The partial eclipse preceding and following the total eclipse, while longer lasting, won’t be nearly as dramatic, but will captivating in its own way, representing key features in the science of the spectacle.
Here’s a short rundown to help you appreciate what you’re about to see.
90 minutes before
The sun will rise as it always does. Until approximately 90 minutes before totality, nothing will seem out of the ordinary. Shortly after 9:15 a.m. Pacific Time, though, those on the West Coast will notice the moon slowly expanding into the top-right edge of the sun. As the moon rapaciously eats away at the solar disk, the bite will grow, taking just over an hour to reach its maximum extent in any given location.
One hour before
Assuming you’re in the path of totality, you’ll be going all the way. However, even through about 70 or 80 percent, you won’t notice any sharp drop-off in ambient light. In fact, the only real way you’ll notice something is up is to look through a solar filter or eclipse glasses.
Shortly thereafter, a gradual decrease in light will occur until totality. As the unhidden portion of the sun shrinks to a fingernail-like crescent, look in the shade of trees, where images of the sun will be projected onto the ground through the tiny slits between leaves. This effect is the same as you would see in a camera obscura.
About five minutes before the eclipse, twilight will begin to form over the western horizon, like a curtain of dusk. As it slowly envelops more land, twilight will begin to fall.
You’ll be able to see the entire shadow of the moon cast on the atmosphere. That’s when you’ll understand the true difference between a 99 percent eclipse and a total solar eclipse.
If you find yourself in a region treated to a mere 99 percent eclipse, you’ll get to see exactly 99 percent of the sun covered. It sounds pretty neat, but it would be like driving 99 percent of the way to your favorite restaurant — sure, you can see it, but you’re not going to experience it.
The difference is literally night and day. It will get 10,000 times darker as the moon covers that last 1 percent of the sun. With a total eclipse, you have to go big or go home.
If you’re shooting for the full splendor of totality, the final 30 seconds before the crucial moment will be filled with several beautiful phases as the last rays of light disappear behind the encroaching moon. During this time, several brief monumental sights can be spotted — but you have to be quick, as they’ll come and go in a flash!
First, the oval-shaped shadow will slide eastward, and overspread the sun. Along either horizon, sunlight will still be seen; since the path of totality is only about 65 miles wide, you may still able to see distant areas basking in full daylight.
Then, the final few rays of sunlight peaking out behind the bottom-left will slowly converge into one large, concentrated speck, resembling a “diamond ring” against the otherwise-black silhouette hollowed out by the moon.
A few seconds later, that spot of sunlight will disintegrate into several minuscule beads streaming down to earth through the peaks and valleys of the moon’s undulating surface. Not only do the patterns of these Baily’s beads reveal information about the moon’s jagged topography, but also the light that makes them up becomes collimated. This term is used to describe a beam of light where all the rays are parallel.
When collimated sunlight shines through the chaotic fluid motions of the atmosphere and is subjected to varying air densities, it produces wavelike ripple patterns — akin to those cast on the bottom of a swimming pool. These “shadow bands” last for about 15 seconds on either side of totality, and can be one the more commonly overlooked treats that comes in the eclipse package.
And just like that, you’re plunged into darkness.
Mabel Loomis Todd wrote about this moment in her 1894 book “Total Eclipses of the Sun.” “With frightful velocity,” Todd wrote, “the actual shadow of the moon is often seen approaching, a tangible darkness advancing almost like a wall.”
As all direct sunlight vanishes and your eyes adjust, the sun’s atmosphere — the corona — will emerge. The corona holds the keys to unlocking vital secrets to the sun’s magnetic field. Meanwhile, scores of stars and a handful of planets will dot the sky, including Jupiter below the moon to the left, and Venus above and right. Along the same line will lie Mercury and Mars, both considerably dimmer but still briefly visible.
Only when the sun is completely obscured is it safe to remove your eclipse glasses; even if only a few pinpricks of sunlight are poking around the sun, harmful ultraviolet rays invisible to us can cause damage to eye tissue.
As totality draws to a close, the process will reverse itself — first Baily’s beads and the shadow bands, then the diamond ring, and eventually the unwinding stages of a partial eclipse. Animals will awaken, roosters will crow, and cattle will return to the fields to graze.
Though the entire event may persist for only a few hours, it’s guaranteed to be an experience that will last a lifetime.
Mashable has posted a terrific augmented reality video that offers a play-by-play of the eclipse, which is worth a look: The Weather Channel shows the value of using mixed reality to explain the solar eclipse