[Note: There is a lot to say about this eclipse. Every time I thought I was done writing this, I remembered something else I had to tell you about! Once it hit 3000 words I figured it was better to split it into two parts. Part 1, todays post, is an introduction to the eclipse: why its a big deal, how it works, and where to go see it. Tomorrow, Part 2, will have information on how to safely observe the eclipse what you can do to see it, and just as importantly what you shouldnt do, as well as equipment you might want to have handy. Ill also have extensive links with more information.]

Get ready, America. The Moon is about to eat the Sun.

Yesterday (Sunday, July 23, 2017) was the new Moon, when the Moon is closest to the Sun in the sky. That means we are just one lunation one complete cycle of lunar phases away from what may be the most viewed eclipse in human history.

I say that with some confidence. For one thing, there are more people alive today than ever before, so we have that going for us. Plus, the path of this eclipse cuts right across the continental United States, including some major cities; for millions of people the farthest they need to travel to see it is to their front yard.

And then theres the internet. I expect the live streaming for this event will be one of the biggest data streams weve ever seen. I wonder how many millions of photos will be taken during the roughly two minutes of totality

This eclipse is a big deal.

For one thing, total solar eclipses in any given spot on the Earth are rare. They happen roughly once or twice a year somewhere on Earth, but its a big planet, and a lot of it is hard to reach. 70% is ocean, and a lot of whats left of the real estate is taken up by places like the Arctic and Antarctic. So getting a total solar eclipse over, say, the U.S. doesnt happen often. The last one was in 1979, and that one cut a shallow chord across the northwest.

For another, total solar eclipses are one of the most beautiful, wondrous, awe-inspiring sights nature provides for us. The Moon slowly covers the Sun, taking nearly 90 minutes. In the last seconds before the Sun is totally covered, the sky grows dark, the air cools, birds fooled into thinking night has fallen stop singing and then the moment arrives.

Totality. The last bit of solar surface is blocked by the Moon, and the glory of the corona is revealed.

Ah, the Suns outer atmosphere, the ethereally thin gas that is normally invisible due to the Suns overwhelming glare. But when the Sun is behind the Moon, the corona is visible, sometimes reaching out for several times the Suns diameter. Shaped by magnetic forces, it can appear wispy, or shot through with tendrils, or as just a smooth glow. It all depends on the Suns magnetic mood at that moment.

I know many people who have seen total solar eclipses, and they all say every last one of them that its one of the most beautiful things they have ever seen in their entire lives. For a few moments, under the shadow of the Moon, people gasp, choke up, even weep openly.

Or so I hear. Ive never seen a total solar eclipse. A partial one, sure, many times, but never total. After all these decades of being an astronomer, this will be my first.

So if its your first too, heres some advice on what to do, where to go, and what youll see.

I have some details about how eclipses work below, but first, I devoted an entire episode of Crash Course Astronomy to eclipses (both solar and lunar), and it has most of the basic info you need to understand the whys and hows of this. Its only a few minutes, so watch!

That was a lot in a short amount of time, I know. In the interest of making sure this is understandable, here are some more details.

The Moon orbits the Earth about once per month. As it does so it passes by the Sun once per month as well, usually getting a degree or two away from it in the sky. But every now and again this celestial dance aligns, and the Moon passes directly in front of the Sun. Thats a solar eclipse. The Moon is casting its shadow on the Earth!

One of the most common questions I get asked is, why dont we get a total solar eclipse every four weeks? I explain it in the Crash Course episode, but this video shows it a bit better:

The green square represents the orbit of the Earth. The Sun is in that plane, far to the left. The blue is the plane of the Moon. Looking down, they seem coincident. But when we view from an angle, we see theyre not (like one hula hoop wedged inside another, they intersect at two opposite points, called nodes). The Moons orbit is tilted by about 5 with respect to the Earths, so usually at new Moon (when the Moon is between the Earth and Sun) it passes above or below the Sun in the sky. But a couple of times a year, the Moon happens to be new just as it passes a node, and you get an eclipse.

So what happens during the actual eclipse?

At first you see a little dip (called first contact), a nibble, taken out of the side of the Sun as the leading edge of the Moon moves onto the Suns face. As the Moon progresses in its orbit you see a deeper and deeper cut into the Sun (the Moon appears dead black during an eclipse because its between us and the Sun, so were seeing its unlit side, plus the Sun is so bright it totally overwhelms the far darker Moon). The Sun appears as thick crescent, then a thinner one and then suddenly the Sun is gone, completely blocked by the Moon.

This is second contact, or more commonly: totality.

The time from first to second contact is roughly 70 90 minutes, depending on your location. Totality lasts only minutes, however, because of a cosmic coincidence

The size of an object on the sky depends on two things: How big it is, and how far away it is. The Moon is 3474 kilometers across, and at the time of the eclipse will be about 366,000 km from the Earths center. The Sun is 1,391,000 km across and will be a little over 151 million kilometers from the Earth at the time of the eclipse.

So the Sun is 400 times wider than the Moon, but will be 412 times farther away. These numbers almost exactly balance out, so the Sun and Moon will appear to be the same size in the sky!

