 In a typical coordinate system, you have at least two reference planes, and then any location in that coordinate system can be explained as the distance from these two planes. So for example, for latitude and longitude, the two reference planes are the equator and the primaridian in Greenwich, England. Latitude is just your angular distance, either north or south, from the equator, and longitude is your angular distance, east or west, from Greenwich. So when I say my coordinates right now are 42 degrees north by 71 degrees east, that means I'm 42 degrees north of the equator and 71 degrees east of the British Royal Observatory in Greenwich, England. Now, you may have never considered why do we start measuring longitude from that specific place in Greenwich. I won't answer that in this video, but I will point you to a great book by Davis Sobell called Longitude for the whole story. Now, let's go ahead and move on to the stars and what we call a celestial coordinate system. And there are lots of possible reference planes for making a celestial coordinate system. One of the earliest used by ancient Greeks was the ecliptic, which is the path that the sun appears to move through throughout the year in the sky. And this is where the zodiac constellations come from. It's where the sun is moving through those constellations in the night sky. So you could make an ecliptic coordinate system, but the more common one today is an equatorial coordinate system. And like the name suggests, this is based on the Earth's equator. Just imagine the Earth's equator extending out into space forever. The other reference plane, other than the equator, is a bit more complex to wrap your head around, is it's both a time and a location. The reason it has to be both a time and a location is we're both rotating around our own axis, but also rotating around the sun, which is why there are different constellations up in the summer, compared to the winter, because we're literally changing where we are and facing a different part of the Milky Way. So the other reference plane is where the sun is located in the night sky at the time of the vernal equinox in March. We then measure where the stars are located eastward from that plane and call this right ascension. So to review the two parts that make up equatorial coordinates for the night sky, our first right ascension, which is measured eastward from where the sun is in the night sky on the vernal equinox, and then from that point, declination is how far off from the equator it is. So up it would be positive degrees, down is negative degrees, the equator is zero degrees. Now to make this just a bit more confusing, declination like latitude is measured in degrees. Right ascension could be measured in degrees, and sometimes is, but more commonly is measured in hours, minutes, and seconds. So instead of breaking the celestial sphere into 360 parts that we would call degrees, we break it into 24 parts we call hours, like a clock. Okay, so that's the equatorial coordinate system. You can use a star chart and what are called setting circles on an equatorial mount. If you want to go full manual, what I think of as GPS of equatorial mounts is a go-to system, where you tell the system that's a computer where on earth you are located in the precise time and date. And then it has a little database and it translates where the stars should be in the sky and automates the pointing of the telescope for you. Based on these RA and DEC coordinates, which again are equatorial coordinates. But there's another kind of coordinate system we use all the time in astronomy called a horizontal coordinate system. More commonly known simply as ALT-AS, which is short for altitude and azimuth. With ALT-AS the two reference planes are north and the horizon at the actual time and location of where you're observing. It's called a horizontal coordinate system, but another way I think of it is just a local coordinate system. It's just for that moment in that location. These coordinates, the ALT-AS coordinates don't really mean anything outside of that particular context. You can see if I change my time of day in Stellarium, the ALT-AS keep changing while the RA and DEC stay consistent. But the reason we use ALT-AS all the time is they're a lot more easy to understand. For instance, if I pull up Albirio here and see the azimuth is 90 degrees, I know that's due east for that location. And if its altitude is 45 degrees, I know that's halfway up from the horizon to the zenith. The zenith just meaning straight up 90 degrees. Now there are some fancy formulas for converting equatorial coordinates to horizontal coordinates. But luckily we live in an age where all that info is included in free software like Stellarium or plenty of apps on your smartphone. Next week I'm going to be talking about the apparent size of objects in space and how calculating your field of view for your camera and your lens or telescope can tell you what kinds of deep sky objects will work best for your gear. Till then, this has been Nico Carver from Nebula Photos, Clear Skies.