Date: December 25, 2010
Title: Why Winter is So Cold
Podcaster: Rob Berthiaume
Organization: www.yorkobservatory.com
www.youdontfreezeinspace.com
Description: Winter is cold. It brings us skating and skiing, but also more car accidents and frostbite. The podcast looks at why seasons occur, and gives a simple experiment you can do with anyone of any age or background to visualize it yourselves.
Bio: Robert Berthiaume is working towards an MSc in atomic physics at York University in Toronto, Canada. When he can get away from researching and developing diode laser systems, he rides his motorcycle when the sun is up and it is warm out, and shares the stars with the public at the observatory when it’s colder and darker.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Mike Smithwick, author of the award winning astronomy software, Distant Suns, now for the iPad and iPhone.
Transcript:
Hi there. I’m Robert Berthiaume bringing you the December 25th edition of the 365 Days of Astronomy Podcast from the York University Observatory in Toronto, Ontario, Canada. Ah, Canada. The great white north. Well, we are in the North, and as I look out my window things are certainly white, but I definitely don’t think that that’s great. It’s so cold. I hate the winter. See, I don’t play hockey, I don’t ski, I don’t bobsled, I don’t own a toque company, I don’t really get anything out of winter. Winter means walking in slush on the sidewalks, spending 5 minutes putting on boots and gloves and jackets just to go to the corner store, I can’t go swimming or ride my motorcycle for another 4 months. And if you have a telescope, you’ll know all the great advantages winter observing has over the summer observing. The nights are way longer and often have better skies…too bad you can’t stay out for more than 15 minutes without your hand freezing onto the tube or your batteries in the hand paddle dying. Yes, you can dress warmly, go swimming in an indoor pool, and psych yourself up for observing, there are workarounds, but it all takes more work. And it is simply because winter is cold. But why is it so cold? Why are there seasons in the first place?
So the reason for the seasons is not too difficult to understand. I mean, if you ever stand next to a campfire or a furnace or a stove, some source of heat, you get warm. The further away you stand from the heat source, the colder you get. Well, if we study the orbits of the planets, we observe that the planets circle the sun, but not in perfect circles. They orbit the sun in ellipses, a squashed circle that’s longer on one side than the other. This makes it so that at one point in the orbit, the planet is further away from the sun, and at another point half an orbit, or half a year later the planet is closest. And the hottest season and coldest season are separated by half a year too, so it seems that putting this all together, we realize that the seasons are caused by the changing distance between the Earth and the Sun.
This is totally incorrect. This ladies and gents is probably the number three misconception in astronomy, trailing only the runner up, “We can never see the dark side of the moon” and the king of all misconceptions, “There’s no gravity in space”.
It seems like good science so far though, based on our theory and observations. But let me throw another observation at you. When it is winter in the North, it is summer in the South, and vice versa. Both hemispheres never experience the same season at the same time, despite the fact that both hemispheres, the entire globe, is always either close to the sun or far from the Sun. And one more fact to jazz things up: The Earth is closest to the Sun, which astronomers call perihelion, on January 4th, which coincides with just about the coldest part of winter here in the northern parts of the globe. So we see that the seasons cannot be caused by the Earth-Sun distance. So something else is at work here.
Well, let’s look at how the Earth moves. We know it orbits the Sun, once a year, but it also spins on its axis once per day. Think of this like a top spinning on a table. But just like a top can be tilted over a bit as it spins flat on the table, the Earth too, spins a bit tilted with respect to its orbit. This means that at some points in its orbit, the top half is pointed towards the sun, and in other parts, it is pointed away. When the top half is pointed towards the sun, the bottom will be pointed away, and vice versa. The part that is pointed towards the Sun has sunlight fall more directly onto the surface, and spends more time in the sun, that’s to say the days are longer. This means more sunlight, therefore more heat, therefore warmer. This is summer for that part of the world. And if that hemisphere is getting lots of sun, the other hemisphere is getting less, meaning less heat, colder, and this means winter.
