Axial tilt and seasons
The axial tilt of Earth isapproximately 23.439281° with the axis of its orbit plane, alwayspointing towards the Celestial Poles. Due to Earth's axial tilt, theamount of sunlight reaching any given point on the surface variesover the course of the year. This causes the seasonal change inclimate, with summer in the Northern Hemisphere occurring when theTropic of Cancer is facing the Sun, and in the Southern Hemispherewhen the Tropic of Capricorn faces the Sun. In each instance, winteroccurs simultaneously in the opposite hemisphere. During the summer,the day lasts longer, and the Sun climbs higher in the sky. Inwinter, the climate becomes cooler and the days shorter. Above theArctic Circle and below the Antarctic Circle there is no daylight atall for part of the year, causing a polar night, and this nightextends for several months at the poles themselves. These samelatitudes also experience a midnight sun, where the sun remainsvisible all day.
By astronomical convention, the fourseasons can be determined by the solstices—the points in the orbitof maximum axial tilt toward or away from the Sun—and theequinoxes, when Earth's rotational axis is aligned with its orbitalaxis. In the Northern Hemisphere, winter solstice currently occursaround 21 December; summer solstice is near 21 June, spring equinoxis around 20 March and autumnal equinox is about 22 or 23 September.In the Southern Hemisphere, the situation is reversed, with thesummer and winter solstices exchanged and the spring and autumnalequinox dates swapped.
The angle of Earth's axial tilt isrelatively stable over long periods of time. Its axial tilt doesundergo nutation; a slight, irregular motion with a main period of18.6 years. The orientation (rather than the angle) of Earth's axisalso changes over time, precessing around in a complete circle overeach 25,800-year cycle; this precession is the reason for thedifference between a sidereal year and a tropical year. Both of thesemotions are caused by the varying attraction of the Sun and the Moonon Earth's equatorial bulge. The poles also migrate a few metersacross Earth's surface. This polar motion has multiple, cyclicalcomponents, which collectively are termed quasi-periodic motion. Inaddition to an annual component to this motion, there is a 14-monthcycle called the Chandler wobble. Earth's rotational velocity alsovaries in a phenomenon known as length-of-day variation.
In modern times, Earth's perihelionoccurs around 3 January, and its aphelion around 4 July. These dateschange over time due to precession and other orbital factors, whichfollow cyclical patterns known as Milankovitch cycles. The changingEarth-Sun distance causes an increase of about 6.8% in solar energyreaching Earth at perihelion relative to aphelion. Because theSouthern Hemisphere is tilted toward the Sun at about the same timethat Earth reaches the closest approach to the Sun, the SouthernHemisphere receives slightly more energy from the Sun than does thenorthern over the course of a year. This effect is much lesssignificant than the total energy change due to the axial tilt, andmost of the excess energy is absorbed by the higher proportion ofwater in the Southern Hemisphere.
Earth–Moon system
Moon
The Moon is a relatively large,terrestrial, planet-like natural satellite, with a diameter aboutone-quarter of Earth's. It is the largest moon in the Solar Systemrelative to the size of its planet, although Charon is largerrelative to the dwarf planet Pluto. The natural satellites of otherplanets are also referred to as "moons", afterEarth's. The most widely accepted theory of the Moon's origin, thegiant-impact hypothesis, states that it formed from the collision ofa Mars-size protoplanet called Theia with the early Earth. Thishypothesis explains (among other things) the Moon's relative lack ofiron and volatile elements and the fact that its composition isnearly identical to that of Earth's crust.
