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The Earth and the Moon

Phases of the Moon - A Popular Misconception Relative Positions of Earth, Sun and Moon The Earth's Shadow and Lunar Eclipses The Moon's Angular Size and Solar Eclipses Phases of the Moon - Understood!
Lunar Resources
 
The Earth and Moon
Earth and Moon from space
Image courstesy of Windows to the Universe

The Moon's Orbit
References: [Wikipedia: Lunar Phase, Robbins and Jeffreys 1988]

After the Sun the Moon is obviously the most familiar celestial body in our sky. The Moon orbits the Earth counterclockwise in an almost perfectly circular (e=0.05) path at an average distance of about 384,400 km. It takes the Moon about 27.3 days to orbit the Earth during time which it also makes a complete turn on its own axis of rotation. As a result only one side or “face” of the Moon is ever visible from Earth, while the “far side” of the Moon is permanently turned away. It wasn’t until 1959 when a Soviet probe photographed the far side that it was ever “seen” from Earth. In fact, up until this time scientists modeled the lunar landscape with one side representing visible craters and mountains, and the other side completely smooth and featureless [source].

Lunar Phases
The Phases of the Moon
Courtesy of Best Animations
While it takes 27.3 days for the Moon to orbit the Earth, in a slightly longer period of about 29.5 days the Moon goes through a series of phases, appearing invisible at the New Moon and completely illuminated at the Full Moon. In all there are eight different phases of the Moon, with the period from New to Full called the “waxing” phases, and those from Full to New called the “waning” phases. It is this period from one lunar phase to the next that is the origin of the word “month” as well as the basis for calendars used by many cultures throughout history.


Phases of the Moon - A Popular Misconception

Another popular misconception in astronomy concerns the phases of the Moon identified in the images below. Many people believe that the lunar phases are caused by the Earth’s shadow passing across the Moon’s surface as the Earth passes between the Moon and the Sun. Taking a cue from everyday life they recognize that any object will appear darker or may even be rendered invisible when blocked from light or when passing through the shadow of a larger object.

The Eight Phases of the Moon
New to Waxing Crescent to 1st Quarter
Waxing Gibbous to Full to Waning Gibbous
Last Quarter to Waning Crescent to New
Moon images courtesy of Wikipedia

However, here are a few astronomy “eye openers". The New Moon or invisible moon phase actually occurs when the Moon lies between the Earth and the Sun! And the Full Moon phase occurs when the Earth lies between the Moon and the Sun!

  • But how can the Moon appear at its fullest when the Earth is between it and the Sun?
  • Doesn’t the Earth cast a shadow or block the Sun’s light in some fashion?
  • And if the Moon lies between the Earth and Sun at least once a month why does it not block the Sun’s light to the Earth?
These questions are answered in the next section which takes a closer look at how the relative positions of the Earth, Moon and Sun affect how we see the lunar phases.

Phases of the Moon - A Popular Misconception Relative Positions of Earth, Sun and Moon The Earth's Shadow and Lunar Eclipses The Moon's Angular Size and Solar Eclipses Phases of the Moon - Understood!
Lunar Resources

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