Partial Solar Eclipse
Unlike during a total solar eclipse, the Moon covers the Sun only partly during a partial solar eclipse, as seen from Earth. This phenomenon is visible when only the lunar penumbra (the partially shaded outer region of the Moon's shadow) touches the Earth. The extent of the eclipse varies depending on the observer’s location but regardless of a viewer’s position, the Sun is only partially eclipsed.
Partial solar eclipses explained
Partial solar eclipses can be observed more often than total solar eclipses because the Moon's distance from Earth is irrelevant and the area in which a partial eclipse can be viewed is much larger. A partly obscured Sun can be seen from Earth during New Moon, when
- the Sun is near one of the nodes of the lunar orbit, so Earth, Sun and Moon roughly form a straight line,
- and the observer is located in the Moon's penumbra.
The Moon's orbit and lunar nodes
The Earth revolves around the Sun, and the Moon circles the Earth. During New Moon, the Moon passes roughly between Earth and Sun. However, in most cases the three celestial bodies do not form a straight line, so the Sun is not eclipsed.
The reason why solar eclipses do not happen every New Moon is that the lunar orbital plane - the imaginary flat surface whose outer rim is formed by the Moon's path around Earth - runs at an angle of approximately 5 degrees to the Earth's orbital plane around the Sun (ecliptic). The points where the two orbital planes meet are called lunar nodes. Only if the Sun appears near one of the two lunar nodes during New Moon so Earth, Moon and Sun form a straight line can a solar eclipse be observed from Earth.
The Moon's shadow
Like any other object's shadow, the Moon's shadow consists of three different areas: the innermost and darkest part (umbra), the lighter, outer part (penumbra), and an area beyond the umbra that is only visible from Earth when the Moon is at apogee (antumbra). To observe a partial solar eclipse, the observer must be located in the penumbra.
Types of Eclipses
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