Astronomical Unit or A.U.) by observing the 1^st^ quarter moon. At first
quarter, the Earth-moon-sun angle should be exactly 90 degrees. Then, by
measuring the moon-Earth-sun angle, one could, in principle calculate
the distance to the sun in Earth-moon units (figure ??). Aristachus
concluded that the sun was about 20 times more distant than the moon.
Unfortunately, the moon-Earth-sun angle was a very difficult measurement
to make accurately yielding a result a factor of 20 too small. However,
from this somewhat low estimate, he concluded that the sun was
approximately seven Earth diameters wide. He was then, the first to
suggest that the sun was very far away and much larger than the Earth.
With the sun then so large and observably bright, he reasoned that it
was logical that the sun, not the Earth was at the center of the solar
system. Though his logic was flawed, his conclusion was correct yet it
would be almost two millennia before this fundamental idea precept would
acquie a solid foothold in scientific circles. One of the most powerful
arguments against this sun centered model was the problem that if the
Earth did move around the sun, why did not the stars seem to change
position over the course of the year? It would take great minds of
people like Brahe, Kepler, and Galileo and better instruments to answer
this question.

In the 2^nd^ century AD, Ptolemy, a Greek astronomer, would develop the
theory of cosmic motion that would be the prevailing view, the bible,
the fundamental text that all astronomers would read and accept as fact
for over 1500 years. His great work, published in "The Almagest" was
based on voluminous and painstaking measurements of the position and
motion of the planets and the stars. In the Almagest, Ptolemy put forth
a model of the workings of the universe with the Earth at the center,
neither rotating nor revolving in an orbit and all celestial objects
including the sun orbiting the Earth on transparent crystalline spheres.
Of course, there were problems with his model. Why, for example, did
Mars as well as other planets, exhibit periodic retrograde motion? Why
did Venus change in brightness over time and exhibit phases like the
moon? If it revolved around the Earth in a perfect circle, Venus should
maintain the same apparent brightness. In order to account for these
confusing departures from observation, Ptolemy introduced the concept of
epicycles, smaller spheres that the planets orbited around as they moved
around the Earth. Over time, his successors would add more and more
spheres attempting to account for still unexplained departures from
observational evidence. It would take such great minds as Galileo with
his detailed observations of Venus and the moons of Jupiter around 1610
and Kepler through his work analyzing many planetary positional
measurements of Ptolomy and Kepler's mentor, Tycho Brahe to finally get
it right. The planets all orbit the sun in elliptical orbits. Though
many students of astronomy in 17^th^ century Europe still subscribed to
the Ptolemaic system, the stage was set to begin the quest for the one
of astronomy's holy grails -- the true size of the solar system and
eventually, distances to the stars.

The transit of Venus was to supply astronomers with the ideal tool to
accomplish the first of these feats, The solution to the Astronomical
Unit was a crucial element in their attempts to derive distances to the
nearest stars as well as the size of the solar system. Using the method
of parallax (see text box), Jeremiah Horrocks (viewing the 1639 transit)
and Edmond Halley (who developed the first detailed plan for calculating
the AU from a transit of Venus) were to lead the charge in deriving a
much improved sun-Earth distance.

**TEXT BOX WHAT'S GOING ON?**

All nine planets orbit the sun in the same direction though not all in
quite the same plane. The reference plane or "ecliptic" is defined by
the Earth's orbit around the sun. Venus' orbit is tilted by 3.5 degrees
to the ecliptic. Remembering that the sun subtends only about ½ degree
in the sky, it is easy to see why there is not a transit of Venus every
time Venus is at inferior conjunction (see diagram). Usually, Venus is
either too low or too high to be seen against the sun. The two orbital
planes intersect in two points called the ascending and descending
nodes. It is near these two points and only these two points that a
Venus Transit can occur. So Venus must reach inferior conjunction with
the Earth at one of these two points in their orbits. Since the orbital
periods of Venus (224.701 days) and the Earth (365.256 days) are in an
eight year resonance with each other (Venus makes thirteen revolutions
around the sun in the same time that Earth completes eight), you might
conclude that a transit of Venus would occur at least every eight years.
But because Venus reaches its ascending or descending node in slightly
less than eight Earth years (2921.11 days vs. 8\*365.256=2922.05 days)
Venus and the Earth must wait between 105.5 and 121.5 years to be back
in the proper alignment. Chuck Beuter's Paper Plate Astronomy
(http://analyzer.depaul.edu/paperplate/credits.htm) provides an easy and
fun activity demonstrating these concepts.

