Solar System, Earth Rotation and Seasons

The solar system and the Earth — navigation basics

Understanding the Earth's motion in the solar system matters for navigation because:

• Sun position determines daylight and visibility,
• Seasons affect sunrise and sunset times,
• Earth rotation is the basis of time measurement,
• Magnetic variation changes slowly due to core circulation.

The Earth

Structure:

• Core (solid inner + liquid outer, mostly iron),
• Mantle (silicate rock),
• Crust (continental + oceanic).

Shape:

• Geoid — actual shape, slightly flattened at the poles.
• Ellipsoid — simplified mathematical model (WGS-84 in aviation).
• Equatorial radius: 6 378 km.
• Polar radius: 6 357 km.
• Flattening: about 1/298.

Axis:

• The Earth's axis passes through the geographic poles.
• Tilted about 23.5° from the perpendicular to the orbital plane (ecliptic).
• This tilt is the main cause of the seasons.

Earth rotation — day and night

Properties:

• Earth rotates from west to east (counter-clockwise viewed from above the North Pole).
• One complete rotation takes ~23 h 56 min 4 s (sidereal day).
• Mean solar day (noon to noon) = 24 h (4 minutes longer due to orbital motion).
• Rotational speed at the equator: ~1 670 km/h (465 m/s).

Terminator (day-night line):

• Separates lit and dark hemispheres.
• Moves westward across the globe at 360°/24 h = 15°/h.

Example: New York is 5 hours behind Berlin → 5 × 15° = 75° west.

Earth orbit around the Sun

Properties:

• One orbit (revolution) takes ~365.25 days (leap year correction).
• Orbit is elliptical, not circular.
• Perihelion (closest to Sun): ~3 January — Earth 147 Mkm from Sun.
• Aphelion (farthest): ~4 July — Earth 152 Mkm from Sun.

The Earth's axial tilt causes the seasons — NOT the varying distance from the Sun!

The seasons

Because of the 23.5° tilt, each pole sees different amounts of sunlight during the year:

Northern Hemisphere

Summer solstice — about 21 June:

• North Pole tilted toward the Sun.
• Longest day in the north.
• Sun reaches the Tropic of Cancer (23.5° N).
• Midnight sun north of the Arctic Circle (66.5° N).

Autumn equinox — about 23 September:

• Sun directly over the equator.
• Day = night worldwide.

Winter solstice — about 22 December:

• North Pole tilted away from the Sun.
• Shortest day in the north.
• Sun reaches the Tropic of Capricorn (23.5° S).
• Polar night north of the Arctic Circle.

Spring equinox — about 21 March:

• Sun again over the equator.

Southern Hemisphere

Mirror image of the north — when Berlin has summer, Buenos Aires has winter.

Implications for pilots

Daylight duration

Mid latitudes (e.g. 50° N, central Europe):

• Summer: up to 17 h day (21 June).
• Winter: only 7.5 h day (22 December).

Consequence: cross-country flights in winter require strict timing relative to sunset.

Sun elevation

Sun elevation at noon (observer site, sun in the south):

• Summer solstice: 90° − latitude + 23.5°. Berlin (52.5° N): 90° − 52.5° + 23.5° = 61°.
• Winter solstice: 90° − latitude − 23.5°. Berlin: 90° − 52.5° − 23.5° = 14°.

Consequence: in winter the sun is low in the south → significant glare on approaches to southern runways.

Solar wind and magnetic variation

Solar activity (sunspots, corona, coronal mass ejections) influences:

• Ionosphere → HF radio propagation,
• Earth magnetic field → slight magnetic-variation drift,
• GPS accuracy during severe solar events.

In a solar maximum (every ~11 years) HF communication is disrupted and GPS may show reduced accuracy.

Polar night and polar day

In regions north of the Arctic Circle (66.5° N):

• Polar night in winter — no Sun for weeks.
• Midnight sun in summer — no night.
• Relevant for flights in northern Scandinavia, Iceland, Greenland.

Solar declination

Solar declination δ is the angular position of the Sun north/south of the equator:

Declination is used in astronomical almanacs and sunrise/sunset tables.

Moon orbit and tides (background)

The Moon orbits the Earth in ~27.3 days and together with the Sun causes the tides. Not directly relevant to standard PPL navigation, except:

• Moonlight at night — a full moon can greatly improve VFR visibility.
• Spring/neap tides for seaplane operations.