Time, Sunrise and Twilight

Time and longitude are two ways of expressing the same thing. The Earth turns through 360° relative to the Sun in 24 hours, so every meridian has its own local time, and the difference between the times of two places is simply their difference in longitude converted at 15° to the hour. Navigators have used this since long before radio: a position could be fixed only as accurately as the time was known.

For today's pilot, time matters in three ways. All aviation messages, flight plans and clearances use Coordinated Universal Time (UTC), so local times must be converted before any planning starts. Sunrise and sunset tables entered by latitude give local mean time and need an arc-to-time correction. And the legal boundary between day and night, which decides whether a day-only pilot or aeroplane may fly, is set by civil twilight rather than by sunset. The subject forms the time section of EASA 061 General Navigation, and the FAA knowledge tests ask the same conversions together with the FAA's own night definitions.

Solar and sidereal days

A day can be measured against two references. A sidereal day is one rotation of the Earth relative to the distant stars and lasts about 23 h 56 min 04 s. A solar day is the interval between two successive passages of the Sun across the same meridian. During one rotation the Earth also travels roughly a degree along its orbit, so it must turn a little more than 360° before the Sun is back on the meridian. The sidereal day versus mean solar day difference is therefore about four minutes, the solar day being the longer.

The distinction reaches the flight deck through inertial navigation. The gyros of an inertial reference system must be corrected for the Earth's rotation in space, the sidereal rate of about 15.04° per hour, slightly faster than the 15° per hour of the solar clock.

Apparent time, mean time and the equation of time

Apparent solar time is time kept by the real Sun: apparent noon is the moment the Sun crosses the local meridian, and a sundial reads apparent time. The real Sun is a poor clock. The Earth's orbit is an ellipse, so the Earth moves faster when nearer the Sun, and the Earth's axis is tilted about 23½° to the plane of its orbit, so the Sun's apparent path is inclined to the equator. Apparent solar days are therefore of unequal length through the year.

Mean solar time replaces the real Sun with a fictitious mean Sun that moves at a uniform rate along the celestial equator. Every mean solar day lasts exactly 24 hours, and clocks keep mean time. The difference between apparent and mean time is the equation of time. It follows a smooth annual cycle, reaching about 16 minutes in November and about 14 minutes, in the opposite sense, in February.

The tilt of the axis also produces the Sun's declination, its angular distance north or south of the celestial equator. Declination is about 23½°N at the June solstice, about 23½°S at the December solstice and zero at the equinoxes in late March and late September. For a given date, the declination and the observer's latitude decide how long the Sun is above the horizon.

Arc-to-time conversion and local mean time

Local mean time (LMT) is the mean time of the observer's own meridian. Because the mean Sun crosses 15° of longitude every hour, the arc-to-time conversion is fixed:

At any instant, places east of Greenwich have a later LMT than UTC and places to the west an earlier one, because the Sun reaches them first or last. The rule is longitude east, UTC least; longitude west, UTC best.

• At 0800 UTC, overhead 123°E: 123° is 8 h 12 min, added because the longitude is east, giving 1612 LMT.
• Sunset tabulated as 2005 LMT at an aerodrome on 010°W: 10° is 40 minutes, and UTC is the later time in the west, so sunset is at 2045 UTC.
• Between aerodromes on 017°W and 026°E the change of longitude is 43°, or 2 h 52 min. When LMT at the western one is 1140, it is 1432 at the eastern one.

UTC, standard time and daylight saving

Coordinated Universal Time (UTC) is the international time reference. For everyday purposes it equals mean time at the Greenwich meridian, and it replaced Greenwich Mean Time (GMT) in aviation usage; the suffix Z, spoken Zulu, marks a UTC time. ICAO requires times to be given in UTC in hours and minutes of the 24-hour day beginning at midnight, and air traffic services give time checks to the nearest half minute.

