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 - The Moon's Angular Size and Solar Eclipses
References: [Robbins and Jeffreys 1988, Wikipedia: Solar Eclipse]

NOTE: Some Images May Not Appear in Internet Explorer

In the previous section the alignment of the Sun, Earth and Full Moon in the Earth’s orbital plane was shown to result in a lunar eclipse, a phenomenon during which the Earth blocks all direct sunlight to the Moon. With direct sunlight blocked the only sunlight that reaches the Moon is light that is “bent” or refracted around the Earth’s atmosphere giving the Moon a dull, reddish appearance. In this section the discussion turns to the solar eclipse, a phenomenon during which a similar alignment of celestial bodies occurs with the Sun, the New Moon and the Earth aligned in the Earth’s orbital plane as shown in the figure. Recall that the New Moon is defined as that lunar phase during which the “far side” of the Moon is illuminated while the Earth-facing side lies in darkness.

Celestial Alignment During Total Solar Eclipse
Globe image courtesy of GraphicMaps.com

Total Solar Eclipse
Total Eclipse France 1999
Eclipse image courtesy of Wikipedia
As with lunar eclipses, solar eclipses can occur only when the Moon’s inclined orbit around the Earth passes through the plane of the Earth’s orbit around the Sun at one of two orbital nodes. If the Moon is in its New Moon phase as it passes through a node the Sun, Moon and Earth will align causing the Moon to block direct sunlight to part of the Earth’s surface. Notice in the figure above that the Moon’s darkly shadowed umbra and its partially shadowed penumbra reach the surface of the Earth.
Partial Solar Eclipse
Partial Solar Eclipse
Eclipse image courtesy of NASA JPL-Caltech
Someone positioned within the Moon’s umbra during this alignment will witness a total solar eclipse during which the Moon’s apparent size completely blocks the Sun’s rays and only the Sun’s corona is visible. The figure on the left was taken during a total solar eclipse as seen from France in 1999. For someone positioned within the Moon’s penumbra the celestial alignment is witnessed as a partial solar eclipse during which the Moon only partially blocks the Sun rays. The image on the right was taken during a partial solar eclipse as seen from Florida in 2001.

The Moon's Angular Size

Notice in the discussion above that emphasis was placed on the role of the Moon’s “apparent” size in a solar eclipse. The apparent or angular size of a celestial body is the size or diameter that the body appears to approximate in the sky expressed in breadth of degrees. The angular or apparent size of a celestial object is related to its true size or diameter and its distance from the Earth by the following equation:

Angular Size to Apparent Size Equation

Something very interesting occurs when the Sun’s and Moon’s respective true diameters and distances from Earth are plugged into this equation. The Sun’s true diameter is approximately 1.39 million km and the Moon’s true diameter is 3,476 km. The Sun’s average distance from the Earth is 149.6 million km, and the Moon’s distance from the Earth varies between from 356,400 km at perigee (closest approach to Earth) to 406,700 km at apogee (farthest distance from Earth) during its orbit around the Earth. Based on these numbers the Sun’s angular or apparent size is approximately 0.53 degree. At its closest approach to the Earth the Moon’s angular or apparent size is 0.56 degree, while at its farthest point the Moon’s angular size is 0.49 degree. This means that the apparent Moon’s size in the sky is approximately the same as the Sun’s! As a result the Moon is able to block the Sun’s rays very effectively when the three celestial bodies are aligned during a solar eclipse.

Annular Eclipses

But these numbers reveal another more interesting facet to the solar eclipse phenomenon. Notice that when the Moon is at apogee or its farthest distance from the Earth the Moon’s angular size of 0.49 degree is actually smaller than the Sun’s angular of 0.53 degree. When the Sun, Moon and Earth align for a solar eclipse with the Moon near its farthest point from the Earth the result is called an annular solar eclipse as illustrated in the figure below.

Celestial Alignment During Annular Eclipse
New Moon aligns for Annular Solar Eclipse

Annular Eclipse
Angular Size to Apparent Size Equation
Eclipse image courtesy of Wikipedia
When the Moon is at a sufficiently great distance from the Earth the Moon’s shadow includes a third region called the antumbra. The Moon’s antumbra begins where the umbra ends, is bordered on both sides by the penumbra and is sometimes called the “negative shadow”. The antumbra is created because the Moon’s angular size of 0.49 degree is not large enough to obscure the entire disc of the Sun at 0.52 degree.As a result sunlight from a ring or annular region around the Moon reaches those parts of the Earth lying within the Moon’s antumbra and an annular eclipse is observed. The photo on the right is an example from NASA’s archives. An annular eclipse, therefore, requires a very special alignment of the Sun, Moon and Earth with the New Moon passing through an orbital node while near apogee or at some sufficiently large distance from the Earth.

The final section summarizes key facts about the phases of the Moon and the celestial motion of the Earth, Moon and Sun. In addition a a number of interesting online resources are suggested for further study of 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

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


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


Disclaimer