Sunday, 16 December 2012

The Science of Christmas

The latest in the University of Nottingham’s Public Science Lecture Series was entitled “The Science of Christmas” and presented by Michael Merrifield, Professor of Astronomy, Faculty of Science.

Prof Merrifield took a nurmber of iconic images from traditional Christmas scenes and looked at some of the science behind them, beginning with….

Christmas is in the middle of winter (for northern hemisphere residents, at least) and the Prof took a moment to consider why that was.

Surprisingly, there are many people who still believe that the reason it is cold in winter is because this is the time at which the Earth is furthest from the Sun.

This is wrong on a two levels. Firstly, of course, the seasons are actually caused by the tilt of the earths axis, but secondly, winter is when the earth is actually closest to the sun.

The Earth's orbit around the Sun

The Prof also explained that, aside from the reduced hours of daylight, temperatures were lower in northern latitude winters because the sun was lower in the sky, resulting in its heat being spread over a larger area, as shown below:

How sunlight at northern latitudes is spread out over a larger area

Christmas Trees
Another icon of the UK Christmas season is the appearance of Christmas trees, which were actually introduced into the country relatively recently by the Royal family due to their connections with Germany.

Prof Merrifield had wondered why it was that fir trees had their characteristic triangular shape, and a little research revealed that this was because, essentially, the branches and trunk grew at relatively constant rates - meaning that the branches that had been growing for the longest time (the ones at the bottom of the tree) were also the ones that had reached the longest lengths.

In addition, a triangular shape meant that the trees could catch a larger amount of sunlight.

But if this was so advantageous to fir trees, why don’t all trees have this shape?

Prof Merrifield admitted that he initially had no idea. A look on Google revealed that this was one the few questions to which Google does not know the answer, no matter which search terms you use.

However, one of the advantages of working in a University was that there was someone, somewhere, was an expert on pretty much every subject - and in the University of Nottingham that someone turned out to be Marcus Eichhorn, Lecturer in Ecology, Faculty of Medicine & Health Sciences

Marcus explained that there was a fundamental difference in the water transport mechanisms of fir trees and deciduous trees. Fir trees could take up water through the whole thickness of their trunks, while deciduous trees could only do so through the outer part of the trunk.

His meant that deciduous trees had to manage the location of their branches more carefully - too many branches too low down would mean that there was not enough water for branches higher up the tree.

The Christmas Star
The star on the top of Christmas trees refers, of course, to the Christian story of the Star of Bethlehem, and there have been many attempts to determine what this star might have been.

The artist Giotto was one of many who thought that it might have been a comet, as shown in his 1306 fresco “The Adoration of the Magi” nativity scene has a comet arcing across the sky. It seems likely that Giotto used this image because Halleys comet had passed just a few years before he painted this work.

The Adoration of the Magi by Giotto

And some measure of the influence of the paining can be gained from the fact that the European space probe sent to intercept Halleys comet was called “Giotto” in recognition of the artist. (although the closest pass of Halleys comet was in 12BC, which is a few years too early for it to have been the Star of Bethlehem.

The astronomical records of China and Korea, both keen stargazing nations, recorded some kind of appearance in the sky in 4BC (which is pretty much exactly the right time). They noted that the object was fuzzy in appearance, did not move, and was visible for a long period of time (perhaps 70 days).

An astronomer named Tipler recently suggested that the story of the Star of Bethlehem should be interpreted literally, which implied that the star was directly overhead. It turns out that there are not many things in that part of the sky at the latitude of Bethlehem, but one of the things that is there is the galaxy Andromeda and Tipler proposed that there was a supernova in Andromeda at that time, and further, that it must have been a Type 1a or Ic supernova as any other types would have been too dim to see, or so bright that there would have been more widespread records. Tipler also said that its remnants could still be discerned, given the right equipment and technology (which astronomers do not yet have).

Prof Merrifield put up a picture of the Andromeda Galaxy as he told Tiplers story, touchingly saying that the only reason he mentioned this rather far-fetched theory was that it gave him an excuse to show a picture of a galaxy as they were so beautiful.

NSB wonders how much more beautiful they must be for astronomer like Prof Merrifield, who understands their complexity, scale and jaw-dropping physics, than they are for ordinary Joes like NSB.

And, let there be no mistake, NSB thinks galaxies are very, very beautiful indeed.

The Beautiful Andromeda Galaxy

Continuing with a look at candidates for the Star of Bethlehem, one of the most likely possibilities is a so-called “triple conjunction” of Jupiter and Saturn.

