From Perigee to Apogee and Back

The Earth’s moon is a huge ball of rock with a diameter 2,000 miles across and 4.5 billion years old.  The origins of the moon are still a mystery.  But there are many things astronomers do know about the Earth’s only natural satellite.  The lunar cycles and phases are predictable, as are eclipses caused by the moon’s orbit around the planet.  


The moon is an average of 238,855 miles away from the earth, according to NASA.  The orbit isn’t perfectly circular as the heavenly body swings between a closest approach (perigee) of 225,623 miles and its farthest reach (apogee) of 252,088 miles.  The irregular orbit is called its eccentricity, which is valued at around .05.  That means the ellipse is very nearly circular.  By comparison, the Earth’s eccentricity is about .017.

How phases work

Phases of the moon depend upon its orbit.  The moon rotates exactly once during its 27.3-day journey.  As such, the same side of the moon is always facing the sun, according to Cornell University.  Because the moon orbits its mother planet at a different speed than the Earth spins on its own axis, humans at night observe the moon at varying degrees of brightness.  

When the moon is in the sky at the exact same time as the sun the phase is called the new moon.  The sun is so bright it blots out any possibility of seeing the lunar landscape.  The dark side of the moon is facing the Earth but humans can’t see it because any latent light reflecting from the planet is too dim.

The moon is in the night sky as the antithesis to the sun during its full phase. This is when the full face of the moon is lit by sunlight.  The reasons the Earth doesn’t usually darken the moon are two-fold.  One is that the lunar orbit takes the satellite far enough away from the Earth’s shadow.  Two is that the moon’s orbit is five degrees inclined as compared to Earth’s orbit around the sun.  

The cycles complete in one lunar month.  A new moon is totally dark before the moon waxes, or gradually gets brighter.  After the full moon in roughly two weeks, the waning phases begin until it goes down to a new moon and the cycle repeats.


Eclipses happen when the moon, sun and Earth offer perfect alignments and one body is in the shadow of another.  Solar eclipses occur when the moon gets in between the Earth and the sun, temporarily darkening the sunlight.  Lunar eclipses are when the Earth casts its shadow on a full moon to prevent direct sunlight from reaching it.

The perfect alignment happens roughly every 18 months.  The moon’s five percent difference in orbit from the plane of the Earth’s keeps it out of an eclipse mode most of the time.  But every so often, the heavenly bodies make a spectacular show.  If the moon and Earth were on the same orbital plane, eclipses would happen much more often.  

Apogee and perigee  

Apogee is the furthest point in the moon’s orbit around the Earth.  The moon appears to be 14 percent smaller to humans on the ground because it literally is farther away.  At perigee, the moon is roughly 13,000 miles closer to its planet than average.  Tides, which are affected by the gravity of the sun and moon, vary ever so slightly when the moon is closer to us or farther away.

Even with all of the science behind the closest neighbor in outer space, there is still mystery and romance surrounding the moon.  Going for moonlit walks, listening to coyotes howl and seeing the Man in the Moon are all parts of our human nature to make the world around us a more livable place.  Science tells people the Earth’s only natural satellite is a lump of rock.  Yet it still provides mankind with a sense of wonder every time they see the moon in the sky.