On the morning of August 21, 2017, a total solar eclipse will be visible across the continental United States. The Moon’s shadow will also pass through Idaho, and visitors from all around the world will flock to our state.

Join the Boise State Physics Department in the Multi-Purpose Classroom Building on Aug 4 at 7:30p to learn more about this historic event and how to view it safely.

After the presentation, we will distribute eclipse shades and do some stargazing on top of the Brady Street Parking Garage.

This event is free and open to the public. No need for reservations.

Last night, I visited the town of Stanley, a small tourist spot so lovely every view looks like a postcard. Since Stanley is smack in the middle of the eclipse track, and so they are anticipating tens of thousands of visitors in August.

At the invitation of Sarah Cawley of Sawtooth Interpretive and Historical Association, I spoke at the Stanley Museum to a crowd of nearly 100 about the eclipse.

As usual, folks asked some brilliant questions, including the classic “Why does the Moon move west to east during the eclipse, instead of the usual east to west motion?“.

The answer (which I tried to demonstrate by spinning on my axis) is that the Moon’s motion during the eclipse involves both the Earth’s rotation – that causes the Moon and Sun to move together east to west – and the Moon’s orbital motion – that causes the Moon move west to east relative to the Sun.

After the presentation (posted below), my hosts and I adjourned to the Redfish Lake Lodge, where I enjoyed the grilled trout and one of my hosts attempted to teach me to fold a napkin flower. I have to admit that the trout came out much better than my flower.

From https://upload.wikimedia.org/wikipedia/commons/d/d4/Keplers_supernova.jpg.

A supernova is an astronomical event that occurs during the last stellar evolutionary stages of a massive star’s life, whose dramatic and catastrophic destruction is marked by one final titanic explosion.

Join the Boise State Physics Department and Prof. Irwin Horowitz from College of Western Idaho to learn about the theoretical understanding of the most violent and energetic events in the universe as well as the observational support for these models and see Dr. Horowitz “blow up a star”!

The event will take place on Friday, July 7 and start at 7:30PM in the Multipurpose Classroom Building, room 101 on Boise State’s campus. After Prof. Horowitz’s presentation, we will head to the top of Brady Street Parking Garage to stargaze.

UPDATE (2017 Aug 2) – I’ve had more than person ask whether they’ll be able to see the solar corona, the tenuous and very hot outermost layer of the Sun’s atmosphere, during the partial eclipse in Boise. I believe the answer is no.

This website indicates that the corona is a million times dimmer than the Sun’s photosphere. During the darkest part of the partial eclipse in Boise, the photosphere, the part of the Sun we usually see, will be 200,000 times dimmer than usual (see below).

So my guess is, from Boise, not enough of the photosphere will be occulted to reveal the corona.

Millions of people will travel from around the world to observe the total solar eclipse on August 21 in the path of totality, a band extending across the US in which the Moon will completely occult the Sun. Everyone NOT inside that path will observe a partial solar eclipse, and the closer you are to the path of totality, the darker your partial eclipse.

During our statewide tour talking about the eclipse, I have been asked again and again “Is it worthwhile trying to get to the path of totality?” The honest truth is that there are some aspects of the eclipse that you will miss out on if you don’t get to the path of totality.

BUT getting to the path of totality can be difficult – in Idaho, there are reports that all hotels and campsites are booked up, and driving to the path on the day of the eclipse will probably involve sitting in traffic for many hours. So travelers should be prepared with food and water.

So what will you see if you CAN’T get to the path? You can still enjoy a nice eclipse, but you MUST use eye protection during the ENTIRE eclipse. Staring at a partial eclipse at any level can permanently damage your eyes.

Eclipse shades, the best way to safely observe the solar eclipse.

What will you see during a partial eclipse? That depends on where you observe from. For example, Boise is just south of totality and will see a partial eclipse with 99.555% obscuration (check out this amazing map to see eclipse conditions anywhere in the world). That means the Moon will block all but 0.00445% of the Sun’s disk, making the Sun more than 200,000 times darker than usual.

