There are at least three different ways to define "night" according to the Code of Federal Regulations, Chapter 14 (the one that deals mostly with aviation). Each definition serves a specific purpose.
*The Air Almanac for 2018 can be found here, all 900-plus pages of it.
This is the Air Almanac page for today, and I'll be honest -- I have no idea how to read it, although I understand it helps to have a sextant.
It is simpler and more convenient to simply search for your local sunrise/sunset times in local news sources, or on the Internet.
By the way -- for almost every FAA definition of "night," there's another way of defining it for Alaska, since there are places in Alaska where the sun literally never sets at certain times of the year, and barely rises at others. So really, there are more like five or six ways to define "night" depending on why you need to define it.
When referring to drone operations, the definition in 14 CFR 1.1 is the one that matters: "the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time."
But here are three concepts every drone operator needs to be familiar with, because they define when the rules change for flying:
Between sunrise and sunset, drone operators may operate as normal. That's daytime.
During morning and evening civil twilight, certain rules go into effect about how much lighting you are required to have on your drone. Tonight, civil twilight lasted from 5:24 PM until 5:54 PM; I just added 30 to the time of official sunset. Most drone operations can still take place during civil twilight.
Night operations usually require additional lighting and safety measures, and a waiver if you're working commercially.
Hobbyists must follow the rules in 14 CFR 101 Subpart E, which includes being required to follow the safety guidelines of a community-based organization (like the Academy of Model Aeronautics or the Drone Users Group Network). Since 101 Subpart E doesn't specifically include any guidance on flying at night, operators need to look at the safety code they've chosen to follow. Both the AMA and the DUGN allow flying at night, as long as the aircraft is equipped with lighting that provides the pilot "with a clear view of the model’s attitude and orientation at all times" (AMA) or "sufficient for [the pilot] to see the orientation and flight direction of the drone" (DUGN).
There is no official definition of "night" for hobbyists, but since drones are considered aircraft by the FAA, it's safe to assume that the definition in 14 CFR 1.1 will suffice. There also aren't any requirements for how bright the navigation lighting needs to be; it's up to the pilot's discretion.
By the way, the DUGN code also suggests evaluating the flight area before it gets too dark to mark any possible obstacles or complications. This is an excellent practice.
Commercial drone operations are restricted to periods of daylight by 14 CFR 107.29:
So here we see that commercial UAS operations are not permitted after civil twilight has ended, and that during civil twilight, the drones must be equipped with anti-collision lighting of a defined intensity. People seeking more specific instructions for flash rate, light color, and visibility can look at 14 CFR 91 and 14 CFR 27, which provide guidance for lighting systems on manned aircraft.
A helpful diagram on anti-collision lighting requirements for manned aircraft from Whelen.
Of course, under 14 CFR 107.205, it's possible to apply to have the daylight requirement waived and complete commercial operations at night as defined under 14 CFR 1.1. The process involves taking steps to mitigate the risks the FAA perceives in operating small UAS at night. Until the person or company wishing to fly commercially at night holds that waiver for 107.29, he or she needs to cease operations by end of evening civil twilight, or begin after the start of morning civil twilight.
The nuances and requirements of the 107.29 waiver application will need to wait for another post.
Above all, fly safe.