(The first two chapters are just for flavour, you can safely skip to the TL;DR near the end.)
This puzzle in situated Helsinki for a reason: for the city's "200 years as capital" celebration, the massive granite church in Kallio was illuminated for several nights by shining a powerful blue laser from the Helsinki Observatory, located several kilometres away on the other side of the downtown Helsinki. The installation highlighted (quite literally) an interesting intentional design in the city's planning: Along Unioninkatu, there is an unobstructed line of sight between the two prominent hilltops that are home to those two buildings.
Now then, let's imagine that someone, a lucky drunken university student most likely, happened to gain access to the laser, and after shining it at various buildings, birds, and other targets of opportunity, left the laser so that it happened point exactly to east, and illuminated a ship somewhere near the horizon. (For the purposes of this puzzle, let's assume all these things are possible.) The question is: which end of the laser beam is the northernmost one?
TL;DR: If you point a perfectly straight beam of finite length from Helsinki towards the true east, which end of the beam is the northernmost one?
NB: This is supposed to be a 3D geometry and visualisation problem, or a maths problem without any calculations, if you like. So, although I'm a staunch advocate of loopholes and lateral thinking, please don't post such answers to this puzzle. Well, unless they are particularly excellent, of course.
For extra credit: Would the answer change, if the laser was pointed west instead of east? What if the laser was in Sydney?
EDIT: To avoid any confusion, there is absolutely no trickery involved here. Every term is used in its usual meaning, and there aren't any traps. Some examples:
- Northernmost means "having the largest northern latitude". Latitude is measured relative to the equator. The magnetic north pole has nothing to do with this puzzle.
- Any point that is above ground level is considered to have the same longitude and latitude as the unique point on earth's surface directly below it. (A helicopter directly above the Eiffel Tower is just as north as the Eiffel Tower.)
- East means true east, the direction where earth's rotation is taking every (non-axle) point at every time. At every point, "east" is exactly parallel with the latitude line (or rather, the latitude circle) passing through that point. No compass measurements are involved.
- If you want to, you can choose to include an optional vertical angle between -90 and 90 degrees into your definition of east. ("Point your telescope towards east, and 30 degrees up.") This makes it a lot easier to hit the ship at sea level with a laser on a hilltop. You can also choose to not include the vertical angle; it won't change the answer. (You'll need a somewhat larger ship for the flavour story to make sense, though.)
- This is a pure geometry puzzle, so you know the directions and straightnesses more or less magically; there's no need to account for measurement errors, natural phenomena, blocked lines of sight, or anything of that sort.
- If you make an honest effort to answer the question, I'll of course try to help you wherever I think you might need help, so go on ahead even if you're not sure of your answer!
I think one reasonable definition for a "correct answer" might be "the first factually correct answer that would convince even a somewhat sceptical person."