Why They Happen / How to Watch Safely

What is a Solar Eclipse?

A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth. A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and Earth and its shadows fall upon Earth’s surface. According to NASA, a full solar eclipse occurs every 18 months on average. For any given region, though, a total solar eclipse only happens, on average, once every 375 years. A total eclipse of the sun can only be seen from within what is known as the path of totality, a narrow path the moon’s inner shadow travels as it glides across the Earth. Any point on Earth may experience no more than one total solar eclipse in three to four centuries.

Total Solar Eclipse

A total solar eclipse occurs when the moon is perfectly aligned with both the sun and the Earth, so it appears from our perspective that the sun is completely blocked. In terms of sheer size, the moon could never totally block out the sun from our view because the sun’s 864,000-mile diameter is 400 times greater than that of the moon, which has a diameter of just 2,160 miles. Our perspective from Earth comes from the fact that the moon also happens to be approximately 400 times closer to the sun than the Earth. As a result, when the two orbital plans intersect each other and the distances align favorably, the moon can indeed appear to totally blot out the entire sun.

The Umbra and Penumbra

The shadow of the Moon consists of two overlapping parts that are conical in shape – umbra and penumbra. The umbra is the dark, slender, cone-shaped part of the shadow where all sunlight is completely blocked out. Those who are fortunate enough to be in the direct path of the umbra will see the fullness of the sun diminish into a crescent as the moon’s shadow covers the landscape. The sun – with the exception of its outer atmosphere, known as the corona – is completely covered during this period of totality. The penumbra is a lighter, funnel-shaped shadow from which sunlight is just partially obscured. The penumbra extends for about 9,941 miles and it may be over 1,864 miles wide.

Partial, Annular and Hybrid

A partial solar eclipse occurs when only the penumbra passes over you. In these cases, a part of the sun always remains in view during the eclipse. During an annular eclipse, the antumbra, a theoretical continuation of the umbra, reaches the ground, and anyone situated within it can look up past either side of the umbra and see an annulus, or “ring of fire” around the moon. A hybrid eclipse starts as an annular eclipse because the tip of the umbra falls just short of making contact with Earth; then it becomes total, because the roundness of the planet reaches up and intercepts the shadow tip near the middle of the path, then finally it returns to annular toward the end of the path.

Eclipse Viewing Tips

The safest way to view the Sun is certainly by projection, but having a direct protected view of the eclipse stages is more enjoyable. This can be done with approved safe solar filters which provide a beautiful orange image of the Sun against a black background.

Most of these filters feature a thin layer of chromium alloy or aluminum deposited on their surfaces to reduce both visible and near-infrared radiation. One of the most widely available filters for safe solar viewing is shade number 14 welder’s glasses, which can be obtained from welding supply outlets.  An inexpensive alternative is cardboard-framed glasses with lenses coated in aluminized Mylar manufactured specifically for solar observation.

Free solar eclipse viewing glasses for your entire family!