The solar system, seasons and the day
Navigation · Chapter 14 · DGCA CPL and ATPL
How the Earth moves round the Sun, why there are seasons, and why days and years are not all the same length.
Remember
- Orbit is an ellipse; perihelion early Jan (fastest), aphelion early Jul.
- Axis 66.5° to the orbit plane (23.5° from upright): the tilt makes the seasons.
- Sun overhead 23.5°N about 21 Jun, 23.5°S about 21 Dec, the equator at the equinoxes.
- Sidereal day < apparent solar day; mean solar day = 24 h.
- GHA is measured westwards: ≤ 180 west, > 180 is 360 − GHA east. LHA = GHA − long W (+ long E).
- Equation of time: the real Sun is up to ~16 min early (Nov) or ~14 min late (Feb).
What path does the Earth follow round the Sun?
The Earth from space
- The Sun and the Earth • From far out in space: the Sun (it makes its own light) and the Earth (lit on one side only) • The dark half of the Earth is night. (Both are drawn far bigger than real: at this distance the real Earth would be a speck.)
- One trip round: one year • The Earth goes round the Sun anticlockwise seen from above the North Pole • One full trip takes a year: watch the date
- Not a circle: an ellipse • The path is an ellipse, with the Sun not in the middle but at one focus • Kepler's first law. (Drawn more oval than real: the real one looks almost round.)
- Closest and farthest • perihelion: closest, about 4 January • aphelion: farthest, about 4 July • The distance changes only about 3 % (91.4 to 94.6 million statute miles)
- Faster when closer • The line Sun–Earth sweeps equal areas in equal times: Kepler's second law • The two 30-day slices cover the same area, so near the Sun the Earth moves faster
Which way does the Earth's axis point?
The tilted Earth
- Ride along with the Earth • Close up: the Earth spins about its axis, a line through the poles (white) • The blue line stands upright to the orbit
- Leaning 23.5° • The axis leans 23.5° from upright • So it makes 66.5° with the orbit plane
- Always the same way • All year the axis points the same way in space (towards the Pole Star) • So half the year the north leans towards the Sun, half the year away
- Two planes, 23.5° apart • The orbit's plane is the ecliptic (blue) • The equator's plane (gold) is tilted 23.5° to it
Why is it summer in June in India but winter in Australia?
Why there are seasons
This lesson is part of the Navigation plan. See plans
How does the Sun's overhead latitude change through the year?
The Sun's declination
This lesson is part of the Navigation plan. See plans
Why is a day by the Sun longer than one turn of the Earth?
A day by the stars, a day by the Sun
This lesson is part of the Navigation plan. See plans
How do clocks and calendars deal with uneven days and years?
Mean time, and the years
This lesson is part of the Navigation plan. See plans
How is the Sun's position given as a "longitude in the sky"?
Greenwich hour angle
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 14
Test yourself
This lesson is part of the Navigation plan. See plans