Well, almost. The Sun is actually more than 400 times farther away, so it appears fractionally smaller than the Moon. Thats good news for us! If they were exactly the same size, totality would last a fraction of a second. But because the Sun looks smaller, it takes time for the Moon to move across it. For this eclipse, given their sizes and distances, and how fast the Moon moves across the sky (about 1.1 degrees every hour), this all shakes out to totality lasting roughly two minutes.

Ill get back to that in a sec. But once those two minutes or so are up, the Moons trailing edge uncovers the Sun, and boom! Totality is over. Thats called third contact. Then, over the course of the next 70 - 90 minutes the whole thing plays out in reverse. The Sun looks like a thin crescent, then a thicker one and finally the trailing edge of the Moon leaves the Sun altogether. Thats fourth contact, but more importantly, it means the whole thing is done.

But totality is the big show. Thats due to combination of factors. One is environmental: During an eclipse, it gets dark. I mean, duh, but this is really something! It gets dark during the middle of the day, which is weird. This doesnt happen until minutes before totality, actually; even when the Sun is half covered or more you might not notice. But in the minutes leading up things around you start to change.

And once the Sun is totally covered, things change immediately. Thats when the sky gets actually dark, like a deep twilight. You might see stars, and some planets (like Mars and Venus toward the west [to the right in the sky], Mercury very close to the Sun [below and to the left] and Jupiter and Saturn to the east [left] this sky map should help). And of course, the solar corona.

The corona is invisible right up until the last moment before totality. But then it pops into view, far fainter than the Sun but obvious once the Sun is gone. This is what Im looking forward to seeing the most. Ive only seen pictures of it, and itll be very cool to say the least! to see it for myself.

There are tons of details about what to look for during those precious brief minutes of totality. I talk a little bit about them in the Crash Course video (the diamond ring effect, Bailys beads, and more) but the American Astronomical Society has a nice brief synopsis of what to watch out for. Theres enough there to get you started, and a good Google search will fill in the blanks.

So now you know how this works, and what to look for. The next big question is obvious.

In Part 2 of this post Ill go over how to safely observe the eclipse, but to see it at all you need to plan ahead. The Moons shadow on the Earth is relatively small and moves rapidly, so you need to be at the right place at the right time!

This map shows the path of the eclipse. If you go anywhere between the two blue lines, youll see a total eclipse. The red line is the centerline of the path, where the Moon appears to cut most directly across the Sun, and so the closer you are to that line the longer the eclipse will last.

If youre outside the lines, the eclipse wont be total. The farther away from it you are, the less of the Sun will be covered. Youll get a partial eclipse, which is still very cool! But you wont get the glory of totality.

There is an interactive map of the eclipse online (care of NASA and eclipse expert Fred Espenak). You can click on it and itll tell you how much of the Sun is covered from that location, as well as the times of the eclipse events (it might help to check the box labeled Large map on the lower right). Its extremely useful, so check it out! Important: The times listed are in Universal Time, so youll want to make sure you have the right conversion. In August, Pacific time is UT 7 hours, Mountain is UT 6, Central is UT 5, and Eastern UT 4.

Also, heres a video showing the Moons shadow sweeping across the US (note that the local times, duration, latitude and longitude of the shadow center, and the altitude of the Sun over the horizon are shown on the left):

Having said that, heres the bad news: You can bet that pretty much every hotel in the path of totality is booked. You can try to find one, and please do! But I suspect itll be difficult. Many have been booked for a year or more.

Worse, traffic will be very difficult. Because the eclipse happens in the late morning to midday for many locations, a lot of people will get up early and drive to the centerline. A lot of the locations are rural, and not designed to handle thousands of cars all at once. So be prepared: If you get stuck in a traffic jam five kilometers north of the line, youll miss totality! This website has traffic info and has links to real-time traffic data. It should prove useful. Apparently there are still campsites and RVC parks available; check here for more.

Also, be aware of weather. If its cloudy, you wont see it (though itll get completely dark, like nightfall, which is kinda cool). Theres a map online with historic cloud cover of the sky that will show you where the best places are to see it, statistically speaking.

Now, if you dont want to or cannot travel far (or you already live in the eclipse path), you still have options. For one thing, there will be a ton of live feeds streamed online, and Ill have links to some in Part 2.

You can also find out if theres a museum, a planetarium, a university, or an astronomy club near you. I strongly suspect many of them in the country will be holding viewing parties at the time of the eclipse. This has lots of advantages: experts on tap, access to observing equipment (and itll be far more likely to be safe to use, too; see below), live feeds from the centerline, and what will no doubt be a festive atmosphere for the event.

I expect a lot of schools may be holding events as well for the students. If youre a parent, see if theyll allow you to attend maybe even volunteer to help out! They may need help distributing safe viewing glasses, talking to the students, and especially making sure everyone stays safe and views the event in the correct manner so no one damages their eyes.

And that brings me to the next part observing this rare and wonderful astronomical occurrence in a responsible manner that still maximizes the experience.

But thats for tomorrow, in Part 2. Stay tuned!

Here is the original post:

The Great American Solar Eclipse of August 21, 2017 (Part 1) - SYFY WIRE (blog)