 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
 |
|
|
|
|
|
|
|
|
|
Understanding a Solar Eclipses One consequence of the Moon's orbit about the Earth is that the Moon can shadow the Sun's light as viewed from the Earth, or the Moon can pass through the shadow cast by the Earth. The former is called a solar eclipse and the later is called a lunar eclipse. The small tilt of the Moon's orbit with respect to the plane of the ecliptic and the small eccentricity of the lunar orbit make such eclipses much less common than they would be otherwise, but partial or total eclipses are actually rather frequent. Frequency of Eclipses For example there will be 18 solar eclipses from 1996-2020 for which the eclipse will be total on some part of the Earth's surface. The common perception that eclipses are infrequent is because the observation of a total eclipse from a given point on the surface of the Earth is not a common occurrence. For example, it will be two decades before the next total solar eclipse visible in North America occurs. |
|||||||
|
|
|||||||
|
Geometry of Solar Eclipses The geometry associated with solar eclipses is illustrated in the following figure (which, like most figures in this and the next section, is illustrative and not to scale).
The shadow cast by the Moon can be divided by geometry into the completely shadowed umbra and the partially shadowed penumbra. Types of Solar Eclipses The preceding figure allows three general classes of solar eclipses (as observed from any particular point on the Earth) to be defined:
As illustrated in the figure, in a total eclipse the surface of the Sun is completely blocked by the Moon, in a partial eclipse it is only partially blocked, and in an annular eclipse the eclipse is partial, but such that the apparent diameter of the Moon can be seen completely against the (larger) apparent diameter of the Sun. A given solar eclipse may be all three of the above for different observers. For example, in the path of totality (the track of the umbra on the Earth's surface) the eclipse will be total, in a band on either side of the path of totality the shadow cast by the penumbra leads to a partial eclipse, and in some eclipses the path of totality extends into a path associated with an annular eclipse because for that part of the path the umbra does not reach the Earth's surface. Total Solar Eclipses A total solar eclipse requires the umbra of the Moon's shadow to touch the surface of the Earth. Because of the relative sizes of the Moon and Sun and their relative distances from Earth, the path of totality is usually very narrow (hundreds of kilometers across). The following figure illustrates the path of totality produced by the umbra of the Moon's shadow. (We do not show the penumbra, which will produce a partial eclipse in a much larger region on either side of the path of totality; we also illustrate in this figure the umbra of the Earth's shadow, which will be responsible for total lunar eclipses to be discussed in the next section.)
As noted above, the images that we show in discussing eclipses are illustrative but not drawn to scale. The true relative sizes of the Sun and Earth and Moon, and their distances, are very different than in the above figure. Animations of Solar Eclipses Here are three animations that illustrate observations in a solar eclipse. The first demonstrates generally the case of a total solar eclipse; the next two are simulated views of two recent solar eclipses from unusual vantage points, one from the Moon and one from the Sun |
|||||||
|
|
|||||||