A solar eclipse takes place when the Moon passes between the Earth and the Sun and the lunar shadow falls onto the Earth’s surface. Although the Sun is considerably larger than the Moon, it is also much further away and, in spite of the great difference in their actual diameters, when viewed from the Earth, the Sun and Moon have roughly the same apparent size. As a result, during an exact alignment of the three bodies (see below), the lunar disc neatly covers the Sun as seen from the Earth.
There are three types of solar eclipse, the one that occurs depending both on how precisely the Earth, Moon and Sun are aligned, and upon the distance of the Moon from the Earth at the time. The lunar shadow has two regions, these being a dark, central area of total shadow known as the umbra surrounding which is an area of partial shadow called the penumbra. A solar eclipse may be total (when the observer is located in the Moon’s umbra), partial (when the observer is located in the Moon’s penumbra) or annular.
If the three bodies are exactly aligned and the lunar disc completely obscures the Sun, a total solar eclipse will result (see diagram – top). To an observer on the Earth’s surface standing within the umbra of the lunar shadow, the Sun will temporarily disappear from view. The lunar shadow travels across the Earth’s surface due to a combination of the Moon’s orbital motion and the axial rotation of our planet. The route taken by the umbra is referred to as the path of totality and observers located within this path will, at some stage, see a total solar eclipse (subject to cloud cover, of course). The total phase of the eclipse can last for anything up to several minutes and will be preceded and followed by partial phases.
When the Earth simply passes through the penumbra a partial solar eclipse will be seen with only part of the solar disc being covered. A similar effect is seen by observers at either side of the path of totality during a total eclipse.
The third type of solar eclipse arises when the Moon is at or near the furthest point in its orbit from us (apogee). Because it is further away, its apparent diameter will be correspondingly reduced and it will not completely cover the Sun even during an exact alignment. The Sun will be visible as a bright ring around the lunar disc. These so-called ‘annular’ eclipses (see diagram – bottom) are named as such from the Latin word annulus meaning ‘ring’.
It has often been said that, during total solar eclipses, day is turned into night, and indeed the sky does become quite dark with several of the brighter stars springing into view. Any of the bright planets which happen to be above the horizon at the time may also become visible, as will the Sun’s outer atmosphere, or corona, which can be seen surrounding the lunar disc. The corona is visible to the unaided eye only during total solar eclipses, the glare of the Sun normally swamping its relatively feeble glow. A total solar eclipse is a stunning spectacle and it is understandable therefore that, in spite of the fact that the total phase is only visible from the restricted region of the path of totality, the prospect of seeing such an event prompts many astronomers to travel great distances in order to view it.
Lunar eclipses occur at Full Moon when the Sun and the Moon are exactly opposite each other in the sky and the Moon passes into the shadow cast by the Earth. The Earth’s shadow, like that of the Moon, consists of umbral and penumbral regions. As described below, a lunar eclipse may be total (the Moon passing completely through the Earth’s umbra), partial (the Moon passing only partially through the Earth’s umbra at maximum eclipse) or penumbral (the Moon passing only through the Earth’s penumbra).
Total lunar eclipses (see diagram) are seen on those occasions when the Moon moves completely into the umbra, or darkest region of the Earth’s shadow. An observer standing on the lunar surface within the umbra of the Earth’s shadow during an eclipse of this type would see the Sun disappear completely behind the body of our planet.
When the Moon passes into the umbra, the curved shadow of the Earth can be seen crossing the lunar disc. The sunlight which normally illuminates the lunar surface is cut off resulting in our satellite being plunged into darkness. However, it is only very rarely that the Moon disappears completely from view. A small amount of sunlight is usually bent, or refracted, onto the lunar surface by the Earth’s atmosphere, resulting in the Moon taking on a deep coppery-red colour as seen here.
If only part of the lunar disc enters the umbral region of the Earth’s shadow, a partial lunar eclipse will occur. Total and partial lunar eclipses are more readily spotted than penumbral lunar eclipses, which take place when the Moon does not enter the umbra at all and only passes through the penumbra. From an observational point of view, it helps to know when a penumbral eclipse is due to take place as the effect of darkening on the lunar surface is only very slight.
THE INFREQUENCY OF ECLIPSES
Solar eclipses can only take place at New Moon, when the Sun and Moon are in the same area of sky. Lunar eclipses can only occur at Full Moon, when the Sun and Moon are exactly opposite each other in the sky. At first it may be thought that an eclipse should occur every month, although that is not the case, the reason being that the plane of the Moon’s orbit around the Earth is inclined to the plane of the Earth’s orbit around the Sun.
The upshot of this is that the Moon usually passes either just above or below the Sun in the sky at time of New Moon and just above or below the cone of Earth’s shadow at Full Moon. An eclipse can only occur if the Moon is either New or Full and is positioned at, or very near, either the ascending or descending node (see diagram) of its orbit around our planet. It is at these two points when the plane of its orbit crosses the ecliptic (the plane of the Earth’s orbit around the Sun) and the three bodies are aligned with each other.