The other day in the workshop, early in the morning, sun streaming in through the glass door at the back and the Green Man's mirror sitting there prettily, I recalled an experiment from schooldays and bethought me to waste a bit of time before knuckling down to some real work.

Those of you of about my age or older (and, even, perhaps just a little bit younger?) will, no doubt, have sat in on discussions between their elders - parents, aunts, uncles and others. No doubt some of those discussions will have been about education - “In My day, it was education for the sake of education, for the youngsters themselves and subsequently for the community, spreading out into the nation and into the world as a whole but, nowadays, there's far too much emphasis on career - or even just jobs”.

Yes, well, it may have been thus though I don't remember it as such - and I wonder if today's youngsters think it's so (and, do you)?

That thought set my curiosity in motion “Do they do things like this in today's schools?” I found undertaking it (while putting off starting work) an interesting refresher . Maybe some of you will, too – and maybe a few of you have missed out by not having done anything like this?

Watching the Sun Move

Ingredients:

. One smallish mirror (an adjustable shaving mirror on gimbals is ideal)

. A sunny day (do we get as many of those, still?).

. Perhaps a rather larger second mirror, too.

Note: it doesn't need to be early in the morning nor in a garage-converted-to-workshop .

Procedure:

Take your mirror to a patch of sunlight and adjust it to reflect a patch of that sunlight (an image) onto, say a wall or ceiling or somesuch. Make minor adjustments so that one side edge of it is just touching (or just not touching) a suitable feature such as a cupboard edge, the end of a brick, a light fitting, whatever. Sit for a while and watch.

Now, we know that the sun goes round the earth once in twenty four hours (well, of course what we actually know is that the earth rotates once each day, making it seem to us that the sun rotates around us). We also know that once around a circle is 360 degrees of rotation. That means that, as we rotate, the sun seems, to us, to be moving 360degrees in 24hours, 180degrees in 12 hours, 15degrees in 1 hour, 1degree in 4 minutes.

As you're sitting watching, it might occur to you that that patch of reflected light is moving faster than the edge of direct sunlight coming through the door or past a tree or fence or whatever. Yes, it is. In fact it's moving twice as fast – 1 degree in 2 minutes. Why? How?

The light from the sun that is striking your mirror is arriving at some angle which is called the Angle of Incidence, while the angle that it's reflected from your mirror (don't use the magnifying side, you won't get a satisfactory result - and be careful with it!) is the Angle of Reflection. And both, would you believe, are the same angle, so that the Angle of Incidence and the Angle of Reflection being the same, the resulting image on your wall, or wherever, arrives there at the sum of both angles - twice the incident angle.

By extension we can see that, when the sun moves across the sky, your image will move across your wall at twice the speed of that direct shadow, yes? Yes; and that means you can watch it move without staring at it for so long.

If you now put another mirror to reflect your image back (onto the opposite wall?), that new image will move at twice the speed of the first - four times the sun's speed, – 1 degree in 1 minute which makes it, of course, much easier to watch it in motion. Another mirror? Yet another? Pretty soon you'd be running around making tiny adjustments to all of 'em and you can explain why – yes?

One thing that will spoil the effect of using several other mirrors is that the images will tend to become fuzzier and fuzzier mirror by mirror, due to tiny imperfections within the reflective coatings and the glass covers - and we'll leave you to figure out the effect that the sun's diverging light has.

Why is your image moving up (or down) a bit, as well as across? Which? Why?
Will that change around the day? Around the year?

There is one thing, however, that you Should Not Do and that is to reflect your image directly back at the sun else eight-and-a-bit minutes later. It (the sun) will overheat and explode (Why eight-and-a-bit minutes?) and eight-and-a-bit minutes minutes after that, you'll have the satisfaction of knowing that you have, finally, destroyed Our Earth along with everything on it and in it.

Well no, of course you won't – and we'll leave you to figure out why not.

Articles by Rod

17.03.14 Front Page

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