Hello, and welcome back to the National Solar Observatory. People are excited and gearing up for the total solar eclipse happening August 21st, 2017. It’s going to pass from coast to coast from Oregon to South Carolina and pass over 12 States. But have you ever wondered how exactly do we know that? Well today we’re going to be discussing solar eclipse maps – how they are made and who makes them. Now predicting solar eclipses depends on the geometry of the solar system, geometry that must be known so exactly that we can tell ,for example, that North Kansas City will experience exactly one minute and 17 seconds of totality whereas those in the South end of Kansas City will not experience totality at all, there’ll just be a 99.9% partial solar eclipse. Okay so we’re here to talk about eclipses and so far we’ve been concentrating on the Sun but the other critical object we need to consider is the Moon. A new Moon always occurs when the Moon is between the Sun and the Earth. If you think about it, a solar eclipse can only happen during a new Moon because the Moon is blocking the Sun as seen by Earth. Similarly a lunar eclipse can only happen during a full Moon when the Earth is able to cast a shadow on the Moon. But why don’t we have eclipses every month then? The reason is that the orbit of the Moon is tilted by about 5 degrees. Now that might not seem like much but because the Moon is 385,000 kilometers away it travels 33,000 kilometers or 21,000 miles above and below the midline of the Earth – more than enough to clear the size of the Earth. Eclipses happen when the Moon’s orbital tilt aligns with the Earth and the Sun.This happens approximately every 16 to 18 months. But then why don’t we hear about eclipses more often if they occur so frequently? Well the path of the Sun shadow can pass over any part of the surface of the Earth so many of them occur over the ocean. Furthermore, because the orbit of the Moon is not a perfect circle but an ellipse, the Moon is not always the exact same distance away from the Earth well the shape of the shadow is determined from the salient elements basically that’s the difference between projecting onto a flat surface and onto a curved surface like a globe. A total solar eclipse depends on the coincidence that the Sun is about 400 times bigger than the Moon but also 400 times further away if the Moon is a little further away from the Earth it will appear smaller to us and if that happens during the solar eclipse, the Moon may not block the Sun completely, leaving a ring all the way around it where the sunlight still shines through. That’s called an annular eclipse. How do we know when to expect an eclipse? well the Jet Propulsion Laboratory keeps a very precise catalogue of the positions and motions of lots of bodies in our solar system that includes the Sun the Earth the Moon as well as other things like other planets their Moons, satellites, comets, asteroids, all kinds of things. So this catalog is called the ephemeris and it takes into account all of the physics that you need to predict where exactly the Sun and the Earth and the Moon will be at any given time in the past or future and that is what we use to predict when a solar eclipse is going to happen. Don’t forget to check out our new solar eclipse website, and especially our our interactive sun science pages at Eclipse2017.nso.edu/science You can find us on twitter at @NatSolarObs or on Facebook at www.facebook.com/NationalSolarObservatory and you can find us on Instagram at NationalSolarObservatory Finally, don’t forget to subscribe to our YouTube channel so as you can get notifications when new webcasts are posted. See you next month, when we’ll be discussing space weather, how the Sun’s activity has a direct impact on our everyday lives and how we use man-made eclipses to keep an eye on what the Sun is doing.