Evil aliens have decided to kill everyone on Earth! Fortunately, they've given you two minutes to save as many as you can. All you have to do is name a country to save everyone in it.
Start by naming any country that France borders. Then name any country that borders any of those countries. Then keep going until there are no more countries left to name.
I wondered the same thing. Here's what Wikipedia says: "The Tropic of Cancer position is not fixed, but varies in a complicated manner over time. It drifts south almost half a second (0.47″) of latitude per year (it was at exactly 23° 27′ in year 1917)."
Right. And in that year, the nation of Chad was not touching the Tropic of Cancer. That will happen again at some point, and because of that fluctuation, Chad is not included.