Recently, I was in an exchange with a girl in London. In my novel, I mentioned it getting dark around 9:00 PM in mid June. She commented that it does not get dark in mid June until 10:00PM. So, who was right? If you have a knowledge of physical geography, you know we both were.
The discrepancy comes from the fact the difference in the lengths of days by season is greater in the high latitudes (areas closer to the poles) than the low latitudes (areas closer to the equator).
The novel I was writing takes place in the Ozarks, approximate latitude 37 degrees north, while London is at latitude 52 degrees north. If you know the latitude of a location, it's easy to calculate sunrise and sunset for that location by using an online generator such as the one here: http://aa.usno.navy.mil/data/docs/RS_OneDay.php . Sunset at 52 degrees north (London, UK) occurs at 8:24 on June 21. At latitude 37 degrees north (Springfield, MO) the sun sets at 7:23 on this date.
I have compiled a small table mentioning selected cities, their latitudes, and the time of sunrise and sunset on June 21 and December 21. No adjustment for how far east or west of their time zone has been made.
City Lat. June 21 Dec. 21
Rise Set Rise Set
London UK 52 3:40 am 8:24 pm 7:25 am 4:31 pm
New York NY 41 4:28 am 7:36 pm 6:50 am 5:06 pm
Springifeld MO 37 4:41 am 7:23 pm 6:40 am 5:16 pm
New Orleans LA 30 4:59 am 7:04 pm 6:25 am 5:32 pm
As you can see from the table, the sun rises an hour earlier in London than it does in Springfield and sets an hour later. In winter it's nearly reversed, with sunrise forty-five minutes later in London and setting forty-five minutes later. Furthermore, the length of days varies more for high-latitude locations than low-latitude locations.
It does not get instantly dark the moment the sun sets. There is a period of twilight. As a general rule there is enough twilight to read a book a half hour before sunrise or after sunset, there's enough twilight to make out basic land features a full hour before sunrise or after sunset. Twilight lasts longer the closer to the poles you get. Of course, this assumes a clear sky. Depending on cloud cover, the twilight could be nonexistent.
While studying to make your novel as authentic as possible, don't forget about the effects of latitude on your setting. Now that you know the secret, you have the knowledge to make your novel that much more realistic.