Thanksgiving

Canadians have many strange and bizarre habits. They buy milk in bags, describe a six inch snowfall as 'light' and have a chain of automotive stores that are also the place to go to for alarm clocks and space heaters. Then in the second week of October, they have a holiday devoted entirely to food.

Unlike the USA counterpart which remembers the arrival of the pilgrims in the New World, the Canadian Thanksgiving celebrates the harvest. I should mention I discovered this historical fact through google, since raising the topic at the lunch table merely produced a discussion surrounding plans for family visits and huge quantities of food. In common with their near neighbours, Canadians often travel to visit kin for the long weekend which meant many people were due to be out of town.

The exact count of absentees, however, was smaller than might be suspected due to the number of department members from other countries. The postdoctoral community in particular is very international, with five out of the seven postdocs in Physics & Astronomy coming from outside North America. This phenomenon is not limited to McMaster; while I was at the University of Florida, there was a standard joke that only one postdoc at a time was allowed to be an American.

Postdoctoral positions are short, normally between 1 and 3 years, and are designed to give young researchers experience by working with senior members of their field. Due to the nature of the job (skilled and fixed term), visas are relatively easy, if sometimes tedious, to acquire and therefore many people take the opportunity to work in another country. This is especially true in Astronomy, where the scientific instruments are incredibly expensive and the number of professionals in the field is relatively small (the International Astronomical Union has around 10,000 members). This results in strong collaborations between countries being an essential requirement of the work. Parts of Europe go as far as to expressly discourage their own nationals for applying to postdoctoral positions in their country, desiring them to gain experience abroad. This multicultural mix feeds through to the faculty level professors who, while all now settled in Canada, have their roots from all over the world.

From a personal perspective, this is something I enjoy most about being an Astronomer. Since graduating from my doctorate in 2005, I have worked in New York, Florida, Australia and Japan before starting my post here in Canada last November. The chance to experience life in a different country is truly amazing and something I have wanted to do since being a small child. Of course, there are downsides. Permanent faculty jobs are sparse and highly competitive and potentially moving countries every few years with a partner or family while you try and secure a position can take its toll. I have had several friends who left the field entirely for just this reason.

My immediate problem though, was not the country for my next job but the location for my dinner. It seemed to me that eating a modest meal on my own at Thanksgiving was clearly not embracing the culture. Fortunately, rescue was at hand in the form of Ben Jackel, a graduate student working on the creation of magnetic fields in disks around massive objects with Professor Ethan Vishniac. Ben volunteered his apartment for a Thanksgiving dinner, complete with a large ham. I joined Ben, his wife Margaret and two other graduate students, Tara and Leo, to more food than I could fit on a plate in one go.

Moving was more challenging than galaxy formation for the next 24 hours. I was told this was normal.

The moon illusion

"So why do you think the moon looks larger on the horizon?"

The question was posed by Rob Cockcroft, a graduate student who was also a presenter at the University's McCallion Planetarium. He was about to present a show dedicated to the moon and thought it possible he would be asked this question by one of the audience.

I blinked and looked up from my sandwich. "It does?"

I realized immediately I had just failed as an Astronomer. Yet the truth of the matter was that if an astronomical object appeared too large in my simulations, I had probably messed up the units in my calculation and caused the Universe to expand too slowly. Actual objects were not really my thing.

Fortunately the rest of the lunch table were more use.

"Isn't it because the moon is closer to other objects, such as trees, when it is low in the sky?" another graduate student asked. "Compared to those the moon will appear bigger."

This was a logical guess and one Rob himself had held until he had looked into the matter. The effect is known as the Ponzo Illusion and it is an optical effect that causes the human mind to judge the size of objects based on their background. Simple diagrams such as the ones shown here easily demonstrate this effect exists, but it is not the cause for the moon illusion, which has been proven to occur even on a featureless plain such as the ocean.

Another popular myth is the moon illusion is caused by the distortion of light in the Earth's air. It is true that as the moon sinks towards the horizon, you view it through a thicker layer of atmosphere, but the bending of light this produces actually causes the moon to appear smaller, not bigger.

In fact, Rob's investigations turned up no definite reason for the moon to appear larger; it seems no one knows for sure. The most common explanation, however, is that our brains have a view of the sky that is not a perfect hemisphere (like a planetarium) but a squished, shallower arc, more like a soup bowl. This causes us to believe that an object on the horizon is further away from us than when it was directly overhead. Since the moon's size actually hasn't changed, our brains assume that is must be larger at the horizon since it is apparently at a greater distance.

Quite why our brains would do this is a mystery. One argument suggests that we have evolved to be better at judging the size and proportion of objects that are close to us, since we are far more likely to be eaten by them than something at the bottom of a cliff or high in the sky. No one though, is entirely sure and some people reportedly don't experience the moon illusion at all. For those super interested, a good description of the moon illusion, including the flaw in the soup-bowl sky theory can be read here.

As I finished my lunch, I wondered if I was one of those rare people unaffected by the moon illusion. That would turn my folly at not knowing about it into the product of superior perception, not incompetence. It was a long shot, but I made a mental note to check next time there was a full moon.