THE MOON
The moon is Earth's
only natural satellite. The moon is a cold, dry orb whose surface is
studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles.
The
same side of the moon always faces the Earth. The far side of the moon
was first observed by humans in 1959 when the unmanned Soviet Luna 3
mission orbited the moon and photographed it. Neil Armstrong
and Buzz Aldrin (on NASA's Apollo 11 mission, which also included
Michael Collins) were the first people to walk on the moon, on July 20,
1969.
If
you were standing on the moon, the sky would always appear dark, even
during the daytime. Also, from any spot on the moon (except on the far
side of the moon where you cannot see the Earth), the Earth
would always be in the same place in the sky; the phase of the Earth
changes and the Earth rotates, displaying various continents.
THE MOON'S ORBIT
The moon is about 238,900 miles (384,000 km) from Earth on average. At its closest approach (the lunar perigee)
the moon is 221,460 miles (356,410 km) from the Earth. At its farthest
approach (its apogee) the moon is 252,700 miles (406,700 km) from the
Earth.
The moon revolves around the Earth in about one month (27
days 8 hours). It rotates around its own axis in the same amount of
time. The same side of the moon always faces the Earth; it is in a
synchronous rotation with the Earth.
The
Moon's orbit is expanding over time as it slows down (the Earth is also
slowing down as it loses energy). For example, a billion years ago, the
Moon was much closer to the Earth (roughly 200,000 kilometers) and took
only 20 days to orbit the Earth. Also, one Earth 'day' was about 18
hours long (instead of our 24 hour day). The tides on Earth were also
much stronger since the moon was closer to the Earth.
SAROS
The saros is the roughly 18-year periodic cycle of the
Earth-Moon-Sun system. Every 6,585 days, the Earth, Moon and Sun are in
exactly the same position. When there is a lunar eclipse, there will
also be one exactly 6,585 days later.
SIZE
The moon's diameter is 2,140 miles (3,476 km), 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter).
The gravitational tidal influence of the Moon on the Earth is about twice as strong as the Sun's
gravitational tidal influence. The Earth:moon size ratio is quite small
in comparison to ratios of most other planet:moon systems (for most
planets in our Solar System, the moons are much smaller in comparison
to the planet and have less of an effect on the planet).
MASS AND GRAVITY
The moon's mass is (7.35 x 10 22 kg), about 1/81 of the Earth's mass.
The moon's gravitational force is only 17% of the Earth's gravity. For
example, a 100 pound (45 kg) person would weigh only 17 pounds (7.6 kg)
on the Moon.
The moon's density is 3340 kg/m 3. This is about 3/5 the density of the Earth.
TEMPERATURE
The temperature on the Moon ranges from daytime highs of about 130°C = 265°F to nighttime lows of about -110°C = -170°F
ATMOSPHERE
The moon has no atmosphere. On the moon, the sky
is always appears dark, even on the bright side (because there is no
atmosphere). Also, since sound waves travel through air, the moon is
silent; there can be no sound transmission on the moon.
MARE
Mare (plural maria) means "sea," but maria on the moon are plains on
the moon. They are called maria because very early astronomers thought
that these areas on the moon were great seas. The first moon landing
was in the Mare Tranquillitatis (the Sea of Tranquility). Maria are
concentrated on the side of the moon that faces the Earth; the far side
has very few of these plains. Scientists don't know why this is so.
CRATERS AND RILLES
The surface of the moon is scarred by millions of
(mostly circular) impact craters, caused by asteroids, comets, and
meteorites. There is no atmosphere on the moon to help protect it from
bombardment from potential impactors (most objects from space burn up
in our atmosphere). Also, there is no erosion (wind or precipitation)
and little geologic activity to wear away these craters, so they remain
unchanged until another new impact changes it.
The
lunar crater Aristarchus ( on the NW edge of the Oceanus Procellarum).
This huge, circular crater is 25 miles (40 km) in diameter and 2.2
miles (3.6 km) deep (from rim to floor). There is a lot of ejecta
(material thrown from the crater at impact) surrounding the crater.
These craters range in size up to many hundreds of kilometers,
but the most enormous craters have been flooded by lava, and only parts
of the outline are visible. The low elevation maria (seas) have fewer
craters than other areas. This is because these areas formed more
recently, and have had less time to be hit. The biggest intact lunar
crater is Clavius which is 100 miles (160 km) in diameter.
A
rille is a long, narrow valley on the surface of the moon. Hadley Rille
is a long valley on the surface of the moon. This rille is 75 miles
(125 km) long, 1300 feet (400 m) deep, and almost 1 mile (1500 m) wide
at its widest point. It was formed by molten basaltic lava that carved
out a steep channel along the base of the Apennine Front (which was
explored by the Apollo 15 astronauts in 1971).
MOON OR DOUBLE PLANET?
The Earth and the Moon are relatively
close in size (4:1 in diameter, 81:1 in mass), unlike most planet/moon
systems. Many people consider the Earth and Moon to be a double planet
system (rather than a planet/moon system). The moon does not actually
revolve around the Earth; it revolves around the Sun in concert with
the Earth (like a double planet system).
LIBRATION
Libration is a rocking movement of the Moon.
Librations cause us to view the Moon from different angles at different
times, enabling us to see about 59 percent of the Moon's surface from Earth,
even though the same side always faces us. There are librations due to
variations in the rate of the Moon's orbital motion (longitudinal
libration) and to the inclination of the Moon's equator with respect to
its orbital plane (latitudinal libration). There is also an apparent
libration due to an observer on Earth viewing the Moon from different
angles as the Earth rotates (diurnal libration, which occurs each day).
TWO LUNAR MONTHS
The sidereal and synodic
lunar months have different lengths. The sidereal month is the amount
of time it takes the Moon to return to the same position in the sky
with respect to the stars; the sidereal month is 27.321 days long. The
synodic month is the time between similar lunar phases (e.g., between
two full moons); the synodic month is 29.530 days long.
LUNAR EXPLORATION
Astronaut Buzz Aldrin's footprint on the moon's Sea of Tranquility, from the Apollo 11 mission in 1969.
There have been many missions to the moon, including orbiters missions and moon landings. NASA's Apollo missions
sent people to the moon for the first time. Apollo 11's LEM (Lunar
Excursion Module) landed on the moon on July 20, 1969 with Neil
Armstrong and Edwin "Buzz" Aldrin (Michael Collins was in the orbiter).
Neil Armstrong was the first person to set foot on the moon. His first
words upon stepping down the Lunar Module's ladder onto the lunar
surface were, "That's one small step for man, one giant leap for
mankind." Aldrin described the lunar scenery as "magnificent
desolation." Apollo 12-17 continued lunar exploration.
MOON ROCKS
NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in
many missions), which have been closely studied. The composition of the
moon rocks is very similar to that of Earth rocks. Using radioisotope
dating, it has been found that moon rocks are about 4.3 billion years
old.
THE ORIGIN OF THE MOON
Most scientists believe that the moon was formed from the ejected
material after the Earth collided with a Mars-sized object. This
ejected material coalesced into the moon that went into orbit around th
Earth. This catastrophic collision occurred about 60 million years
after Earth itself formed (about 4.3 billion years ago). This is
determined by the radioisotope dating of moon rocks
BLUE MOON
When
two full moons occur in a single month, the second full moon is called
a "Blue Moon." Another definition of the blue moon is the third full
moon that occurs in a season of the year which has four full moons
(usually each season has only three full moons.)