This is where Jupiter and Saturn appear to move back and forth across each other three times. The phenomena is caused by the fact that the Earth has a faster orbit that Jupiter or Saturn so periodically “overtakes” them. When this happens they appear to move backwards in the night sky for a period of time.

An Astronomical Conjunction

A triple conjunction fits a number of criteria for being the Star of Bethlehem in that :

i) It is a known astrological event
ii) It involves significant objects (Saturn was a sign of Kingship)
iii) It is visible to the naked eye
iv) It is predictable - so the “Three Wise Men” could have set off on their journey well ahead of time
v) It is rare, the last one being in 1998 and the next in 2238.

There is also an astronomical link to Gold, one of the presents brought by the “Three Wise Men” in that all the Gold on the planet (and indeed all other heavy elements) have been formed in Supernovae that occurred billions of years ago.

NSB can feel his brain melting every time he thinks about this for more than a few seconds.

One might think that there is little or no science behind presents, but one would be wrong..

The Prof pointed out that a little digging in the literature had revealed that there was a whole branch of economics called “Scrooganomics” which dealt with the economics of present giving. This school of thought suggested that giving presents was economically very inefficient as most presents were not what the recipient wanted and ended up being thrown away or lying unloved at the back of a cupboard. It would be better, this theory suggested, for everyone to simply spend the money on themselves as this way they would at least get what they really desired.

On the other hand, Prof Merrifield suggested, there was another view which held that whilst many presents weren’t what the recipient wanted, a few were presents that the recipient really did want but didn’t know it. The Prof gave an example from his own experience when he had been a little unnerved on receiving a present of a parachute jump (not least because he was afraid of heights) but he took the plunge, as it were, and discovered that he actually liked parachuting very much, to the extent that he continued jumping out of planes for several years thereafter.

Lastly, a paper entitled “Asymmetric beliefs about gift giving and feelings of appreciation” by Flynn and Adams was mentioned as this had investigated how present givers and receivers felt about the price of a present. It turns out that present givers attach a lot of value to the price of a present, while present receivers do not really care about the monetary value, only that the giver has actually given them a present. The moral of this research, suggested Prof Merrifield, is to buy cheap!

The symbol of a snowflake is another icon of Christmas time and Prof Merrifield, in an impressive jump from astronomy to chemical physics, did rather a good job of explaining why ice has a six fold nature and why ice crystals grow in a hexagonal way. The explanations for these phenomena, which are much easier told by way of diagram than by words, can be found at the “Story of Snow”.

Incidentally, microscope images of snowflakes show that they DO NOT have exact symmetry on all six sides.

Snowflakes, not quite symmetrical

Santa Claus
Inevitably, the last section of the talk looked at the science behind Santa Claus, or Father Christmas as he is often known.

Here comes the maths part :

Assume 2 billion children in the world, and 900million homes Distance : If all the homes were spread evenly over the surface of the earth (~480million km2) the distance between each home would be about 0.7km apart, making a total distance that Santa has to travel of 600million km.

Time : Assuming that Santa takes full advantage of the international date line, travels westwards, drops each present through the chimney with pin-point accuracy, does not stop for mince pies and delivers the presents between the hours of 10pm and 6am - then he has 34hrs to play with.

Speed : 600million km in 34hrs represents a speed of 17million km/hr, or Mach14,000.


Prof Merrifield pointed out that this was much faster than the speed of the Space Shuttle during re-entry (about Mach 20) and was indeed a lot faster than the speed required to escape the earth’s gravity (about Mach 400).

Achieving this kind of speed in the lower atmosphere would also result in severe frictional heating issues.

Which left NSB wondering whether, rather than a sleigh and reindeer, Santa Claus was actually using high downforce technology adopted from Formula 1 in order to avoid being flung out into space.

Or perhaps Santa uses active cooling techniques such as those used in the Apollo Moonshot missions where cryogenic fuels were pumped around the rocket nozzles to keep them from burning up?

Does Santa Claus take aerodynamic tips from Formula 1?

As is customary, there was a short question and answer session after the talk, and it was Santa Claus that had clearly caught the imagination of the audience, to the extent that Prof Merrifield shook his head at one point, wondering aloud how it had some to pass that he was discussing the merits of gift delivery methodologies with an audience at a science lecture.

Update Feb 2014
A beautiful timelapse video of snowflakes forming in the lab can be found here

Image Sources
Earth orbit, Latitudes, Giotto, Andromeda, Conjunctions, Snowflake, Formula1

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