What Is An Eclipse?

Solar Animation Courtesy NASALooking forward to the 2017 solar eclipse, but not quite sure what to expect? We’ve got you covered! Our guide to what exactly a solar eclipse is will have you ready to pull up a front row seat to this amazing natural phenomenon.

What Is A Total Solar Eclipse?

From our perspective on Earth, a solar eclipse occurs when the moon passes between the sun and the Earth.  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 actuality, though, this is what is sometimes referred to as a “happy accident of nature.”

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.

This effect produces two types of shadows – the umbra and the penumbra.  The umbra is the dark, slender, cone-shaped part of the shadow where all sunlight is completely blocked out, while the penumbra is a lighter, funnel-shaped shadow from which sunlight is just partially obscured.  Total eclipses occur along the section of the moon's orbit called perigee, where the moon is closest to the Earth.  At the other end of the moon's orbit, called apogee the moon is further from the Earth and appears smaller than the Sun.

The moon casts its umbra upon the Earth’s surface during a total solar eclipse, creating a shadow which can sweep a third of the way around the planet in the span of a few hours.  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. 

Frequency And Length of an Eclipse

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.  Any point on Earth may, on the average, experience no more than one total solar eclipse in three to four centuries.

Eclipses happen only during the new moon phase, when the moon moves to the side of the Earth facing the sun.  Because the moon orbits Earth at a slight angle, however, the three bodies will only periodically line up on the same plane to create an eclipse.

While the 2017 eclipse will have a totality of 2 minutes, 40 seconds, totality may last as long as 7 minutes, 31 seconds.  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.

Total eclipses occur every one or two years, while total and partial eclipses together average about two-and-a-half incidences per year. Because they are visible from such a small area on Earth each time, however, the chance of observing a total eclipse from any single spot is less than once in a lifetime. 

Illustration courtesy NASA