Question

Can a sundial work correctly if its gnomon casts its shadow not on a horizontal surface but on a vertical one, e.g. the wall of a house? Explain.
Suppose you have built a really big sundial, big enough to have divisions for minutes between the hour lines. You have corrected it for your position in your time zone and are taking the equation of time into account. What else may affect its accuracy?
At high latitudes, close to the pole--Alaska, Canada, Scandinavia etc.--the Sun is never far from the horizon. In the summer it moves around the horizon and may be visible 18, 20 or even 24 hours of th