The Moon


Did you ever stop to think about how the Moon moves in relationship to the Earth?  Why do we sometimes see the moon during the day, and other times at night?  Did you ever notice how rapidly the phase of the moon changes?  What causes eclipses of the Sun and Moon?

The moon is about 13 degrees, or 780 miles further to the East each day, although, like ALL the other heavenly bodies, actually travels across the sky to the West during each 24 hour day..... (OK, I know the earth rotates towards the East against a stationary heaven, but it's much easier to visualize what is going on if you think that the heavens move against a stationary earth while exploring the subject of the "movement" of the heavens.)


Moon and Sun RelationshipWhen the Moon is on the same side of the Earth as the Sun, we are presented with the darkened side or New Moon.  Thus, at the time of New Moon, the Moon and the Sun rise in the East and set in the West at the same moment.  The Moon rises and sets in roughly the same directions as the Sun, but about 49 minutes later each night than the night, or day before, depending on the phase.  Therefore, the Waxing Moon is seen first in the morning just moments after Sunrise as a little crescent sliver, becoming a larger and larger crescent, further and further to the East of the Sun, later and later each day.  Finally, the Full Moon rises just as the Sun sets.  Then we see the Moon rise, later and later after Sunset and then set, later and later after Sunrise, each day as it Wanes, showing less and less of it's surface illuminated, becoming less and less of a crescent each day.  The Half Moon always rises at Midnight when it is Waning" and at Noon when Waxing.

Moon PhasesWhen Moon is “On The Equator” or it’s declination is 0°, (as related to it’s physical position in the Heavens exactly above the Earth’s equator), we have either an Eclipse of the Sun at the time of New Moon, or an Eclipse of the Moon at the time of Full Moon.  This is a rare occurrence as the Moon's orbit is so out of sync with the Earth's rotation and orbit around the Sun.

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 Full Moon is actually only displaying a half illuminated sphere - A Half Moon is actually displaying only a quarter of the sphere illuminated - A Quarter Moon is actually displaying only an eighth of the sphere illuminated!  (See the attached photos.)

The interesting thing to me is that the moon goes around the earth so quickly.  On average, both the rotation of the Moon and its revolution around Earth takes 27 days 7 hours 43 minutes.  The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time. The time from New Moon to Full Moon is only about 14.75 days!  A bit over 2 weeks.  The time from a New Moon to a Half Moon, or Half Moon to Full Moon, is only about 7.4 days, or about one week.  We often do not notice how quickly the Moon changes, or how fast it moves across the sky, or how often we see the moon during the day.... half the time!  (Note: - The Earth’s rotational speed is 900 Knots or 1,035.7 miles an hour at the equator!  The Earth also revolves in orbit about 1° around the Sun each day. Since there is 365 1\4 days in a year and 360° in a circle, the Earth rotates about 361° every 24 hours in the Sun/Earth relationship.)

When we "see" the New Moon, we actually do not see it at all as it is so near the sun that we are often blinded by the sunlight's glare when looking for it. It's not at night that the New Moon presents itself.  The reason we don't see the moon at night during the New Moon phase is because it isn't there!   It's on the other side of the Earth from us!

Light and Dark MoonThere is also not a dark side to the Moon; it’s the other side, which we never see.  The Moon rotates in synchronism with its orbit around the Earth. The same side of the moon always faces the earth.  A person standing on the same spot on the Moon would always look towards the Earth, but would go through one day and one night period about every 27.32 days. (See drawing) The only reason we can’t see the other side of the moon is because it is only lit during the times when that side faces away from the Earth   During the Moon’s 27.32 day cycle, one half of the Moon’s surface is always lit, as every 13.66 days each side tales it’s turn in full sunlight. (Perhaps, occasionally giving those green folks on Mars a nice look at the other side.)  We do see a little bit of the edge of the other side of the Moon because of the Moon’s orbit about the Earth is both elliptical and 5.49% eccentric.  (ie: although the moon rotates at a constant rate, it's distance from earth, and speed in its orbit, varies quite a bit.)

P.S.     So what is the "so-called" effect that the Moon has on humans when it is Full?  Is there such a thing as a Full Moon making humans act strange?  The Moon is always fully there, all month, every day.  It's just seen from Earth half illuminated, (the “Full Moon”), once every 29.5 days.  Sometimes the Moon appears in our sky at night, sometimes during the day.  The Earth’s tides are affected by the Moon’s orbit character

and the moon’s positional relationship with the sun about the Earth.  But as there are not conclusive blind study verifications that this has any effect on Earth’s life forms, scientists claim they can prove that it's just another “Urban Legend”.  Actual long term case studies of hospital emergency room and police blotter records show no relationship in the overall activity of life with the phase of the moon.





Mark S. Anderson – October 21, 2010


Moon Statistics

Mass (kg)


Mass (Earth = 1)


Equatorial radius (km)


Equatorial radius (Earth = 1)


Mean density (gm/cm^3)


Mean distance from Earth (km)


Rotational period (days)


Orbital period (days)


Average length of lunar day (days)


Mean orbital velocity (km/sec)


Orbital eccentricity


Tilt of axis (degrees)


Orbital inclination (degrees)


Equatorial surface gravity (m/sec^2)


Equatorial escape velocity (km/sec)


Visual geometric albedo


Magnitude (Vo)


Mean surface temperature (day)


Mean surface temperature (night)


Maximum surface temperature


Minimum surface temperature