What is an Eclipse

While ‘lunar eclipses’ and ‘solar’ eclipses are the most spectacular form of eclipse mankind sees, there are far more than just two kinds of eclipses. Put simply, an eclipse happens when any object of mass passes through the shadow cast by another object. One person can ‘eclipse’ another person standing behind them, and the shadow of a computer can eclipse the keyboard below it.

Yet stellar eclipses are far more spectacular in scope, and far more important to astronomical studies. Of these, the ‘solar eclipse’ is the most well known.

‘Solar’ eclipses happen when Earth’s moon passes between the Earth and the sun. Earth’s shadow falls upon the moon either in part or full. Actually, as the earth is casting its shadow on the moon, not the moon on the earth, a Solar ‘eclipse’ is not a true eclipse at all. Perhaps a more correct term would be ‘Lunar Occultation’; Occultation refers to the phenomena of an object passing in front of another object and thereby obscuring the view of other bodies behind it. The moon blocking earth’s view of the stars behind it is a common example. (During ‘total’ solar eclipses, the deepest part of the lunar shadow does fall upon the earth – but then it is the earth being eclipsed and not the sun.)

In most cases, however, the moon passing between the earth and sun is called a ‘solar eclipse’. These eclipses take three main forms: total, partial, and annular. To various observers on the Earth’s surface, the eclipse may be a combination of two or even three of these forms. A partial eclipse occurs when, from the observation point, the disc of the moon does not entirely cover the disk of the sun. If the disc of the moon appears smaller than that of the sun, the eclipse is known as an ‘annular’ solar eclipse. Finally, when the apparent diameter of the moon covers that of the sun, a ‘total’ eclipse occurs. In all three cases, eclipses are still not safe to be viewed by the unprotected eye. Live eclipses are best observed when wearing protective glasses or by reflecting the image of the eclipse onto a screen and observing that.

Total solar eclipses begin to observers as partial eclipses. The sun appears to be ‘eaten’ up more and more by the moon as it covers the sun and approaches ‘totality’, or full coverage. This process can last over an hour, but totality never lasts longer than seven and a half minutes in any one location. This is due to speed at which the moon’s shadow travels, and that only in the direct shadow of the moon can the total eclipse be seen.

The sky becomes darker as more and more light from the sun is blocked, causing twilight to fall even in the daytime. Just before totality, the moon may appear to be surrounded by a ring of beads. The ‘Bailey’s Beads’ effect is caused by sunlight striking lunar valleys. Just before complete coverage, observers can see a brilliant burst of light. Combined with Bailey’s Beads, the apparent sphere of light looks like the gem on a diamond ring.

When the moon completely covers the sun, the faint outer atmosphere of the sun becomes visible as a ring of light. This is the sun’s coronasphere or ‘corona’. The corona cannot normally be seen as the sun is so bright, but the total eclipse allows a rare chance for its observation. The moon occultates most of the sun’s light allowing the fainter corona to become visible. The study of the corona during total eclipses has provided a much deeper understanding of the sun to become possible.

In partial and annular eclipses, too much light from the sun can still be seen and the corona is not visible. Still, such eclipses are interesting to observe and will cause partial ‘night to fall in the middle of the day.

‘Lunar’ eclipses occur when the moon is eclipsed by the shadow of the earth. A ‘total’ lunar eclipse occurs when the moon is completely within the umbral, or deepest part, of the Earth’s shadow. When the moon is only partially in the umbral shadow it is only a ‘partial lunar eclipse’. Partial eclipses are difficult to observe without equipment as the moon’s apparent brightness, or its magnitude, does not appear to dim significantly.

In a total lunar eclipse, the moon appears darkened or black, like a new moon – but some light is refracted through the Earth’s atmosphere and can cause the eclipsed moon to appear reddish. The total phase of the eclipse lasts less than two hours, but the partial eclipse can last almost four hours.

Unlike solar eclipses, lunar eclipses are safe to observe with the unprotected eye. Every year, two to seven eclipses occur. Ancient Babylonians monitored the pattern of stellar eclipses and charted their recurrence, as eclipses are predictable. The exact cycle of lunar and solar eclipses repeats itself every two hundred and twenty months in a pattern known as the Saros.

For further study on eclipses and to find out when the next one will occur and where, visit NASA’s eclipse wepsite at: