Earth and the Moon


 

Earth and the Moon as seen from Mars, at only a quarter of the maximum possible Earth-Moon separation

Earth and the Moon as seen from Mars, at only a quarter of the maximum possible Earth-Moon separation     [Source]

Earth (also known as Terra) is a terrestrial planet featuring vast oceans on most of its surface. It has one natural satellite, The Moon (also called Luna). It is the largest moon relative to the diameter of its planet in the Solar System. The Moon is tidally locked to Earth, meaning one side (the near side) always faces the planet.

There is no permanent “dark side” of the Moon, only a far side. The Moon exhibits phases (caused by sunlight), which cycle and repeat every 29.5 Earth days. Below is a listing of this cycle and approximate times for moonrise and moonset (this does not account for Daylight Saving Time).

Moon phase: Highest at: Rise/set time:
New 12:00 pm 6:00 am – 6:00 pm
Waxing crescent 3:00 pm 9:00 am – 9:00 pm
First quarter 6:00 pm 12:00 pm – 12:00 am
Waxing gibbous 9:00 pm 3:00 pm – 3:00 am
Full 12:00 am 6:00 pm – 6:00 am
Waning gibbous 3:00 am 9:00 pm – 9:00 am
Last quarter 6:00 am 12:00 am – 12:00 pm
Waning crescent 9:00 am 3:00 am – 3:00 pm
New 12:00 pm 6:00 am – 6:00 pm
A time lapse of the lunar cycle, with the Full Moon near perigee; due to libration, 59% of the Moon's surface can be seen from Earth

A time lapse of the lunar cycle, with the Full Moon near perigee; due to libration, 59% of the Moon’s surface can be seen from Earth

An eclipse can occur only during a Full Moon or a New Moon, each which could potentially produce a lunar eclipse or a solar eclipse respectively. When the Moon hides in Earth’s shadow opposite the Sun, it is known as a lunar eclipse. Three different types of lunar eclipses exist: total, partial, and penumbral. Total lunar eclipses do not render the Moon invisible however, as some sunlight is refracted through Earth’s atmosphere. This results in a weak reddish glow. (For information on solar eclipses, see “The Sun” page.)

The Moon has two distinct zones on its surface: the maria (Latin for “seas”) and the highlands. The maria (singular: mare) are visible as smooth, dark plains. They contain basalt from the Moon’s volcanic past. The maria exist almost entirely on the Moon’s near side, possibly due to a thicker crust on the far side. The heavily-cratered highlands are much more reflective than the maria. Still yet, the Moon’s overall albedo (0.136) is no greater than that of asphalt.

Overlapping craters Theophilus (right) and Cyrillus (left)

Overlapping craters Theophilus (right) and Cyrillus (left)     [Source]

The Moon has essentially no atmosphere, so no erosion occurs on its surface. This allows much of its history to be preserved as an artifact. By observing overlapping impact craters (such as Theophilus and Cyrillus), one can easily gauge which crater was created first. Additionally, by assuming a constant crater distribution rate one can also compare the age of different parts of the surface, particularly the maria versus the highlands.

Earth has an even higher variety in its surface characteristics. The planet features a 23.4° axial tilt, resulting in seasonal changes of its climate as the Sun’s position varies from -23.4° to 23.4° in celestial latitude (each extreme representing a solstice). When the Sun crosses the celestial equator, it represents an equinox.

Month: Northern Hemisphere: Southern Hemisphere: Latitude of direct sun illumination:
March Vernal equinox Autumnal equinox Earth’s equator
June Summer solstice Winter solstice Tropic of Cancer
September Autumnal equinox Vernal equinox Earth’s equator
December Winter solstice Summer solstice Tropic of Capricorn

Earth’s distance from the Sun defines the astronomical unit (AU), equivalent to 150 billion meters (1.50×1011 m). It is a unit of length commonly used for measurements within the Solar System. It also has a trigonometric relationship to the parsec (pc), a different unit of distance related to stellar parallax.

Due to a molten iron core, Earth has a strong magnetic field. This forms the magnetosphere, which repels particles in the solar wind. However, unusually high amounts of solar activity can lead to a dazzling display of aurorae, typically directed into the atmosphere as a ring offset 10-20° around the magnetic north and south poles. Strong geomagnetic storms can result in aurorae even further from the poles.

A brilliant display of the aurora borealis from Iceland

A brilliant display of the aurora borealis from Iceland     [Source]

Much of sentient life on the planet once believed they occupied the ultimate place in the Universe, and that it was created entirely for their pleasure alone. In a certain sense, it is not surprising that much of the planet’s recent history (within the last few thousand years) has consisted of selfish warfare rather than a general unifying quest for knowledge.

 


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