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Why is the Sky Blue?

Entry 1019, on 2009-05-27 at 22:33:20 (Rating 2, Science)

Why is the sky blue? Why is the Earth hotter during summer than it is during winter? These are two basic science questions that many intelligent, educated people can't answer.

The subject arose as part of a discussion in a science podcast related to the gap between the arts and science but I think it is equally applicable to the gap between people who are scientists, technologists, or just scientifically literate for some other reason, and everyone else.

According to the program, when the entire graduating class of Harvard University was asked why the Earth is hotter in summer only a few gave the right answer. Not only is does this show a huge failure to know some basic facts (this is usually taught in school at around age 13, I think) but the answer most people give makes no sense if you think about it a bit.

The most common answer is that the Earth is closer to the Sun during summer. This seems reasonable until you realise that during the northern summer the southern hemisphere has its winter. Since both hemispheres are part of the same planet how can one be closer and experience summer while the other is more distant and be experiencing winter?

If Harvard graduates (who most people would say are fairly intelligent) can't even see this simple problem with their answer then what chance does everyone else have?

By the way, the correct answer is that its the direction of tilt of the Earth which changes. The Earth is tilted at about 23 degrees in relation to the Sun and the direction the Earth points stays the same as the Earth orbits the Sun.

So when the Earth is on one side of its orbit and the northern hemisphere is pointing more towards the Sun during December and January it is hotter but six months later on the other side of the orbit when it is pointing more away it is colder. Of course when the northern hemisphere is pointing away the southern is pointing towards the Sun (and vice versa) so its seasons are opposite. That's not too hard, is it?

If people can't even handle that concept then what chance do they have with more subtle and abstract ideas, like quantum physics, evolution, and cosmological theories? And what about complex and contentious ideas like global warming?

Does this all really matter though? Well yes, because people would be called ignorant if they didn't know a few basic facts about literature (some lines from Shakespeare maybe) or some simple geography (the capital of major countries) or some history (approximate dates of the world wars) but most people don't seem to worry as much about scientific ignorance even though it is of greater practical importance.

Actually, looking at the paragraph above I'm not so sure I'm right because I suspect a lot of people wouldn't have any clues about those non-science questions either. In fact I seem to remember a survey which showed many people didn't even know which century World War I was in! So maybe I shouldn't re railing against scientific ignorance as much as pure, generic ignorance!

Oh yes, I haven't told you why the sky is blue yet, have I? The molecules in air scatter shorter (bluer) wavelengths of light more than longer (redder) so they get diverted down from the sky to where we see them. The red stuff goes right through. At sunset the sky seems redder for the same reason: the blue is being diverted away and the red is coming through in a straight line.

But that explanation is really too complex. Exactly the same effect occurs in a tank of coloured liquid or gas and we don't engage in complex explanations for that. So really the reason the sky is blue is simply because air is blue!


Comment 1 (2042) by SBFL on 2009-05-28 at 09:02:27:

I didn't know about the reason for a blue sky but the axis of the earth should be known to most (re seasons). On seasons I am confused by your comment "So when the Earth is on one side of its orbit and the northern hemisphere is pointing more towards the Sun during December and January it is hotter but six months later on the other side of the orbit when it is pointing more away it is colder." Eh? Surely you mean when the side of Earth's orbit is closer to the sun it is summer (Dec-Jan for the northern hemisphere, Jun-Jul for the southern). If so, I think your choice of wording could be better.

As for the Harvard graduates, well if you are a Harvard graduate of music I wouldn't expect them to know any better. Stop generalising please.

On the matter of people in general being ignorant of basic general knowledge, well I am also astounded by the results of studies reported. Mostly they come from the USA so I assume that since Americans think the world revolves around them, the results are not so surprising (and yes, the most vociferous creationists are Americans!)


Comment 2 (2048) by OJB on 2009-05-28 at 17:33:08:

It has nothing to do with the distance from the Earth to the Sun (well a tiny bit, but that can be ignored). Its which hemisphere is tilted towards the Sun which determines which will have winter and which summer.

Why would a graduate of music not know a few basic science facts? I'm sure most science graduates know a few music facts. I wasn't trying to imply that Harvard graduates are thick, quite the opposite in fact: I was trying to show that even intelligent people are ignorant of really basic stuff outside their area of expertise.

I can't remember where I saw it now but I'm sure I can remember similar results form a survey carried out in the UK, so its not just Americans who suffer form this. And I didn't even mention creationists. Why would you bring up that subject? :)


Comment 3 (2057) by SBFL on 2009-06-04 at 07:31:40:

Isn't that what I was referring to?

Hmmm, well IŽll take you word on that. Have to admit there are few basic facts on some subjects I know noting of. As a uni graduate, I can't tell you much (or anything actually) about art history.

Call it intuition...


Comment 4 (2068) by OJB on 2009-06-04 at 20:32:44:

Yes well, no one knows a lot about everything. We do live in an age of specialisation. That's why I'm not sure whether the scientific knowledge thing really matters.


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