A MIDNIGHT SUN? Not where I live

I was working with some 6th Graders. Some have impressive general knowledge, but it turned out that there was little awareness that there are places on Earth where the sun shines 24 hours a day. Their home is just off the Tropic of Cancer, so to get an idea of how much variation of sunshine time there can be as one moves away from the equator, we chose some data from http://www.timeanddate.com/  (see below)

AT THE EQUATOR the daylength differs from one solstice to the other by only two minutes. Year round, the day is 12 hours and 7 minutes long, within a minute.

AT THE TROPIC of CANCER the June sun is up for 20 minutes short of 14 hours. Add the fact that sun passes overhead and the days are very hot, and the nights are not long enough to allow much cooling. The December sun is up for three hours less and does not even reach an altitude of 45° Evenings can be romantic; they are often cool and calm

INSIDE THE ARCTIC CIRCLE at Tromsǿ, the summer Solstice sun has not set for five weeks and will not for a further five. Ten weeks of Midnight Sun and only 2° inside the Arctic Circle.

The December solstice is in the middle of a 7 week period wherein the sun will not be seen

So for any given date, how long is a day on this planet? It depends on the season and the latitude.

Location	Latitude	Date	Day length	Time Solar Noon	Altitude Solar Noon
Quito	0°15S (equator)	21 Jun	12h 06m 32s	12:16	66.3°
Quito	0°15S	21 Dec	12h 08m 18s	12:12	66.8°
Abu Dhabi	24.8°	21 Jun	13h 39m 11s	12:24	89.0°
Abu Dhabi	24.8°	22 Dec	10h 37m 23s	12:21	42.1°
Tromsǿ (Norway)	69.7° N	21 Jun	24 hours	12:46	43.8°
Tromsǿ	69.7° N	22 Dec	Zero hours	11:42	1.9° below

Say IF . . . THEN . . . and you are talking Science, even if you are 10 yrs old and saying the earth is round

Over a few years, our family had several weeks of holidays in a "house" 50 metres from a rocky shore. I was about 10. No electricity.

A luxury was water that ran from a tap.  Sometimes we had to clear a blockage at the intake pipe, about 30 metres up a steep stream behind the house. After sorting this one morning, I sat down and looked out to sea. Surprise! From a few metres up a hill, the horizon was above a particular rock, yet from the water’s edge, the top of that rock was above the horizon. 

Another time, at the water's edge there was a ship at the horizon with a blue hull. Yet climb just a couple of metres and the ship had a red stripe along the water line.

Meaning?

IF THE HORIZON shifts away from you when you climb even a couple of metres, THEN THE SURFACE of the water between you and the horizon is curved. THE EARTH is ROUND!