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Each time I look at a shadow cast by a sunlit object it seems as if the shadow doesn't move at all. But if I look at it after say 5 minutes, I can notice that the shadow has actually moved a bit. This allows to say "yes, the Sun has moved in the sky". But this doesn't make it all that obvious that "the Sun is continuously moving all the time".

So I'd like to make some optical configuration which would allow me to watch a spot of sunlight or a shadow moving much faster — say, 1 mm/s — still being from the Sun, not from an artificial light source. I've tried doing this with a cylindrical silvery surface like that of a kitchen rail system, sticking black tape on it and leaving only a lateral strip of reflecting surface, then then letting Sun rays to hit it almost tangentially, and to observe the reflection. But either this doesn't give good enough magnification to make Sun's motion apparent, or the reflected spot just becomes too faint for the naked eye.

So, what can I do to effectively magnify angular motion of the Sun to observe it (almost) directly at the scale of seconds? I's OK if it could be visible for e.g. a minute and required me to adjust my instrument to view the next piece of motion. But I'd like to do it as simply as possible, without too expensive hardware.

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The obvious answer is to use a telephoto lens -- or any other magnification scheme. As an extreme example, if you had a post 1 km tall, the shadow of its top (think sundial) would move pretty clearly. If you re-image a short post, or just re-image the sun itself, which enough magnification, you'll see the motion.

This is kind of the same as trying to observe a celestical object with a high-power telescope. If you don't have an equatorial mount, everything in your field of view will move quite noticeablly.

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  • $\begingroup$ What do you mean by re-image? Project object's light onto a screen? $\endgroup$ – Ruslan Jan 4 '17 at 14:32
  • $\begingroup$ Sure. Or project onto anything you want :-) $\endgroup$ – Carl Witthoft Jan 4 '17 at 15:20
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    $\begingroup$ OK, just tried with a newly-purchased telescope. The image of the Sun on the ceiling, with diameter of the image of about 30 cm, is indeed quite visibly moving, despite all the noise of atmospheric refraction. $\endgroup$ – Ruslan Jan 5 '17 at 13:12
  • $\begingroup$ Good to hear. Now all you need is a video recording of the image :-) $\endgroup$ – Carl Witthoft Jan 5 '17 at 13:41
  • $\begingroup$ I do have recorded it, but the quality is poor, since it's done with a Nokia N95's camera in a hurry 30 minutes before sunset :). If you really want to have a look, here it is. $\endgroup$ – Ruslan Jan 5 '17 at 14:16

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