HiSET Science: Earth-Moon-Sun System

Earth's Rotation
The Earth rotates west to east about its axis, an imaginary straight line that runs nearly vertically through the center of the planet. This rotation (which takes 23 hours, 56 minutes, and 5 seconds) places each section of the Earth's surface in a position facing the Sun for a period of time, thus creating the alternating periods of light and darkness we experience as day and night. This rotation constitutes a sidereal day; it is measured as the amount of time required for a reference star to cross the meridian (an imaginary north-south line above an observer). Each star crosses the meridian once every (sidereal) day. Since the speed at which Earth rotates is not exactly constant, we use the mean solar day (a 24-hour period) in timekeeping rather than the slightly variable sidereal day.

Sun
The Sun is the vital force of life on Earth; it is also the central component of our solar system.
It is basically a sphere of extremely hot gases (close to 15 million degrees at the core) held together by gravity. Some of these gaseous molecules are ionized due to the high temperatures. The balance between its gravitational force and the pressure produced by the hot gases is called hydrostatic equilibrium.
The source of the solar energy that keeps the Sun alive and plays a key role in the perpetuation of life on Earth is located in the Sun's core, where nucleosynthesis produces heat energy and photons. The Sun's atmosphere consists of the photosphere, the surface visible from Earth, the chronosphere, a layer outside of and hotter than the photosphere, the transition zone (the region where temperatures rise between the chronosphere and the corona), and the corona, which is best viewed at X-ray wavelengths. A solar flare is an explosive emission of ionized particles from the Sun's surface.

Earth's Revolution Around the Sun
Like all celestial objects in our solar system, planet Earth revolves around the Sun. This process takes approximately 365 1/4 days, the period of time that constitutes a calendar year. The path of the orbit of Earth around the Sun is not circular but elliptical.
Therefore, the distances between the Earth and the Sun at points on either extreme of this counterclockwise orbit are not equal. In other words, the distance between the two objects varies over the course of a year. At perihelion, the minimum heliocentric distance, Earth is 147 million kilometers from the Sun. At aphelion, the maximum heliocentric distance, Earth is 152 million kilometers from the Sun. This movement of the Earth is responsible for the apparent annual motions of the Sun (in a path referred to as the ecliptic) and other celestial objects visible from Earth's surface.

Seasons
The combined effects of Earth's revolution around the Sun and the tilt of the planet's rotational axis create the seasons. Earth's axis is not perfectly perpendicular to its orbital plane; rather, it is tilted about 23.5 degrees. Thus, at different times of the year, certain areas of the surface receive different amounts of sunlight. For example, during the period of time in Earth's orbit when the Northern Hemisphere is tipped toward the Sun, it is exposed to higher amounts of nearly direct sunlight than at any other time of year (days are longer, and the direction of Sun's rays striking the surface is nearly perpendicular). This period of time is summer in the Northern
Hemisphere and winter in the Southern Hemisphere; on the opposite side of the orbit, the seasons are reversed in each hemisphere.

Summer and Winter Solstices
The summer solstice occurs when Earth's orbital position and axial tilt point the North Pole most directly toward the Sun. This happens on or near June 21 each year. On this day in the Northern Hemisphere, the Sun appears to be directly overhead (at its zenith) at 12:00 noon. The entire Arctic Circle (the north polar region above approximately 66.5 degrees north latitude) is bathed in sunlight for a complete solar day. The North Pole itself experiences constant daylight for six full months. Conversely, the winter solstice occurs when the South Pole is oriented most directly toward the Sun.
This phenomenon, which falls on or near December 22 each year, orients the Sun as viewed from the Northern Hemisphere at its lowest point above the horizon.

Equinoxes
The ecliptic (the
Sun's apparent path through the sky) crosses Earth's equatorial plane twice during the year; these intersections occur when the North Pole is at a right angle from the line connecting the Earth and the Sun. At these times, the two hemispheres experience equal periods of light and dark. These two points in time are respectively referred to as the vernal (spring) equinox (on or about March 21) and the autumnal (fall) equinox (on or about September 23). A calendar year is measured as the length of time between vernal equinoxes.