So what will that look like? The visualization below shows an approximation as seen through eclipse shades. As one-two-hundredth its normal brightness, the Sun will appear a little dimmer than the sky at twilight* and about ten times as bright as the Sun normally appears from Pluto.

What a 99% partial eclipse looks like through eclipse shades. From http://americaneclipseusa.com/seeing-the-eclipse-from-home/.

At this level, though, you probably won’t be able to see the solar corona, and other eclipse effects will be substantially more muted. But for those who can’t take off of work or skip school – August 21 is a Monday, after all – the partial eclipse viewed from near the path of totality will still be a once-in-a-lifetime event.

*At 0.00445%, the usual solar flux (1300 W/m^2) will be reduced to about 0.06 W/m^2. This paper indicates that when the Sun is at a zenith angle of 105 degrees (i.e. a while after the Sun has set), the sky brightness is about 0.1 W/m^2.

Figure from Mocquet et al. (2014) show how a gaseous planet might evolve into a dense, rocky core.

Another blast from the past, Mocquet et al. (2014) was the topic of our journal club this week, a paper that seeks to answer the question “What would Jupiter look like if you took away its atmosphere?”.

Given the enormous number of gas-rich exoplanets very close to their host stars discovered in recent years, many astronomers (including myself) have wondered whether such planets could have their atmospheres completely removed.

We certainly see some very hot planets where intense sunlight is blasting away their atmospheres, and in other cases, the star’s gravity can rip off the atmosphere. And so it’s not crazy to think some gaseous planets might completely lose their atmospheres.

Would anything be left over? Astronomers think that gas giants like Jupiter are like big cherries, with a squishy outer layer of gas wrapped around a dense pit of rock. Indeed, the Juno mission currently in orbit around Jupiter is designed to measure the size of Jupiter’s core by measuring its gravitational field very precisely.

And so the cores of gas giants are under enormous pressure – for instance, the core of Jupiter is being squeezed by 45,000 times the pressure at the bottom of the Mariana Trench on Earth.

In their study, Mocquet and colleagues explore what happens to a rocky core under such large pressures. Not surprisingly, they find that such a core would have an enormous density, perhaps three times larger than the Earth’s.

But what is surprisingly is that their results suggest the core might retain a very large density even if you removed the overlying atmosphere. It’s as if you squeezed down a nerf ball and then let it go – instead of springing back immediately, the nerf ball would take a few billion years to decompress. This means that we might be able to identify the cores of former gas giants by looking for planets roughly the size of Earth but with anomalously high densities.

And in fact, such planets have been found – the planet Kepler-57 b has a mass more than 100 times Earth’s but squeezed into a volume only ten times larger, giving a density of almost 44 grams per cubic centimeter – twice the density of the densest element on Earth, osmium.

So in their search for fossils, gas giant paleontologists should keep in mind that the bones of extinct gas giants may have distinctively large densities, almost as dense as adamantium.

Along with my student Karan Davis, I enjoyed a visit last night to the town of Cambridge, about a two hours north of Boise, to talk to folks there about the August 21st solar eclipse.

We were invited by Nina Hawkins, one of the librarians at the public library there in town, and we met her before the presentation at the newly refurbished Country Coffee Cabin in Midvale.

A BLT and fingersteak basket later, Nina led us up the nine-mile winding road to Cambridge, where we were greeted by a few dozen Cantabrigians at the library. For about an hour, Karan and I described the upcoming event and answered questions from the public. As always, I was impressed by how engaged and interested everyone was, especially late on a Thursday evening.

After passing out eclipse shades to the attendees, we packed up our road show and drove back down to Boise, just as the Sun set in the cloud-strewn pink sky, a preview of the twilight effect we will experience during the eclipse in August.

The presentation I gave in Cambridge is posted below.