For private pilot flight training, I’m about to start night flying. Should be easy right? In brief, the requirements are:

3 hours of night flight training in a single-engine airplane that includes one cross-country flight of over 100 nautical miles total distance; and 10 takeoffs and 10 landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport.

Simple, right? But… when is it “night”?

I know rules of thumb that work to qualify for flying at “night” (e.g., at Boston’s latitude, half an hour after sunset to before sunrise suppsoedly works), but I was curious about the precise definitions, which led me down a crazy rabbit hole.

And then I had to know exactly how good the rule of thumb was (TL;DR – in Boston, use 36+ minutes to always be in compliance).

Here we go (links to references provided).

(Photo “Sunset” by ICU『 I BE4 see you 』 is licensed under CC BY 2.0.)

Flight training and nighttime

Private pilot certificate requirements

§ 61.109 Aeronautical experience.: 14 CFR 61.109(a)(2)

Except as provided in § 61.110 of this part, 3 hours of night flight training in a single-engine airplane that includes

1. One cross-country flight of over 100 nautical miles total distance; and
1. 10 takeoffs and 10 landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport.

(§ 61.110 has exceptions for Alaska)

Definitions and Info

Night: 14 CFR 1.1 “Night means 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.”
Civil Twilight: Air Almanac “Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon.” (Does not state this is relative to sunrise and sunset); National Weather Service “[Civil twilight] begins in the morning, or ends in the evening, when the geometric center of the sun is 6 degrees below the horizon. Therefore morning civil twilight begins when the geometric center of the sun is 6 degrees below the horizon, and ends at sunrise. Evening civil twilight begins at sunset, and ends when the geometric center of the sun is 6 degrees below the horizon.”
Sunset and Sunrise: Air Almanac “Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth’s surface.”; “Sunrise and sunset. For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun’s center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes).”
Angular size of sun: Approximation “The sun is 1.39 X 10^6 km in linear diameter and 1.50 X 10^8 km from Earth. If you put these numbers into the small-angle formula, you will discover that the sun has an angular diameter of 1900 seconds of arc, which is 32 minutes of arc or about 0.5°.”

Night

So each civil twilight is from sunset and sunrise until the sun is roughly 12 sun diameters below the horizon, and night is the time in between.

It almost makes me want to obtain a sextant and learn celestial navigation.

And since you can’t actually see how far the sun is below the horizon, you have to look up twilight times in the Air Almanac. (Or a normal person would just use a rule of thumb. I am not a normal person.)

Air almanac annual tables for Sunrise/Sunset, Moonrise/Moonset, or Twilight Times

Air Almanac Tables
Air Almanac, 2024 twilight table
In Boston, UTC-4 is daylight time; UTC-5 is standard time

                                                              BOSTON, MA                               Astronomical Applications Dept.
Location: W071° 05', N42° 19'                           Civil Twilight for 2024                        U. S. Naval Observatory        
                                                                                                       Washington, DC  20392-5420     
Zone:  5h West of Greenwich**
Add one hour for daylight time, if and when in use.

