Earth Geometry, Seasons, and Magnetic Variation

061-01-01Pablo Asensio Martínez2026-04-013 min

The Earth is a dynamic system whose relationship with the Sun determines fundamental aspects for aviation, such as day length, seasons, and navigation. Below are the key concepts about our planet, solar radiation, and terrestrial magnetism.

Earth Orbit and Seasons

Insolation and Seasons

Insolation is the measure of solar radiation energy received on a given surface over a specific time. This intensity depends on the angle at which the Sun’s rays arrive, which varies according to latitude, time of day, and season of the year.

The main cause of the seasons and the variation in daylight duration is the tilt of the Earth's axis (approximately 66.5° relative to the plane of its orbit, or 23.5° relative to the perpendicular to that plane, known as the obliquity of the ecliptic).

  • When the Northern Hemisphere tilts toward the Sun (June), it experiences summer, with longer days and shorter nights.
  • When it tilts away from the Sun (December), it experiences winter, with shorter days.

At the Equator, daylight does not vary significantly throughout the year. In contrast, at the poles the difference is dramatic: during their respective summers, the Sun may not set, and in winter, it may not rise.

The Ecliptic and the Orbit

The ecliptic is the median plane of the apparent path of the Sun on the celestial sphere throughout the year. All planetary orbits, including Earth’s, are elliptical, with the Sun positioned at one of the foci.

  • Perihelion: The point where Earth is closest to the Sun (January), moving at its maximum orbital speed.
  • Aphelion: The point where it is farthest (July), moving at its minimum speed.

The solstices (June and December) mark the points where the Sun reaches its maximum north or south declination (Tropics of Cancer and Capricorn). The equinoxes (March and September) are the moments when the Sun is directly over the Equator and day and night have equal duration at all latitudes.

Navigation: Circles and Distances

For air navigation, we distinguish between two types of circles on Earth’s surface:

  • Great Circle: A circle whose plane passes through the Earth's center (e.g., the Equator).
  • Small Circle: Any circle that is not a great circle (e.g., all latitude parallels except the Equator).

A practical concept is computing the distance when flying around the Earth at a constant latitude (a small circle). The formula for the distance (Departure) is: $ \text{Distance (NM)} = \text{Change in Longitude (min)} \times \cos(\text{Latitude}) $ For example, for an aircraft flying around the world (360° of longitude change) at a latitude of 88°: $ 21600' \times \cos(88^\circ) = 753.83 \text{ NM} $

Terrestrial Magnetism

Earth behaves like a giant magnet, but its magnetic poles do not coincide with the geographic poles.

  • Magnetic Variation (or magnetic declination): The angle between the true meridian (Geographic North) and the magnetic meridian (Magnetic North).
  • Isogonic Lines: Lines connecting points with equal magnetic variation.
  • Agonic Line: The line where variation is zero (true north and magnetic north are aligned).

Because of the dynamic nature of Earth’s interior, the Magnetic North Pole moves slowly, causing variation to change over time. It is essential for pilots to use updated navigation charts to correct their magnetic headings.

Shape of the Earth (WGS-84)

Earth is not a perfect sphere but an oblate spheroid (flattened at the poles and bulged at the equator) due to its rotation. In the WGS-84 (World Geodetic System 1984) reference system, used for GPS navigation, this ellipsoidal shape is the basis for calculating precise positions. In this model, the only true great circle aligned with latitude is the Equator.