solar system


The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter).

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The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter). Because of its smaller size, the Moon's gravity is one-sixth of the Earth's gravity, as we saw demonstrated by the giant leaps of the Apollo astronauts. While there are only two basic types of regions on the Moon's surface, there are many interesting surface features such as craters, mountain ranges, rilles, and lava plains. The structure of the Moon's interior is more difficult to study. The Moon's top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone.

Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon's surface and interior, geologists can learn about the Moon's geological history and its formation. The footprints left by Apollo astronauts will last for centuries because there is no wind on the Moon. The Moon does not possesses any atmosphere, so there is no weather as we are used to on Earth. Because there is no atmosphere to trap heat, the temperatures on the Moon are extreme, ranging from 100° C at noon to -173° C at night. The Moon doesn't produce its own light, but looks bright because it reflects light from the Sun. Think of the Sun as a light bulb, and the Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the Moon orbits the Earth and different portions of its surface are illuminated by the Sun.

Seventh Cosmic Ray : Linking Through - Aquarius

"A need to materialise on earth all that exists above, by making realities out of other men's dreams, motivates these subjects. Therefore, they act as they think, feel and will. They are extremely aware of the separateness of each thing, and function naturally as a complete, self-sufficient unity. This makes them very independent and earthy with the capacity to do without other people entirely. This independence, unlike the first ray subject's freedom from restrictions, indicates complete attachment to material surroundings. Through their unity of being and detachment from others they can become self-centred and find it difficult not to use other people for their own purposes. They are materially minded and sow seeds solely that they can gather the fruit - always expecting a return from their investment and favours. These subjects have to be the dominant in their relationships with others and may develop a tendency to treat everybody as either possessions or servants.

Physical characteristics

Physical characteristics

Mean radius 1,737.10 km
(0.273 Earths)
Equatorial radius 1,738.14 km (0.273 Earths)
Polar radius 1,735.97 km (0.273 Earths)
Flattening 0.001 25
Circumference 10,921 km (equatorial)
Surface area 3.793 × 107 km²
(0.074 Earths)
Volume 2.195 8 × 1010 km³
(0.020 Earths)
Mass 7.347 7 × 1022 kg
(0.012 3 Earths[1])
Mean density 3,346.4 kg/m³[1]
Equatorial surface gravity 1.622 m/s²
(0.165 4 g)
Escape velocity 2.38 km/s
Sidereal rotation period 27.321 582 d (synchronous)
Equatorial rotation velocity 4.627 m/s
Axial tilt 1.542 4° (to ecliptic)
6.687° (to orbit plane)
Albedo 0.12
Surface temp. min mean max
equator 100 K 220 K 390 K
85°N 70 K 130 K 230 K
Apparent magnitude magnitude -2.5 to -12.9
-12.74 (mean full moon)
Angular diameter 29.3 to 34.1 arcminutes

Moon's Orbit and Rotation

Earth and Moon

The Earth's Moon is the fifth largest in the whole solar system, and is bigger than the planet Pluto. The Moon has a nearly circular orbit (e=0.05) which is tilted about 50 to the plane of the Earth's orbit. Its average distance from the Earth is 384,400 km. The combination of the Moon's size and its distance from the Earth causes the Moon to appear the same size in the sky as the Sun, which is one reason we can have total solar eclipses. It takes the Moon 27.322 days to go around the Earth once. Because of this motion, the Moon appears to move about 130 against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases.

Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. In other words, it takes the Moon the same amount of time to rotate around once as it does for the Moon to go around the Earth once. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit. moon
Copernicus Copernicus is 93 km wide and is located within the Mare Imbrium Basin, northern nearside of the Moon (10 degrees N., 20 degrees W.). Image shows crater floor, floor mounds, rim, and rayed ejecta. Rays from the ejecta are superposed on all other surrounding terrains which places the crater in its namesake age group: the Copernican system, established as the youngest assemblage of rocks on the Moon (Shoemaker and Hackman, 1962, The Moon: London, Academic Press, p.289- 300).
The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is off in the distance, lighting the Earth-Moon system. At any position, half of the Moon is illuminated by the Sun (the light side of the Moon) and half is not (the dark side). Also, half of the Moon is visible to the Earth (the near side of the Moon) and half is not (the far side). As the Moon moves around the Earth, we can see different fractions of the illuminated half of the Moon. Moon phases
When the Moon is between the Earth and the Sun (1), the near side of the Moon is the dark side. The Moon cannot be seen. We call this New Moon, the beginning of a new cycle of lunar phases. When the Earth is between the Sun and the Moon (5), the near side is the light side. We call this Full Moon, even though we only see half the Moon.

Halfway in between these times (3 & 7), only half of the near side of the moon is illuminated by the Sun. So we can only see one quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.

How Do the Phases Get Their Names?

phase names When the Moon appears smaller than a quarter, we call it a crescent. When the Moon appears larger than a quarter, we call it gibbous. When the moon is getting bigger (phases New to Full) it is waxing. When it is getting smaller (phases Full to New) it is waning. For example, if today the Moon were a waxing crescent, then tomorrow the crescent shape would continue to grow larger, approaching first quarter. After first quarter, the Moon would be a waxing gibbous, and continue growing until it reached full. The Moon would then begin to shrink, becoming first a waning gibbous and eventually reaching third quarter. Following third quarter it becomes a waning crescent, and continues to shrink until it becomes invisible at new Moon.
Just in case you can't remember all of this, there are a few handy ways to recognize whether the Moon is growing or shrinking. A crescent moon which looks like a "C" is shrinking (C for collapsing!). If it looks like a "D", then it is growing. This is true for a gibbous Moon as well, but it's a bit trickier to picture. If the edge of the Moon (the real edge of the Moon, not the edge of night on the moon) is curved like a "C", the gibbous Moon is shrinking. Another way to think of it is that the Moon always grows or shrinks from the right to the left.

The Earth and Moon were imaged by Mariner 10 from 2.6 million km while completing the first ever Earth-Moon encounter by a spacecraft capable of returning high resolution digital color image data. These images have been combined at right to illustrate the relative sizes of the two bodies. From this particular viewpoint the Earth appears to be a water planet!

How The Moon Appears In The Southern Hemisphere

Moon - Southern Hemisphere

Photograph By wingmaker732 - 01/02/2010

Orbital characteristics - Moon

Moons orbit
Perigee 363,104 km (0.002 4 AU)
Apogee 405,696 km (0.002 7 AU)
Semi-major axis 384,399 km (0.002 57 AU)
Eccentricity 0.054 9
Orbital period 27.321 582 d
(27 d 7 h 43.1 min)
Synodic period 29.530 588 d
(29 d 12 h 44.0 min)
Average orbital speed 1.022 km/s
Inclination 5.1450 to the ecliptic
(between 18.290 and 28.580
to Earth's equator)
Longitude of ascending node regressing by one revolution in 18.6 years
Argument of perigee progressing by one revolution in 8.85 years
Satellite of Earth

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