The seasons are caused by the tilt of the Earth’s rotational axis, not the changed in orbital distance. If you want to visualize this without a globe handy, grab a coffee mug, something long and sharp, and something round that’s bigger than the opening of the mug. Lots of things will work, like knitting needles, BBQ skewers, or nails with apples, styrofoam balls, or even a crumpled up ball of newspaper or a ball of yarn. I’ll explain using a pencil and an orange.
Step one: Jab the pencil into the orange, and rest it in the opening of the coffee mug. Next tilt the pencil so it hangs over the mug’s handle, and then finally, put it on the edge of a table. You’ve got your orange representing the Earth, the pencil showing the Earth’s axis, the outside edge of the table showing the path of the Earth’s orbit, and imagine the centerpiece in the middle of the table as the sun. You can move the mug around the edge of the table, making sure to keep the mug’s handle always pointing towards the same doorway or window or some point of reference. At different points around the table you can spin the orange around and look at what parts are pointing towards the middle of the table and for how long each time you spin it around. Have fun with this: change the angle of the tilt, change the reference point where the handle is facing, change the rates at which you move around the table or spin the orange, and discover how seasons are the same and different for different setups. Finally, take out the pencil and eat the orange. You can avoid the middle part where the pencil went in if you’d like, and definitely skip this entire step if you used yarn instead of an orange.
Things you’ll notice: if your orange planet is tilted at all, then there will always be one point in the orbit where the top gets more direct sunlight than the bottom, and that half an orbit later things are the opposite. You’ll notice that the very top and very bottom, at these points, are actually always, or never in the sunlight. These are what we call the solstices, the first days of summer and winter. In between these points, we have the equinoxes, where no matter the tilt, every point on the planet gets exactly half a day in light, and half a day in dark.
As much as I hate winter, I do love thinking about the seasons, because they cause these neat effects on planets. On Earth, for instance, The North and South poles see one sunrise and one sunset a year. In the North the Sun rises around March 21st, and all summer long there is one big, long day. Then it finally sets on September 21st, and the whole place sinks in to simultaneous winter and night. On Uranus, this effect is huge: Uranus is tilted almost 90 degrees, and so seasonal effects are as large as they could possibly be. But try this configuration out with your model. You’ll see that the popular idea of Uranus “rolling” along its orbit is not totally true. Sometimes it does roll along, but other times it is corkscrewing , and most of the time, it is a combination of the two.
I should mention that Northern winters are colder than southern winters generally speaking, but again, not because of the Earth’s orbit, only because of the geography of our planet. There is more land mass closer to the north pole than there is close to the south pole. This implies there is more water around the southern hemisphere than the northern one, and since water has a great ability to hold on to heat, changes are much more gradual and things never get as cold down in the south.
Seasons are more than just some physical phenomenon though. They have had and continue to have, some very significant influence over life here on Earth. The cyclical temperature, daylight, and weather changes helped keep track of long periods of time for early cultures. The seasons obviously had a large impact on civilization since finding, or producing food in agriculture, is linked heavily to the seasons. And most importantly, changing seasons probably helped to drive evolution towards more diverse, and maybe more complex forms of life. The spatial and temporal variation in local climate and environment could very well have helped support different versions of the same species, or demanded that the lifeforms were able to survive and flourish in more than one set of conditions.
Given the cool scenarios that the seasons create in our solar system, and no doubt beyond, their cultural importance, and the idea that we humans might owe our entire existence to them, I have to stop myself and think: Seasons may be wonderful, but I still hate the winter.
Thanks for listening to today’s podcast. I’m Robert Berthiaume off to shovel the walkway and then trudge across a slippery sidewalk to buy a space heater for my bedroom. Merry Christmas to those who are celebrating it today, and here’s wishing all the best to every single one of you, your friends and family this happy, joyous, and cold season.
End of podcast:
365 Days of Astronomy
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