Moon
Earth's Moon is historically one of the most studied celestial bodies. Its mass is approximately 1.2% of the Earth's mass, and its radius is just over one-fourth of the size of the Earth's radius. Measurements of the Moon's density suggest that its characteristics are similar to those of the rocks that make up Earth's crust. The landscape of the Moon consists mostly of mountains and craters formed by collisions of this surface with meteors and other interplanetary materials. The Moon's crust (estimated to be 50 to 100 kilometers in thickness) is made up of a layer of regolith (lunar soil) supported by a layer of loose rocks and gravel. Beneath the crust is a mantle made up of a solid lithosphere and a semiliquid asthenosphere. The Moon's core (the innermost 500 kilometers of the body) is not as dense as that of the Earth. The Moon is made up mostly of refractory elements with high melting and boiling points with low levels of heavy elements such as iron.

Formation Theories
The fission model of Moon origin suggests that the Moon is actually a piece of the Earth that split off early during the planet's formation. In this model, a portion of the Earth's mantle fissioned off during a liquid stage in its formation, creating the Moon. According to the capture model, the Moon formed elsewhere in the solar system and was subsequently captured by the Earth's gravitational field. The double-impact model states that the Earth and the Moon formed during the same period of time from the same accretion material. Each of these theories has its strengths, but none of them can explain all of the properties of the Moon and its relationship to the Earth. Recently, a fourth (widely accepted) hypothesis has been suggested, which involves the collision between the Earth and a large asteroid.
This hypothetical collision is said to have released a large amount of Earth's crustal material into its orbit; the Moon accreted from that material and the material displaced from the asteroid due to the collision.

Earth-Moon System
While the Moon is commonly referred to as a satellite of the Earth, this is not entirely accurate. The ratio of the masses of the two bodies is much larger than that of any other planet-satellite system. Also, the Moon does not truly revolve around the Earth. Rather, the two bodies revolve around a common center of mass beneath the surface of the Earth (approximately 4,800 kilometers from Earth's core). The orbital planes of the Moon and the Earth are nearly aligned; therefore, the Moon moves close to the ecliptic, as seen from Earth. Due to the Moon's synchronous rotation (its rotation period and orbital period are equal); the same side of the Moon is always facing Earth.
This occurs because of the mutual gravitational pull between the two bodies.

Phases
The sidereal period of the Moon (the time it takes the Moon to orbit the Earth with the fixed stars as reference points) is about 27 days. The lunar month (or synodic period) is the period of time required for the Moon to return to a given alignment as observed from the Earth with the Sun as a reference point; this takes 29 days, 12 hours, 44 minutes, and 28 seconds. A discrepancy exists between the two periods of time because the Earth and the Moon move at the same time. Sunlight reflected off of the Moon's surface at different times during the lunar month causes its apparent shape to change. The sequence of the Moon's shapes is referred to as the phases of the Moon. The full Moon can be viewed when the body is directly opposite from the Sun. The opposite end of the cycle, the new Moon, occurs when the Moon is not visible from Earth because it is situated between the Earth and the Sun.

Configurations
The configurations of the Moon describe its position with respect to the Earth and the Sun. We can thus observe a correlation between the phases of the Moon and its configuration. The Moon is at conjunction at the time of the new Moon—it is situated in the same direction as the Sun. Quadrature (which signals the first quarter phase) is the position of the Moon at a right angle between the Earth-Sun line; we see exactly half of the Moon's sunlit hemisphere. This is the waxing crescent phase, in which we see more of the Moon each night. Then comes opposition (which occurs when the Moon lies in the direction opposite the Sun)—we see the full Moon. After this point, the Moon enters its waning gibbons phase as it travels back toward quadrature. When it reaches that point again, it has entered the third-quarter phase. Finally, as the Moon circles back toward conjunction, it is in its waning crescent phase.