The earth has a single MOON that orbits it at a distance of 238,855 miles. The moon is kept in orbit by the gravitational force of the earth. Because of this gravitational force, the moon orbits the earth on the same fixed side while the further side is the far or dark side which has only been seen by space probes and astronauts. The temperature ranges from 253 ºF when the sun is not shinning to -261 ºF during sunny temperature on the moon.

Most scientists and astronomers think the moon was formed when massive objects, with size of small planets collided. These may have cause enormous amount of rocky debris when joined together, formed a single mass called the MOON.

Phases of the MOON


An image of a full moon captured at 11:00 PM CDT from the Eastern horizon during Lunar Halo.

Lunar Halo

Lunar Halo

Moon/Earth Comparison

Bulk parameters

                                   Moon         Earth      Ratio (Moon/Earth)
Mass (1024 kg)                    0.07342       5.9726     0.0123    
Volume (1010 km3)                 2.1958      108.321      0.0203
Equatorial radius (km)	           1738.1        6378.1    0.2725      
Polar radius (km)                  1736.0        6356.8    0.2731
Volumetric mean radius (km)        1737.1        6371.0    0.2727
Ellipticity (Flattening)          0.0012        0.00335    0.36    
Mean density (kg/m3)               3344          5514      0.606      
Surface gravity (m/s2)            1.62          9.80       0.165    
Surface acceleration (m/s2)       1.62          9.78       0.166    
Escape velocity (km/s)            2.38         11.2        0.213    
GM (x 106 km3/s2)                 0.0049        0.3986     0.0123 
Bond albedo                       0.11          0.306      0.360
Visual geometric albedo           0.12          0.367      0.330    
Visual magnitude V(1,0)          +0.21         -3.86          -
Solar irradiance (W/m2)            1367.6        1367.6    1.000      
Black-body temperature (K)          270.7         254.3    1.064      
Topographic range (km)               16            20      0.800      
Moment of inertia (I/MR2)         0.394         0.3308     1.191
J2 (x 10-6)                     202.7        1082.63        0.187  

Apogee and Perigee of the Moon
Apogee and perigee refer to the distance from the Earth to the moon. Apogee is the farthest point from the earth. Perigee is the closest point to the earth and it is in this stage that the moon appears larger. Looking at the moon in the sky without anything to compare it to, you wouldn’t notice any size difference. But the difference in size can in fact be quite significant.
If you were to photograph a full moon at apogee and perigee (using the same lens), here’s how the two sizes would compare:

Apogee is the furthest distance of Earth to the Moon and Perigee closest distance from Earth to the Moon.

Apogee is the furthest distance of Earth to the Moon and Perigee closest distance from Earth to the Moon.

Astronomers have formulas for computing the exact distance at any point in time, but the average distance from Earth is 237,700 miles (382,500 km).

Effects of Apogee and Perigee

The apogee and perigee of the moon have an effect on the tides here on Earth. When the moon is at apogee, the furthest distance from the Earth, it has less gravitational pull which, along with other factors that influence the tides, can contribute to lower tides or lower variation in the high/low tide level. When the moon is at perigee, closer to the Earth, there is much more gravitational pull which contributes to the opposite effect: higher tides or greater variation in the high and low tide.

Orbital parameters (for orbit about the Earth)

Semimajor axis (106 km)             0.3844                 
Perigee (106 km)*                   0.3633
Apogee (106 km)*                    0.4055
Revolution period (days)           27.3217            
Synodic period (days)              29.53               
Mean orbital velocity (km/s)        1.022                  
Max. orbital velocity (km/s)        1.076
Min. orbital velocity (km/s)        0.964
Inclination to ecliptic (deg)       5.145
Inclination to equator (deg)       18.28 - 28.58                    
Orbit eccentricity                  0.0549              
Sidereal rotation period (hrs)    655.728                
Obliquity to orbit (deg)            6.68                   
Recession rate from Earth (cm/yr)   3.8

Mean values at opposition from Earth
        Distance from Earth (equator, km)   378,000    
        Apparent diameter (seconds of arc)     1896
        Apparent visual magnitude               -12.74     

* These represent mean apogee and perigee for the lunar orbit.  
The orbit changes over the course of the year so the distance 
from the Moon to Earth roughly ranges from 357,000 km to 407,000 km. 

Lunar Atmosphere

Diurnal temperature range: >100 K to <400 K  (roughly -250 F to +250 F)
Total mass of atmosphere:  ~25,000 kg
Surface pressure (night): 3 x 10-15 bar  (2 x 10-12 torr)
Abundance at surface: 2 x 105 particles/cm3

Estimated Composition (particles per cubic cm):
    Helium 4 (4He) - 40,000 ; Neon 20 (20Ne) - 40,000 ; Hydrogen (H2) - 35,000
    Argon 40 (40Ar) - 30,000 ; Neon 22 (22Ne) - 5,000 ; Argon 36 (36Ar) - 2,000 
    Methane - 1000 ; Ammonia - 1000 ; Carbon Dioxide (CO2) - 1000 
    Trace Oxygen (O+), Aluminum (Al+), Silicon (Si+) 
    Possible Phosphorus (P+), Sodium (Na+), Magnesium (Mg+)

Composition of the tenuous lunar atmosphere is poorly known and variable, 
these are estimates of the upper limits of the nighttime ambient atmosphere 
composition.  Daytime levels were difficult to measure due to heating and 
outgassing of Apollo surface experiments.

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