Basics Home | The Earth and the Sun | The Major Planets | Glossary | Site Map

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

Phases of the Moon - Relative Positions of the Earth, Moon and Sun
References: [Wikipedia: Lunar Phase, Arny 2002]

How can the Moon appear at its fullest when the Earth is between it and the Sun, and why is the Moon not visible when it lies between the Earth and Sun? To answer these questions consider first the diagram below of the relative positions of the Earth, Moon and Sun during the eight phases of the Moon.

Earth, Moon and Sun Positions During Lunar Phases
Phases of the Moon
Globe image courstesy of Wikipedia

The diagram shown is typical of those used to describe the phases of the Moon, but it is easy to become confused if the diagram is not interpreted correctly. There are two representations of the Moon shown for each of the eight lunar phases. The smaller half-gray circles represent how the Moon is illuminated by the Sun throughout its orbit. Each symbol is exactly the same indicating that one-half or one hemisphere of the Moon remains illuminated throughout its orbit. The larger white circles, on the other hand, indicate how the Moon appears in the sky during each of the lunar phases. To demonstrate how illumination and appearance are related semi-transparent carets are positioned to indicate which part of the Moon's Earth-facing side is illuminated during each phase. During the New Moon the far side of the Moon is illuminated by the Sun, while the side that faces the Earth lies in darkness so the Moon is invisible. As the Moon follows its path from the New Moon position more and more of the Earth-facing side becomes illuminated resulting in the “waxing” phases. When the Moon is on the opposite side of the Earth from the Sun its Earth-facing side is fully illuminated producing the Full Moon. As the Moon follows its path from the Full Moon position less and less of the Earth-facing side is illuminated resulting in the “waning” phases.

While the diagram above seems to explain why the Moon looks as it does as it orbits the Earth, it doesn’t appear to take into account any shadows that are cast either by the sunlit Earth or the sunlit Moon during their respective orbits. In the next sections the discussion continues as the role that shadowing plays in two of the most interesting celestial phenomena— lunar and solar eclipses —is introduced.


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

Basics Home | The Earth and the Sun | The Major Planets | Glossary | Site Map

Basics of Celestial Motion. Copyright 2006   S. E. ScruggsEmail Address


Disclaimer