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 - Understood!
References: [Robbins and Jeffreys 1988, Wikipedia: Solar Eclipse]

This discussion of the phases of the Moon has (hopefully) dismissed any misconceptions about how the relative positions of the Earth, Moon and Sun affect how the Moon appears as well as the fundamental role that the lunar phases play in lunar and solar eclipses.

  • The Moon orbits the Earth in about 27.3 days and passes through eight distinct lunar phases approximately every 29.5 days. The Moon also rotates about its own axis of rotation in the same 29.5 days with the result that the same side of the Moon is always facing the Earth, and the "far side" is not visible from Earth.

  • At each point in the Moon's orbit only half of its surface is illuminated by the Sun, and the Moon's appearance during each lunar phase is a result of how much of the Earth-facing side is illuminated. At the New Moon the Moon lies between the Earth and the Sun and only the far side of the Moon is illuminated while the Earth-facing side lies in darkness making the Moon invisible. During the "waxing" phases an increasing portion of the Earth-facing side is visible until the Full Moon when the Earth is between the Moon and the Sun and the entirety of the Earth-facing side in illuminated. During the "waning" phases less and less of the Earth-facing side is illuminated until the Moon returns to its New Moon position.

  • The Moon's orbit around the Earth is inclined 5 degrees with respect to the Earth's orbital plane around the Sun, also known as the ecliptic plane. As a result of this inclination the Moon passes through the ecliptic plane at two points called the orbital nodes. A lunar eclipse, during which the Earth's shadow blocks any direct sunlight to the Moon, occurs when the Moon is at the Full Moon phases as it passes through an orbital node, such that the Sun, Earth and Moon are perfectly aligned. During a lunar eclipse the Moon takes on a dull, red coppery appearance because the Earth's atmosphere has observed the blue component of all refracted sunlight.

  • A solar eclipse, during which the Moon's shadow blocks direct sunlight to the Earth, occurs when the Moon is at the New Moon phase as it passes through an orbital node, such that the Sun, Moon and Earth are perfectly aligned. This blocking of the Sun's light is possible because the Moon's apparent or angular size is approximately the same as the Sun's angular size as viewed from Earth. A total solar eclipse is observed from within the Moon's umbra shadow region, a partial solar eclipse is observed from within the Moon's pentumbra shadow region and an annular eclipse is observed from within the Moon's antumbra or "negative shadow" region when the Moon is sufficiently far from the Earth.

Interesting Online "Lunar" Resources

The purpose of this website discussion was to capture the phenomenology of the lunar phases in easy to understand, but accurate scientific terms. However, a more interesting and perhaps more entertaining aspect of the lunar phases can be found in the "mythology" that has been created around the Moon, and especially around solar eclipses, by civilizations all across the globe and throughout history. The online resources are some of many available that describe, in text and images, many aspects of this lunar and eclipse mythology.


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