       Jan.       Feb.       Mar.       Apr.       May        June       July       Aug.       Sept.      Oct.       Nov.       Dec.  
Day Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End
     h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m
01  0642 1654  0628 1728  0551 1803  0458 1839  0408 1915  0336 1949  0337 1959  0406 1935  0441 1846  0514 1753  0549 1706  0623 1644
02  0642 1655  0628 1729  0549 1804  0456 1840  0407 1916  0336 1950  0338 1959  0407 1933  0442 1844  0515 1751  0550 1705  0624 1644
03  0642 1656  0627 1730  0547 1805  0454 1842  0405 1918  0335 1951  0338 1959  0408 1932  0444 1843  0516 1749  0551 1704  0625 1644
04  0642 1657  0625 1731  0546 1807  0452 1843  0404 1919  0335 1951  0339 1958  0409 1931  0445 1841  0517 1748  0553 1703  0626 1644
05  0642 1657  0624 1733  0544 1808  0451 1844  0403 1920  0334 1952  0340 1958  0410 1929  0446 1839  0518 1746  0554 1702  0627 1643
06  0642 1658  0623 1734  0543 1809  0449 1845  0401 1921  0334 1953  0341 1958  0412 1928  0447 1837  0519 1744  0555 1701  0628 1643
07  0642 1659  0622 1735  0541 1810  0447 1846  0400 1922  0334 1953  0341 1957  0413 1927  0448 1835  0521 1743  0556 1659  0629 1643
08  0642 1700  0621 1736  0539 1811  0445 1847  0359 1924  0333 1954  0342 1957  0414 1925  0449 1834  0522 1741  0557 1658  0629 1643
09  0642 1701  0620 1738  0538 1812  0444 1849  0357 1925  0333 1955  0343 1956  0415 1924  0450 1832  0523 1739  0558 1657  0630 1643
10  0641 1702  0619 1739  0536 1814  0442 1850  0356 1926  0333 1955  0344 1956  0416 1922  0451 1830  0524 1738  0600 1656  0631 1644
11  0641 1703  0618 1740  0534 1815  0440 1851  0355 1927  0333 1956  0344 1955  0417 1921  0452 1828  0525 1736  0601 1656  0632 1644
12  0641 1704  0616 1741  0533 1816  0439 1852  0354 1928  0333 1956  0345 1954  0418 1919  0453 1826  0526 1734  0602 1655  0633 1644
13  0641 1705  0615 1743  0531 1817  0437 1853  0353 1930  0332 1957  0346 1954  0420 1918  0454 1825  0527 1733  0603 1654  0633 1644
14  0640 1707  0614 1744  0529 1818  0435 1855  0351 1931  0332 1957  0347 1953  0421 1916  0456 1823  0528 1731  0604 1653  0634 1644
15  0640 1708  0613 1745  0527 1819  0433 1856  0350 1932  0332 1958  0348 1952  0422 1915  0457 1821  0529 1730  0605 1652  0635 1645
16  0640 1709  0611 1746  0526 1821  0432 1857  0349 1933  0332 1958  0349 1951  0423 1913  0458 1819  0531 1728  0607 1651  0635 1645
17  0639 1710  0610 1747  0524 1822  0430 1858  0348 1934  0332 1958  0350 1951  0424 1911  0459 1817  0532 1727  0608 1651  0636 1645
18  0639 1711  0608 1749  0522 1823  0428 1859  0347 1935  0333 1959  0351 1950  0425 1910  0500 1816  0533 1725  0609 1650  0637 1646
19  0638 1712  0607 1750  0520 1824  0427 1901  0346 1936  0333 1959  0352 1949  0427 1908  0501 1814  0534 1724  0610 1649  0637 1646
20  0638 1713  0606 1751  0519 1825  0425 1902  0345 1937  0333 1959  0353 1948  0428 1907  0502 1812  0535 1722  0611 1649  0638 1647
21  0637 1714  0604 1752  0517 1826  0424 1903  0344 1938  0333 1959  0354 1947  0429 1905  0503 1810  0536 1721  0612 1648  0638 1647
22  0636 1716  0603 1753  0515 1828  0422 1904  0343 1940  0333 2000  0355 1946  0430 1903  0504 1809  0537 1719  0613 1647  0639 1648
23  0636 1717  0601 1755  0513 1829  0420 1905  0342 1941  0334 2000  0356 1945  0431 1902  0505 1807  0539 1718  0614 1647  0639 1648
24  0635 1718  0600 1756  0512 1830  0419 1907  0342 1942  0334 2000  0357 1944  0432 1900  0506 1805  0540 1717  0616 1646  0640 1649
25  0634 1719  0558 1757  0510 1831  0417 1908  0341 1943  0334 2000  0358 1943  0433 1858  0507 1803  0541 1715  0617 1646  0640 1649
26  0634 1720  0557 1758  0508 1832  0416 1909  0340 1944  0335 2000  0359 1942  0435 1856  0509 1801  0542 1714  0618 1646  0640 1650
27  0633 1722  0555 1759  0506 1833  0414 1910  0339 1944  0335 2000  0400 1941  0436 1855  0510 1800  0543 1712  0619 1645  0641 1651
28  0632 1723  0554 1801  0505 1835  0413 1912  0339 1945  0336 2000  0401 1940  0437 1853  0511 1758  0544 1711  0620 1645  0641 1651
29  0631 1724  0552 1802  0503 1836  0411 1913  0338 1946  0336 2000  0402 1938  0438 1851  0512 1756  0546 1710  0621 1645  0641 1652
30  0630 1725             0501 1837  0410 1914  0337 1947  0337 1959  0404 1937  0439 1850  0513 1755  0547 1709  0622 1644  0641 1653
31  0629 1726             0459 1838             0337 1948             0405 1936  0440 1848             0548 1707             0642 1654


