Why Pilots Study Time
There are two primary applications for aviation time knowledge:
Standard Time: Different countries operate different local times. You must know how to determine the standard time in any country you operate to or through.
Sunrise/Sunset: You must be able to calculate when it gets light or dark at your destination — particularly if the aerodrome lacks lighting equipment.
Planetary Orbits and Kepler's Laws
The solar system consists of the Sun and its major planets, including Earth. The shape and mechanics of planetary orbits are governed by Kepler's three laws.
Kepler's First Law — Shape of the Orbit
A planet travels around the Sun in an elliptical orbit. The Sun is at one of the two foci of the ellipse (F1). The planet's two extreme positions are:
🔴 Perihelion (P) — closest point to the Sun. Occurs around 4 January. Distance ≈ 91.4 million statute miles.
🔵 Aphelion (A) — furthest point from the Sun. Occurs around 4 July. Distance ≈ 94.6 million statute miles.
Kepler's Second Law — Equal Areas in Equal Times
The radius vector (line from Sun to planet) sweeps out equal areas in equal times. Consequence: the planet moves fastest near perihelion and slowest near aphelion. This varying orbital speed affects our measurement of a day.
Kepler's Third Law — Period vs. Size
The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. This determines the length of a year and the relative orbital periods of all planets.
The Seasons
The seasons are not caused by the Earth's distance from the Sun (the distance difference only changes solar heat by ~3%). The real cause is the inclination (tilt) of the Earth's axis:
The Earth's axis is inclined at 66.5° to its orbital plane — equivalently, 23.5° to the normal (perpendicular) to the orbital plane. This is called the obliquity of the ecliptic.
🌞 Summer Solstice (~21 June)
Sun overhead 23½°N (Tropic of Cancer)
NH: Summer Solstice | SH: Winter Solstice
Longest day in NH, longest night in SH
❄️ Winter Solstice (~21 December)
Sun overhead 23½°S (Tropic of Capricorn)
NH: Winter Solstice | SH: Summer Solstice
Longest night in NH, longest day in SH
🌱 Vernal (Spring) Equinox (~21 March)
Sun crosses Equator South → North
NH: Spring Equinox | SH: Autumn Equinox
Equal day and night worldwide
🍂 Autumnal Equinox (~21 September)
Sun crosses Equator North → South
NH: Autumn Equinox | SH: Spring Equinox
Equal day and night worldwide
Declination
The angle the Sun is above or below the Equator is called Declination. It is analogous to latitude on the Earth. The Sun's declination cycles annually between 23.5°N and 23.5°S.
Measurement of Days and Years
A 'day' is the time taken for the Earth to rotate once about its axis, measured against a celestial body. Measurement against a star produces a sidereal day; against the Sun, a solar day.
Requirements for a Civil Day
✅ Related to light and darkness — 1200 hrs must always be roughly midway between sunrise and sunset (solar-based).
✅ Constant length — must not vary day to day.
Types of Day
| Type | Reference | Length | Civil Use? |
|---|---|---|---|
| Sidereal Day | Distant star | 23 hrs 56 min 04 sec | No — not related to Sun |
| Apparent Solar Day | Real/apparent Sun | Varies throughout year | No — variable length |
| Mean Solar Day | Imaginary mean Sun | 24 hrs (exactly, by definition) | ✅ YES — the civil day |
The Equation of Time
The difference between apparent noon (when the real Sun transits the meridian) and mean noon (1200 LMT) is called the Equation of Time. It results from the Earth's elliptical orbit and the inclination of the ecliptic.
📌 Maximum values: approximately +16 minutes in November and −14 minutes in February.
📌 The equation of time is zero around the equinoxes and solstices.
Types of Year
| Type | Definition | Length |
|---|---|---|
| Sidereal Year | Earth's orbit measured against a distant star | 365 days 6 hrs |
| Tropical Year | One complete cycle of the seasons | 365 d 5 h 48.75 min |
| Calendar Year | Civil year, kept in step with tropical year | Normally 365 days; 366 in leap year |
Leap Year Rule
Step 1: Every 4th year is a leap year (366 days).
Step 2: Centennial years (1800, 1900, 2100…) are NOT leap years — unless the first two digits are divisible by 4.
Step 3: Exception to the exception: 1600, 2000, 2400 are leap years (16, 20, 24 are divisible by 4).
Hour Angle (GHA / LHA)
Hour Angle is a celestial coordinate that is analogous to longitude on Earth. It measures how far westward a celestial body is from a reference meridian.
🌐 Greenwich Hour Angle (GHA) — angle measured westward from the Greenwich Meridian to the meridian of the body (0° to 360°).
📍 Local Hour Angle (LHA) — angle measured westward from the observer's meridian to the body's meridian.
🔗 Relationship: LHA = GHA + East Longitude (or GHA − West Longitude)