What is a Total Solar Eclipse?

The midday Sun has vanished. In its place – a gaping black hole in the sky, encircled by a spiderweb halo of silvery light. Daylight becomes twilight. On the distant horizon, an eerie sunset glow surrounds all. A few bright planets and stars pop into view. Eyewitnesses shout, clap, cry, or are struck silent. It is sensory overload. It is an unimagined experience. It is a total solar eclipse.

Why do we have solar eclipses? At TravelQuest International, we have both a simple and complex answer. Simply put, a total solar eclipse occurs when the Moon passes directly in front of the Sun, completely hiding the Sun’s disk from view. But getting to that precise alignment is complicated, and totality happens only because of a remarkable set of circumstances.

Dancing to Totality

A total eclipse of the Sun would never be possible were it not for an extraordinary coincidence. Although the Sun’s diameter is 400 times larger than the Moon’s, the Sun is also 400 times farther away. This means that, on average, the Sun and Moon appear to be about the same size in the sky—but this alone doesn’t result in solar eclipses, let alone total solar eclipses.

As Earth orbits the Sun, the apparent diameter of the solar disk varies, because Earth’s orbit is slightly elliptical. When our planet is close to the Sun (January), the Sun appears about 3% larger than it does in July, when Earth is farthest from the Sun. The Moon’s orbit around Earth is also elliptical, only more so. When the Moon is closest to Earth (think “super Moon”), its apparent diameter is 14% larger than when it’s at its farthest. But this variability in the apparent size of the Sun and Moon still doesn’t give us solar eclipses.

The Moon and Earth dance around each other, while both swing together around the Sun. Once a month, the Moon passes between Earth and the Sun—a lunar phase called new Moon. This is the only time it’s possible to have a solar eclipse, but we don’t get a solar eclipse every new Moon. That’s because the Moon’s orbit around Earth is tilted by just over five degrees compared to the plane of Earth’s orbit around the Sun. So most times, when the new Moon crosses Earth’s orbital plane, it passes above or below the Sun, and Luna’s shadow misses Earth completely. But roughly every six months, the Moon crosses Earth’s orbital plane in front of the Sun. Now, finally, we see a solar eclipse. However, that eclipse is not always a total solar eclipse. In fact, over a span of 10,000 years, only 26.6% solar eclipses are total solar eclipses. Why is that?

A Cosmic Alignment

A little more than one-third of the time when the Moon crosses the solar face, the two are not in perfect alignment. The Moon clips the Sun but never completely covers it. The result is a partial solar eclipse, which is often visible over a wide swath of Earth’s surface.

About one-third of the time, orbital mechanics come into play. If the solar disk appears larger than average and the lunar disk smaller than average, then the Moon can never completely hide the Sun, even if the two are perfectly aligned. The result, at mid-eclipse, is a brilliant ring of sunlight surrounding the black Moon. This is an annular eclipse; an unusual sight but not as compelling as a total eclipse. A partial eclipse precedes, and follows, annularity.

If all conditions are perfect, the Moon completely hides the Sun—a total solar eclipse! But the Moon’s shadow falls on Earth along only a narrow band known as the path of totality and only for a painfully short period of time. (A partial eclipse always precedes, and follows, totality.) The science of solar eclipses tells us that totality can last anywhere from one second to more than seven minutes, with an ‘average’ duration of two to three minutes. But for those who have never before witnessed a total solar eclipse, totality passes in what seems like the blink of an eye.

Bucket List Experience

No photograph can ever do a total solar eclipse justice. Words are inadequate to describe it. Seeing a total solar eclipse is visceral, emotional, an experience unlike any other. First-timers are often completely unprepared for the range of emotions that engulf them when the Sun vanishes from the sky. Moments later, when a beam of sunlight pierces the darkness and totality ends, the first words on everyone’s lips are: “When is the next one?” To relive this celestial spectacle and the emotions it engenders, people will travel thousands of miles to again stand in the shadow of the Moon.

It’s often said that seeing a total eclipse of the Sun is a once-in-a-lifetime experience. Fortunately, that doesn’t have to be the case. There is a total solar eclipse somewhere on Earth once every 18 months or so. While the path of totality can be challenging to get to, it occasionally occurs within easy reach. During the next few years, there are a number of total solar eclipses. If you have an opportunity to see one, don’t pass it up. Totality is a must-see experience and one you will never forget.