Location: W071° 05', N42° 19'                           Civil Twilight for 2025                        U. S. Naval Observatory

       Jan.       Feb.       Mar.       Apr.       May        June       July       Aug.       Sept.      Oct.       Nov.       Dec.  
Day Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End  Begin End
     h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m
01  0642 1655  0628 1729  0551 1803  0458 1839  0409 1915  0336 1949  0337 1959  0406 1935  0441 1847  0514 1753  0549 1706  0623 1644
02  0642 1655  0627 1730  0549 1804  0456 1840  0407 1916  0336 1950  0338 1959  0407 1934  0442 1845  0515 1751  0550 1705  0624 1644
03  0642 1656  0626 1731  0548 1805  0455 1841  0406 1917  0335 1950  0338 1959  0408 1932  0443 1843  0516 1750  0551 1704  0625 1644
04  0642 1657  0625 1732  0546 1806  0453 1842  0404 1919  0335 1951  0339 1958  0409 1931  0444 1841  0517 1748  0552 1703  0626 1644
05  0642 1658  0624 1734  0545 1807  0451 1844  0403 1920  0334 1952  0340 1958  0410 1930  0445 1839  0518 1746  0553 1702  0627 1643
06  0642 1659  0623 1735  0543 1809  0449 1845  0402 1921  0334 1953  0340 1958  0411 1928  0447 1838  0519 1745  0555 1701  0627 1643
07  0642 1700  0621 1736  0541 1810  0448 1846  0400 1922  0334 1953  0341 1957  0412 1927  0448 1836  0520 1743  0556 1700  0628 1643
08  0642 1701  0620 1737  0540 1811  0446 1847  0359 1923  0333 1954  0342 1957  0414 1925  0449 1834  0521 1741  0557 1659  0629 1643
09  0641 1702  0619 1739  0538 1812  0444 1848  0358 1925  0333 1955  0343 1956  0415 1924  0450 1832  0522 1740  0558 1658  0630 1643
10  0641 1703  0618 1740  0536 1813  0442 1850  0356 1926  0333 1955  0343 1956  0416 1923  0451 1831  0524 1738  0559 1657  0631 1644
11  0641 1704  0617 1741  0535 1814  0441 1851  0355 1927  0333 1956  0344 1955  0417 1921  0452 1829  0525 1736  0601 1656  0632 1644
12  0641 1705  0615 1742  0533 1816  0439 1852  0354 1928  0333 1956  0345 1955  0418 1920  0453 1827  0526 1735  0602 1655  0632 1644
13  0640 1706  0614 1743  0531 1817  0437 1853  0353 1929  0332 1957  0346 1954  0419 1918  0454 1825  0527 1733  0603 1654  0633 1644
14  0640 1707  0613 1745  0530 1818  0436 1854  0352 1930  0332 1957  0347 1953  0420 1916  0455 1823  0528 1732  0604 1653  0634 1644
15  0640 1708  0611 1746  0528 1819  0434 1856  0351 1932  0332 1958  0348 1952  0422 1915  0456 1822  0529 1730  0605 1652  0635 1645
16  0639 1710  0610 1747  0526 1820  0432 1857  0349 1933  0332 1958  0349 1952  0423 1913  0457 1820  0530 1729  0606 1652  0635 1645
17  0639 1711  0609 1748  0524 1821  0430 1858  0348 1934  0332 1958  0350 1951  0424 1912  0459 1818  0531 1727  0607 1651  0636 1645
