Seasons and the Moon
Earth's tilted axis causes the seasons, and the Moon's orbit produces its phases. Both follow from simple geometry.
Earth's tilt
Earth's rotation axis is tilted about 23.4 degrees from perpendicular to its orbit, and it points in nearly the same direction in space throughout the year. As Earth orbits, first one hemisphere and then the other is tilted toward the Sun.
Why seasons happen
When your hemisphere tilts toward the Sun, sunlight strikes more directly (higher in the sky), concentrating energy on each square metre, and days are longer. When it tilts away, sunlight is more spread out and days are shorter. Seasons are not caused by Earth's distance from the Sun: Earth is actually closest to the Sun in early January, during the Northern Hemisphere's winter.
Solstices and equinoxes
At the June solstice (about 21 June), the Sun is farthest north in the sky; the Northern Hemisphere has its longest day. At the December solstice, the reverse. At the equinoxes (about 20 March and 22 September), the Sun crosses the celestial equator and day and night are roughly equal everywhere.
Lunar phases
The Moon shines by reflected sunlight, and half of it is always lit. As it orbits Earth about every 29.5 days (from new moon to new moon), we see different fractions of the lit half: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent. Phases are not caused by Earth's shadow.
Synchronous rotation
The Moon rotates once on its axis in the same time it takes to orbit Earth, so the same side always faces us. This tidal locking resulted from Earth's gravity acting on the Moon over billions of years. The 'far side' is not permanently dark; it receives sunlight during our new moon.
Eclipses
A solar eclipse occurs when the Moon passes between the Sun and Earth, casting its shadow on Earth; it can only happen at new moon. A lunar eclipse occurs when Earth's shadow falls on the Moon, at full moon, turning it a coppery red as sunlight is bent through Earth's atmosphere. Because the Moon's orbit is tilted about 5 degrees, eclipses do not happen every month.
Tides
Tides are caused mainly by the Moon's gravity pulling more strongly on the side of Earth nearest it than on the far side, stretching Earth's oceans into two bulges. Earth rotates through these bulges, producing roughly two high tides a day. When Sun and Moon align (new and full moon), spring tides are larger; at quarter moons, neap tides are smaller.
Worked example
In Sydney, Australia, December is summer. On 21 December, the Southern Hemisphere tilts toward the Sun, so the Sun climbs high at noon and days last over 14 hours. At the same time, Earth is only a few million kilometres closer to the Sun than in July, a small difference that cannot explain the seasons, since the Northern Hemisphere is in winter then.
Key takeaways
Earth's axis is tilted about 23.4 degrees. Seasons result from the angle of sunlight and day length, not distance from the Sun. Solstices mark extreme Sun positions; equinoxes have about equal day and night. Lunar phases come from seeing different parts of the Moon's lit half over 29.5 days. The Moon is tidally locked. Solar eclipses happen at new moon, lunar eclipses at full moon. Tides come mainly from the Moon's differential gravity.
Flashcard
About 23.4 degrees
Check your understanding
What causes Earth's seasons?