The earth is an oblate spheroid that undergoes three basic motions: (1) a
yearly revolution around the Sun, (2) a daily rotation on its axis, and (3) a
slow wobble of its axis called precession.
As Earth makes its yearly revolution around the Sun, it maintains a
generally constant inclination of its axis to the plane of the ecliptic of 66.5°,
or 23.5° from a line perpendicular to the plane. In addition, Earth maintains
a generally constant orientation of its axis to the stars, which always
points in the same direction. The constant inclination and orientation of
the axis, together with Earth's rotation and revolution, produce three effects:
(1) days and nights that vary in length, (2) seasons that change during
the course of a year, and (3) climates that vary with latitude.When
Earth is at a place in its orbit so the axis points toward the Sun, the Northern
Hemisphere experiences the longest days and the summer season.
This begins on June 21 or 22, which is called the summer solstice. Six
months later, the axis points away from the Sun and the Northern Hemisphere
experiences the shortest days and the winter season. This begins
on December 22 or 23 and is called the winter solstice. On March 20 or
21, Earth is halfway between the solstices and has days and nights of equal
length, which is called the spring (or vernal) equinox. On September 22 or
23, the autumnal equinox, another period of equal nights and days, identifies
the beginning of the fall season.Precession is a slow wobbling of the axis as Earth spins. Precession is
produced by the gravitational tugs of the Sun and Moon on Earth's equatorial
bulge.
Lines around Earth that are parallel to the equator are circles called
parallels. The distance from the equator to a point on a parallel is called
the latitude of that point. North and south arcs that intersect at the poles
are called meridians. The meridian that runs through the Greenwich Observatory
is a reference line called the prime meridian. The distance of a
point east or west of the prime meridian is called the longitude of that
point.
The event of time called noon is the instant the Sun appears to move
across the celestial meridian. The instant of noon at a particular location
is called the apparent local noon. The time at a given place that is determined
by a sundial is called the apparent local solar time. It is the basis for
an averaged, uniform standard time called the mean solar time. Mean solar
time is the time used to set clocks.
A sidereal day is the interval between two consecutive crossings of
the celestial meridian by a star. An apparent solar day is the interval between
two consecutive crossings of the celestial meridian by the Sun,
from one apparent solar noon to the next. A mean solar day is twenty-four hours as determined from mean solar time. The equation of time
shows how the local solar time is faster or slower than the clock time during
different days of the year.
Earth's surface is divided into one-hour standard time zones that are
about 15° ofmeridian wide. The international date line is the 180°meridian;
you gain a day if you cross this line while traveling westward and repeat
a day if you are traveling eastward.
A tropical year is the interval between two consecutive spring
equinoxes. A sidereal year is the interval of time between two consecutive
crossings of a star by the Sun. It is the tropical year that is used as a standard
time interval for the calendar year. A sidereal month is the interval of
time between two consecutive crossings of a star by the Moon. The synodic
month is the interval of time from a new moon to the next new
moon. The synodic month is about 29 1/2 days long, which is about 1/12
of a year.
Earth and the Moon act as a system, with both bodies revolving
around a common center of mass located under Earth's surface. This
combined motion around the Sun produces three phenomena: (1) as the
Earth-Moon system revolves around the Sun, different parts of the illuminated
lunar surface, called phases, are visible from Earth; (2) a solar
eclipse is observed where the Moon's shadow falls on Earth, and a lunar
eclipse is observed where Earth's shadow falls on the Moon; and (3) the
tides, a periodic rising and falling of sea level, are produced by gravitational
attractions of the Moon and Sun and by the movement of the
Earth-Moon system.