18  0638 1712  0607 1750  0523 1823  0429 1859  0347 1935  0333 1959  0351 1950  0425 1910  0500 1816  0533 1726  0609 1650  0637 1646
19  0638 1713  0606 1751  0521 1824  0427 1900  0346 1936  0333 1959  0352 1949  0426 1909  0501 1814  0534 1724  0610 1649  0637 1646
20  0637 1714  0605 1752  0519 1825  0426 1902  0345 1937  0333 1959  0353 1948  0427 1907  0502 1813  0535 1723  0611 1649  0638 1647
21  0637 1715  0603 1753  0517 1826  0424 1903  0344 1938  0333 1959  0354 1947  0429 1905  0503 1811  0536 1721  0612 1648  0638 1647
22  0636 1716  0602 1754  0516 1827  0422 1904  0343 1939  0333 2000  0355 1946  0430 1904  0504 1809  0537 1720  0613 1648  0639 1648
23  0635 1718  0600 1756  0514 1828  0421 1905  0343 1940  0334 2000  0356 1945  0431 1902  0505 1807  0538 1718  0614 1647  0639 1648
24  0635 1719  0559 1757  0512 1830  0419 1906  0342 1941  0334 2000  0357 1944  0432 1900  0506 1805  0539 1717  0615 1647  0640 1649
25  0634 1720  0557 1758  0510 1831  0418 1908  0341 1942  0334 2000  0358 1943  0433 1859  0507 1804  0541 1715  0616 1646  0640 1649
26  0633 1721  0556 1759  0509 1832  0416 1909  0340 1943  0335 2000  0359 1942  0434 1857  0508 1802  0542 1714  0618 1646  0640 1650
27  0632 1722  0554 1800  0507 1833  0415 1910  0339 1944  0335 2000  0400 1941  0435 1855  0509 1800  0543 1713  0619 1645  0641 1651
28  0631 1724  0553 1802  0505 1834  0413 1911  0339 1945  0336 2000  0401 1940  0437 1854  0510 1758  0544 1711  0620 1645  0641 1651
29  0631 1725             0503 1835  0412 1912  0338 1946  0336 2000  0402 1939  0438 1852  0512 1757  0545 1710  0621 1645  0641 1652
30  0630 1726             0502 1837  0410 1914  0337 1947  0337 1959  0403 1937  0439 1850  0513 1755  0546 1709  0622 1644  0641 1653
31  0629 1727             0500 1838             0337 1948             0404 1936  0440 1848             0548 1708             0642 1654

How good is the rule of thumb?

So, the local rule of thumb is that night starts around 30 minutes after sunset. I wondered how good that rule was.

I also fetched the times of sunset and sunrise from the Air Almanac, converted the constant width text tables into dataframes (in a messy, non-reproducible way), and calculated the number of minutes between sunrise / sunset and civil twilight.

At Boston’s latitudes for the year 2024:

It’s only a few minutes difference, but it turns out that the rule of thumb of night time starting 30 minutes after sunset around Boston is a bit too optimistic 31% of the time.

The maximum is 35 minutes, so defining night (in Boston) as 36+ minutes after sunset is always safe. (Remember, this is only going to work at the same latitude as Boston.)