Explanation Texts

Solar Eclipses for Beginners

The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon's 29.5 day period?
To many early civilizations, the Moon's monthly cycle was an important tool for measuring the passage of time. In fact many calendars are synchronized to the phases of the Moon. The Hebrew, Muslem and Chinese calendars are all lunar calendars. The New Moon phase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moon's monthly cycle. When the Moon is New, it rises and sets with the Sun because it lies very close to the Sun in the sky. Although we cannot see the Moon during New Moon phase, it has a very special significance with regard to eclipses.
An eclipse of the Sun (or solar eclipse) can only occur at New Moon when the Moon passes between Earth and Sun. If the Moon's shadow happens to fall upon Earth's surface at that time, we see some portion of the Sun's disk covered or 'eclipsed' by the Moon. Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn't happen because the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region.
The Moon's shadow actually has two parts:
Penumbra - Faint outer shadow; partial eclipses are seen from within this shadow.
Umbra- Dark inner shadow; total eclipses are seen from within this shadow.
When only the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region. Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright. You must use special filters or a home-made pinhole projector to safely watch a partial eclipse of the Sun.
However, if the Moon's dark umbral shadow sweeps across Earth's surface, then a total eclipse of the Sun is seen (see figure below). The track of the Moon's shadow across Earth's surface is called the Path of Totality. It is typically 10,000 miles long but only 100 miles or so wide. In order to see the Sun totally eclipsed by the Moon, you must be in the path of totality.
The total phase of a solar eclipse is very brief. It rarely lasts more than several minutes. Nevertheless, it is considered to be one of the most awe inspiring spectacles in all of nature. The sky takes on an eerie twilight as the Sun's bright face is replaced by the black disk of the Moon. Surrounding the Moon is a beautiful gossemer halo. This is the Sun's spectacular solar corona, a super heated plasma two million degrees in temperature. The corona can only be seen during the few brief minutes of totality. To witness such an event is a singularly memorable experience which cannot be conveyed adequately through words or photographs. Nevertheless, you can read more about the Experience of Totality in the first chapter of Totality - Eclipses of the Sun.

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