Geographical Perspective on the Transit of Venus

June 5th, 2012 was the most recent Transit of Venus. The last Transit of Venus occurred June 8th, 2004. The next Transit of Venus occurs December 11th, 2117. The frequency of transits is no doubt interesting but the more proximate interest I have is how Venus was used to determine the size and scale of our Solar System.

An important geographical question is "how far is it?" Well, the answer to the question depends, depends on what "it" is. How far is it to your house? How far is it to the donut shop? How far is it to the beach? How far is it to the Sun? How far is it to Venus? The last two questions are necessary for determining the size and therefore scale of our Solar System. If we can determine distance to Venus we can then determine the distance to the Sun and thus we will have established our position among the inner most planets and we will have a new unit of measurement, the Astronomical Unit (AU).

[caption id="attachment_1040" align="alignleft" width="300"] The geometry can become far more complicated for an accurate answer but the method illustrated gets within 2%[/caption]

The process of using two locations to measure the distance to a single point is called parallax. We use parallax every day to determine distance. Our eyes are a positioned a few centimeters apart. The distance between our eyes coupled with the images our retinas capture provide our brains enough information for most of us to infer properties about the objects around us, the speed of a car, the distance to a stop sign, or the height of a building. The illustration (left) is from the notes of Daniel Fischer, University of Germany - Bonn (link). I found his explanation of the Transit of Venus precisely in tune with the level of understanding I wanted to communicate.

His geometric description of parallax requires little more than a high school appreciation of geometry or freshman level trigonometry course. By following his directions, one can determine the Astronomical Unit (AU), the distance of the earth from the sun and thus have a basic understanding of planetary distance relationships.

One of fantastic side-effects of understanding the process outlined in Dr. Fischer notes deals with exoplanets. Exoplanets are those planets which orbit distant suns (stars). By determining the distance of an exoplanet from its sun we can figure out if the planet exists in the "Goldilocks Zone," the zone not too hot and not too cold for life.

Another benefit of the method described in Fischer notes is our ability to use the idea of parallax to chart the distance to nearby stars. If we consider 1/2 the planetary orbit of the earth around the Sun as our baseline - the base of our triangle - and we collect two sitings 6 months apart, we can sort of calculate a distance. I say "sort of" since our measurement will not be entirely accurate. The Sun is moving through space and the target star is moving through space over the course of those six months, thus the geometry is changing.

I read recently in a cosmology textbook ("Your Cosmic Context") the error inherent in the calculations is minimal due to the shear distance involved. With NPR running two stories (here) and (here) regarding the use of parallax to determine the distance of the earth to the Sun I've been reading more about the use of parallax to measure all sorts of cosmic distances. Some astronomers and cosmologists point out very small changes in angles, merely being off a few arc seconds can create substantial errors in calculations.

I'm neither cosmologist nor astronomer but I do tell my geography students to pay attention to the latitude and longitude calculations as rounding digits can shift the points measured with a GPS significantly. If you have ever looked through a pair of powerful binoculars and noticed how fast your field of view can swiftly change with small movements of your head then you get the idea.

Back in the "day" explorers used parallax to determine distance to far away objects, lighthouses, islands, points, coves, all sorts of places. Navigators had to be really good at angles, distances, direction, and speed - otherwise known as "course." Tools like the astrolabe and secants were used to gauge position and if a series of positions were known then velocity could be calculated. Once velocity and positions were known then distances to places could be calculated. Navigators were very valuable people; they had to be smart. Early maps were developed by cartographers who were given information derived from parallax measurements. As our tools became better, magnetic compasses and precision chronometers introduced, better and more detailed maps were authored.

[caption id="attachment_1049" align="alignleft" width="240"] In 2013 the GAIA satellite will become active and ready to determine distances using parallax methods.[/caption]

The fact cosmic distances are so large should give us some pause. First, parallax only works well for close objects. Right now, the best tool for measuring these distances is the satellite Hipparcos, launched in 1989. Hipparcos can measures distances using parallax to only 1,600 light-years. In 2013, the European Space Agency's Gaia Mission (link) will be able to measure distances in the range of 10,000+ light-years using the parallax method. Astronomers and Astrophysicists and Cosmologist are constantly working out methods to accurately map out our solar system, our stellar neighbor, and our position within the cosmos. They can use radiation from different portions of the electromagnetic spectrum. They can use known properties of electromagnetic radiation to discern distances.

In fact, most cosmic objects cannot be measured using the parallax method simply because the objects are so very distant the parallax method won't work. We can see many extremely distance objects simply standing outside on a clear night. On a clear night, if one knows where to look, 100+ galaxies are visible to the naked eye. Galaxies, not stars in our own Milky Way galaxy but other galaxies outside our own galaxy. Boggling!!!

Now, imagine looking at any one of those galaxies. The light emanating from the galaxy had to cross intergalactic space to reach your eye. That galaxy is not hundreds of light-years away, nor thousands, nor hundreds of thousands of light-years - er…actually, depending on which one you picked it could be a couple of hundred thousand light-years away - but, if you elected to pick out the stunning Andromeda Galaxy, the light from Andromeda took over 2-1/2 million years to reach your eye.

Fascinating stuff!

On a more earthly note, knowing these distances is very humbling. Not only humbling, but also challenging to those with very strong conservation religious beliefs. Many Southerners with whom I am familiar hold dearly to the Biblical notion the earth is only slightly older than the personalities of people detailed in Genesis. According to Creationists, the earth is a little over 6,000 years old. If the age of the earth was merely 6,000 years we would only be able to see those objects in the cosmos 6,000 light-years away or closer. Judging by the shear number of objects and type of objects we see, 6,000 years is simply absurd and insulting to any intelligent person.

Make no mistake, I am not bashing faith or spiritual beliefs, I am bashing religion-based presuppositional nonsense, especially those viewpoints which purport the Bible to be a first-hand eyewitness account of Creation.

I suppose God could have "staged" everything, like a surprise party, decorating and getting all the galaxies and black holes and nebulas in place and stringing all the light rays into place and 6,000 years ago God turned on the lights and yelled, "Surprise!! Look what I did!"

I don't think so.

That's fiction of the Tolkien world-building fantasy milieu.

And, oh yeah, there were living people walking the earth more than 6,poo years ago, too. They did not have dinosaurs as pets, though. The Dinotopia world Creationists dream about didn't exist, either. I have to admit the idea is fun to think about in a purely speculative fiction sort of way.

Pax