# JUNE 8^th^. 2004

On the 8^th^ of June, observers in eastern parts of Canada, United
States, and South America as well as all of Europe and Africa and most
of Asia will be treated to the rare transit of Venus across the sun. In
Eastern Canada, US, and South America; the sun will rise with Venus
already in transit. Observers here will see third and fourth contact and
be able to observe the transit for between about one (Florida) and two
and a half (Maine) hours (less the further west you go). Table ?? and
figure ?? give more details on the duration and locations of the
transit. At 05:13:29 UT, Venus will appear as a small dark sphere just
contacting the south western limb of the sun (1^st^ contact). Its disk
will subtend about one arc minute or about 1/30^th^ the diameter of the
sun so it will be easily visible using simple solar projection devices.
Venus will continue along a chord stretching from the south west to
slightly more north east ending (4^th^ contact) at 11:25:59 UT.

**RESEARCH -- WHAT'S LEFT?**

Almost all of the real scientific value in the transit is historical
providing the yard stick from which the universe could finally be
measured. However, I am aware of at least two research teams who will be
using the transit to probe Venus' thick atmosphere. Tony Mallama and ???
will use the small phase angle produced as Venus nears the solar limb to
explore the particulate properties of the Venusian atmosphere. Venus
should appear brighter at small phase angles due to MIE scattering of
\<100 micron size particles. Mallama et al. hope to model this function
and identify the material responsible for it as well as particle size
and abundance. Best guess is sulfuric acid droplets -- stay tuned!

Jay Pasachoff and Leon Gelob will use TRACE to study the black drop
effect in x-rays to confirm their mathematical model which explains the
origin of the black drop effect in terms of instrument parameters.

**HOW CAN YOU PARTICIPATE?**

Though real research potential is small, the wonder and excitement of
this event has not been lost on NASA who views the transit as a
marvelous opportunity to communicate with the public and provide
educational opportunities for teachers and students in astronomy and
space science through wide reaching multidisciplinary topics such as
math, technology, world history, geography, and even music. To
accomplish this, NASA has entered into partnerships with museums and
planetariums, amateur astronomers, researchers, and schools across the
U.S. and in Europe. One such partnership involves creation of an
observing certificate program for the Astronomical League
([http://astroleague.org](http://astroleague.org/)) where amateur
astronomers can gain recognition for taking measurements of the transit
and holding public seminars. The NASA Sun Earth Connection Education
Forum (SECEF) --
[http://sunearth.gsfc.nasa.gov](http://sunearth.gsfc.nasa.gov/) is
leading the development and deployment of these programs and you can get
in on the fun as a teacher, student, scientist, museum director or just
a generally interested citizen by going to
[http://sunearthday.nasa.gov](http://sunearthday.nasa.gov/) and
registering. Here you will find a mountain of information on the transit
including multimedia resources, activities, local events, educator
guides, historical and cultural connections, instructions for viewing
the transit, and information on the webcast which will be broadcast from
Athens, Greece on June 8th. You can also register online and receive a
packet of materials that you can use with your classroom, museum, or
astronomy club.

**TEXT BOX: MEASURING PARALLAX**

Parallax is a visual effect observed when close by objects are seen in
relation to far away objects. It occurs because of differences in the
viewing angle of the observer. To demonstrate this, hold your finger at
arms length. With one eye closed, observe where your finger appears to
sit against some more distant background. Now, keeping your finger at
arm's length, close the one eye and open the other, Your finger should
appear to move against the background. Blinking back and forth will
demonstrate the magnitude of the displacement. Now do the same thing but
with your finger held much closer to your face. You should observe a
larger displacement. This effect is called parallax and can be used to
figure the distances to celestial as well as Earthly objects. In the
case of the transit of Venus, Venus takes the place of your finger, the
solar disk is the backdrop, and observations from two different places
on Earth represent each of your eyes. The key to deriving the distance
to Venus lies in knowing the angular measure of the sun (about 30 arc
minutes or ½ degree) and the distance between the two observers. The
small angle formula then provides an easy way to compute the distance to
Venus (fig. ??). Once an absolute distance is known, Kepler's third law,
P\*\*2 \~ A\*\*3 can be used to figure the distances to all the planets
and to the sun (using AU and years as units, P\*\*2 = A\*\*3).(fig. ??).