LMT changes continuously with longitude, so no country keeps it. Standard time (zone time) is the legal civil time of a region, normally a whole number of hours from UTC, each hour corresponding nominally to a band of 15° of longitude, with boundaries bent to follow national borders. Some States use half or quarter hours: India keeps UTC+5:30, Nepal UTC+5:45 and Newfoundland UTC−3:30. Daylight saving time (DST), called summer time in Europe, advances clocks by one hour in summer, so a State on UTC+1 keeps UTC+2 in summer. In the United States, Pacific Standard Time is UTC−8 and Pacific Daylight Time UTC−7; Eastern Standard Time is UTC−5 and Eastern Daylight Time UTC−4.

Conversions are safest in a fixed order: local to UTC, add the elapsed time, then UTC to local. A departure at 0930 local from an aerodrome keeping UTC+3, with 4 h 20 min to a destination keeping UTC−1, leaves at 0630 UTC, arrives at 1050 UTC and lands at 0950 local time. Adding the flight time to the departure local time would give 1350, four hours wrong.

The International Date Line

Counted eastwards from Greenwich, 180°E is 12 hours ahead of UTC; counted westwards, 180°W is 12 hours behind it. They are the same meridian, so the two sides of it differ by a whole day. The International Date Line (IDL) is where the date changes. It follows the 180° meridian in open ocean but deviates around island groups and countries, such as Kiribati, Fiji and eastern Russia, so that each keeps a single date.

• Crossing westbound, for example from Hawaii towards Japan, the date is advanced one day.
• Crossing eastbound, from 179°E to 179°W, the date is put back one day: an aeroplane crossing eastbound on 10 March enters 9 March.

The UTC date is unaffected by the crossing. Convert both ends to UTC and the date line cannot cause an error.

Sunrise, sunset and the three twilights

Sunrise and sunset are the moments the upper edge of the Sun's disc appears on, or disappears below, the horizon. Because the sky remains lit after sunset, the evening is divided by the Sun's depression below the horizon:

Morning twilight is the same sequence in reverse. Twilight lasts longest where the Sun's daily path meets the horizon at a shallow angle. Near the equator the Sun sinks almost vertically and twilight is brief; at 60°N in June it slides down at a low angle, evening civil twilight is much longer, and summer nights practically disappear.

For a given date, the LMT of sunrise, sunset and twilight depends only on latitude, because every meridian sees the same event at the same local mean time. Tables are therefore entered with date and latitude, and the pilot applies the arc-to-time correction for longitude to obtain UTC. AIPs publish sunrise, sunset and twilight times for named aerodromes, normally in UTC, in GEN 2.7.

The definitions of night that follow from these events differ between rule books:

A day-only flight is planned backwards from the end of civil twilight at the destination. If civil twilight ends at 1942 UTC, the flight takes 1 h 20 min and the pilot wants 15 minutes in hand, the latest landing is 1927 UTC and the latest take-off 1807 UTC.

Polar day and polar night

Day length varies with latitude and season because of the tilt of the axis, and the variation grows towards the poles. Poleward of the polar circles, at about 66½° (90° less the axial tilt), the Sun stays above the horizon for 24 hours or more around the summer solstice, the midnight Sun or polar day, and stays below it around the winter solstice, the polar night. Just inside the circle the winter Sun is only a little below the horizon at midday, so a spell of twilight can still occur around noon.

Because night is defined by civil twilight, high-latitude aerodromes may see no legal night for weeks in summer and only a few hours of legal day in winter. That limits flights planned by day only, and it matters to crews whose body clocks lose the usual cues of light and dark (see sleep and circadian rhythms).

The Air Almanac and celestial navigation

The Air Almanac is the standard table of astronomical data for air navigation. It gives the positions of the Sun, Moon, planets and selected navigation stars against UTC, together with sunrise, sunset and twilight tables for a range of latitudes. The FAA's definition of night refers to it directly.

Celestial navigation uses those tables to fix position. The navigator measures the altitude of a heavenly body above the horizon, in aircraft with a bubble sextant, and compares it with the altitude computed for an assumed position at the time of the sight. The result is a position line; sights on two or three bodies give a fix. Before inertial systems and GNSS it was the main means of fixing position on long oceanic flights, and it shows why time mattered so much: an error of 4 seconds of time is an error of 1′ of longitude, one nautical mile on the equator.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.