14 CFR §1.1 defines night as: ![[1_1 night.png]] This is the **general** definition of night for 14 CFR; recent flight experience (§61.57) and aircraft lights (§91.209) include self-contained specific definitions of night, but the general definition in §1.1 applies in any other case where a specific definition is not provided. Most pilots use a website or application to determine civil twilight times for a flight that may include night time, but proficiency using the Air Almanac, per §1.1, is theoretically a skill pilots should possess. ## Accessing the Air Almanac The current edition of the Air Almanac can be accessed via the U.S. Naval Observatory Website: https://aa.usno.navy.mil/publications/aira ![[air almanac cover.png]] ## Determining Location Civil twilight times published in the Air Almanac are based on location, expressed in latitude and longitude. ### Airport Locations Finding the latitude and longitude of an airport is fairly straightforward; simply check the airport/facility directory entry in the applicable chart supplement. ![[salem OR chart supps.jpeg]] ### En Route Locations Determining precise en route location can present more of a challenge. Using the charted lines of latitude and longitude on the VFR sectional or IFR en route charts, in association with the estimated position, is the most straightforward means of accomplishing this. Many flight planning applications can also provide latitude/longitude of charted or user-defined waypoints. ## Using the Air Almanac The Air Almanac provides a detailed explanation of how to calculate civil twilight times for a given location: ![[air almanac explanation.png]] As an example, let's calculate the end of evening civil twilight for Salem, OR (KSLE), on March 27th, 2025. This date and location were selected for simplicity in performing the following calculations; any intermediate values (day, latitude, or longitude) can be found using conventional interpolation techniques. First, the date and latitude are used on the appropriate *evening civil twilight* table to determine LMT. Salem is located at 45°N latitude, so for March 27th, 2025, LMT is 1851. ![[evening civil twilight table.png]] Next, the longitude is used on the *arc to time* table to convert LMT to UT (Z). Salem is located at 123°W longitude, so 0812 is added to 1851. The end of evening civil twilight for Salem will be 0303Z on March 28th. ![[arc to time table.png]] Finally, UT can be converted to local time. The airport/facility directory entry for KSLE indicates that 7 hours should be subtracted from UT during daylight saving time. 0303Z minus 0700 is 2003 local time, or 8:03pm on March 27th. Checked against online astronomical data, the result is correct. ![[TAD salem window snip.png]] While determining civil twilight times using the Air Almanac is certainly more labor-intensive than other methods, it provides interesting insights into how latitude, longitude, and season can substantially alter our local observation of night time. ### By Kevin Sakson