The Moon
The moon, Earth's only natural satellite, is large as moons go. It is fifth in diameter among planetary satellites, more than two-thirds as large as Mercury, and more than three times the diameter of the largest asteroid. It is, in fact, over one-fourth the size of the earth, with a diameter of 2160 miles (3476 kilometers).
|
|
This view of the full moon shows its dark seas, or maria, and the two bright-rayed craters Tycho (near bottom) and Copernicus (directly north of Tycho). The main seas are the Oceanus Procellarum (far left), Mare Imbrium (top center), Mare Crisium (far right), Mare Tranquillitatis (directly left of Crisium), Mare Serenitatis (above Tranquillitatis), and Mare Fecundatis (beneath and right of Tranquillitatis). |
|
During the Apollo 17 mission in 1972, astronauts explored the rugged Taurus-Littrow region of the moon, shown here. The astronauts spent 22 hours touring 35 km (22 mi) of the area in a lunar rover. In addition, they collected more than 109 kg (243 lb) of rock samples for scientific analysis. The Apollo 17 mission was the last lunar flight with a crew.NASA |
Distance
Since the moon is a relatively near neighbor, we can measure its distance easily by geometrical methods. The average is 238,857 miles (384,403 kilometers).
Brightness
Next to the sun, the full moon is the brightest object in the heavens. However, its surface is rough and brownish and reflects light very poorly. In fact, the moon is about the poorest reflector in the solar system. The amount of light reflected by a celestial object is called the albedo (Latin: albus, white). The moon reflects only 7% of the sunlight that falls upon it, so the albedo is 0.07.
The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth, on average, in 27 days 7 hours 43 minutes.
The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes thru the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, and then decreases (wanes) to the next new moon.
The edge of the shadow (the terminator) is always curved, being an oblique view of a circle, giving the moon its familiar crescent shape. Because the "horns" of the moon at the ends of the crescent are always facing away from the setting or rising sun, they always point upward in the sky. It is fun to watch for paintings and pictures that show an "impossible moon" with the horns pointed downwards.
The Far Side
People often refer to "the dark side of the moon", but there is no such thing. The sun shines on all sides of it in turn. However, there is a "far side of the moon" which is never seen from the earth. Over the eons, the gravitational forces of the earth have slowed down the moon's rotation about its axis until the rotational period exactly matches the revolution period about the earth.
You can see this effect by using two round objects such as softballs. Hold one of the balls stationary, to represent the earth. Now move the other ball around the "earth" without twisting your wrist. You will see that people on the "earth" would see all sides of the "moon". However, if you slowly spin the "moon" on its trip around the "earth", you will see that you can time it so only one side of the "moon" is ever seen from the "earth". That's why the features you see on the face of the moon never change.
The Blue Moon
A "Blue Moon" is the name given to the second full moon in a calendar month. Because there are roughly 29.5 days between full moons, it is unusual for two full moons to "fit" into a 30 or 31-day month (and impossible to fit into a 28 or 29-day month, so February can never have a Blue Moon). The saying "Once in a Blue Moon" means a rare occurrence, and predates the current astronomical use of the term, which is quite recent. In fact, Blue Moons are not all that rare; on average there will be one Blue Moon every 2.5 years. After 1999, the next Blue Moons will be in November 2001; July 2004; and June 2007. The last one before 1999 was in July 1996.
The term Blue Moon is believed to have originated in 1883 after the eruption of Krakatoa. The volcano put so much dust in the atmosphere that the Moon actually looked blue in color. This was so unusual that the term "once in a Blue Moon" was coined. However, Blue Moon was also used in much the same way we use the term "Harvest Moon". There were twelve names for full moons, one for each month, and the name Blue Moon was used in years which had 13 full moons. It referred to the third full moon of the four occurring between an equinox and solstice in that year. A misinterpretation of this led to a Sky and Telescope Magazine "Star Quiz" in July 1943 followed by an article in March 1946 which stated that the second full moon in any calendar month was called a Blue Moon (attributed to the 1937 Maine Farmers' Almanac), and this definition has now become part of the language.
The Double Blue Moon of 1999
In January and March of 1999, we had a situation which only takes place about four times a century: two Blue Moons occurring in the same year. The last time this happened was in January and April of 1961 and it will not happen again until January and March of 2018. A double Blue Moon most commonly occurs in January/March but is also possible in January/April or January/May and only when there is no full moon at all in February. It is also possible to have a Blue Moon in December of one year and March of the next year; again, there is no full moon in the intervening February. Click on the link below for more details.
http://liftoff.msfc.nasa.gov/Academy/UNIVERSE/MOON.HTML
I see the moon,
The moon sees me,
God bless the moon,
And God bless me.
- Nursery Rhyme
