On a world scale, the Icelandic eruption was "large" (mag. 4), there have been much bigger events (Mt St Helens, Pinatubo etc.)
en.wikipedia.org
Eyjafjallajökull ejected 250 million cubic metres
Mt StHelens ejected 1000 million cubic metres
Pinatubo ejected 10,000 million cubic metres
If the sun rises / sets in different locations, then one of two things must have happened:
- the earths axis has changed (the Nth & Sth poles have moved and the equator has a new location)
- the angle of the earths inclination has changed ( if the angle increased, the tropics of Cancer and Capricorn have moved further apart or vise versa)
If the tropics moved, so would the arctic/antarctic circles resulting in a different number of days of total darkness each winter (I think this would be noticed).
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Also, if the earths inclination had changed, the position of every celestial body would also appear to move. Before GPS, the precision navigation instrument was the Sextant. A good operator can plot a position with a Sextant within half a mile, but only if the stars are where they should be. Sextants are still sold to enthusiasts, as backup navigation on yachts and their use is taught in the Navy. Any systematic error would be be discussed widely and be impossible to keep quiet.
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As for your (or any other) Government keeping secrets, it's never happened before
