We've seen in our discussion of ancient astronomy that you can learn a lot about the universe you live in just by making careful observations over the course of the year, and catching the cycles of the sun, the moon, and the Earth in their various orbits. The first people who looked at the night sky were aware of patterns of stars and they needed to navigate, so they named constellations. Constellations are still some of the most familiar features of the night sky. Obviously, it's hard to see literally people and animals in the night sky. These are mnemonics as aids to navigation and understanding where people were on the surface of the Earth. Some of the constellations date back thousands of years, but the modern constellations have been sanctioned by the International Astronomical Union, and there are a 110 of them. Of these, the 12 that circle the zodiac or the ecliptic, the region taken up by the motions of the sun as seen in the sky, are the most important. The 12 zodiacal constellations gave rise to astrology and star signs. If you were ever lost on the Earth's surface, you might find useful the idea of using constellations and navigation aids. In fact, in the ancient world, people were familiar with the constellations and used their appearance and disappearance throughout the year, to judge when planting cycles, and when navigation would take place, when expeditions were possible and so on. We know some constellations are truly ancient. For example, Ursa Major, the Great Bear. There's record that the Egyptians had a constellation with that name and description, almost 4,000 years ago. This must have come from another culture, because there were no bears in Egypt 4,000 years ago. The best evidence is, that the Great Bear came from Siberian tribes that moved across the land bridge into the Americas and back into Europe. In other words, some of the oldest constellations are among the oldest human artifacts in culture. It's important to remember that constellations have no physical significance. They're patterns or asterism of stars on the plane of the sky. Almost always, a constellation is composed of stars that are very different three-dimensional distances from the Earth or the sun, that just project to small angles apart on the sky and make a particularly memorable pattern. Their significance, of course now, is primarily cultural and historical. Indeed many myths and legends are embedded in the constellations, making for a rich lore in many of the world's cultures. In naked eye astronomy and in general, we can think of the celestial sphere. Even though our modern view of astronomy understands that the Earth is moving around the sun and the stars are very far away in a different distances. The appearance of the celestial sphere as something that orbits the Earth every 24 hours, is a useful construct for doing positional astronomy. In this frame of reference, the fixed points are the celestial poles. In the north celestial pole near the point Polaris, is the point in the northern sky about which all stars appear to orbit. You can imagine the Earth spinning on a top, looking straight up from the top of the Earth at the North Pole, all the stars would appear to make concentric circles above the point directly overhead. Analogously, in the southern hemisphere, the stars would all appear to make concentric circles around a point over your head if you were in the South Pole, rotating in the opposite sense. At intermediate latitudes on the Earth, that point is at a different angle. The celestial equator is the projection of the Earth's equator onto the celestial sphere. In positional astronomy, there are actually three coordinate systems or frames of reference that we need to consider. One is indicated by the spinning Earth. That's what gives us the celestial poles and the celestial equator. The other is given by the ecliptic, which is the apparent path taken by the sun in the sky. Since the Earth is spinning like a tap, with a tilt of 23.5 degrees, relative to the plane of the Earth's sun orbit, the ecliptic is tilted by 23.5 degrees, with respect to the celestial equator. A third frame of reference is given by the position of the Earth and the sun within the larger context of the Milky Way. The Milky Way is a disc-like system of stars, where the plane of the disk is at a different angle than the celestial equator or the ecliptic. This third frame of reference tells us where that band of stars, wheeling overhead in the northern and southern sky lies. That's the plane of the galaxy we inhabit. With professional telescopes or even reasonably good amateur telescopes, it's possible to point and shoot, to target a telescope and go very precisely to a region of sky within less than a degree. But to do observational astronomy or learn the night sky, you can estimate angles very simply without using a telescope. In simple terms, if you want to know angular measurement, where 90 degrees is the angle between the zenith, the point overhead and any horizon, your full hand span held out at arms length is about 20 degrees. Your fist width is about 10 degrees, and an index finger width is about a degree held at arm's length. That's twice the diameter of the full moon, which subtends half a degree, as does the sun. There's a basis for getting angular measurement, and indeed, amateur astronomers and anyone without a telescope, you can step your way around the sky just using those single units of measurement. In positional astronomy, where the celestial sphere is the frame of reference, we can define the position of any star or any object in the night sky with two angles, just as we can define a position on the Earth's surface by latitude and longitude. Astronomers use two different frames of reference for measuring angles. The simplest is altitude and azimuth. Altitude is the elevation directly going straight up from any horizon to the object of interest, and azimuth is the angle around the horizon from some fixed point like due north or due south. Any point in the sky is defined by two unique angles of altitude and azimuth. Professional astronomers use a different coordinate system, based on measurements with respect to the ecliptic or the sun orbital plane. Those measurements are right ascension and declination, but the principle is the same. Any object in the sky is identifiable uniquely by two angles. A lot of good positional astronomy can be done naked eye without a telescope. Thinking about the night sky, it's easy to think in terms of a celestial sphere, where the stars are painted on the surface of a sphere that orbits us every 24 hours, even though we know in truth that the Earth is in orbit around the sun and the stars are all at very different distances. The patterns of the night sky were codified centuries ago in terms of a 110 constellations. The most important of which are the 12 that occupy the region where the sun, the moon and all the planets travel, the ecliptic, those 12 are the constellations of the zodiac. Remember that the stars in those constellations are likely at very different regions in three-dimensional space, they just appear to be close on the sky. In terms of measuring angles, it's easy to hop around constellations using simple distance measurements. The celestial sphere is a useful tool for understanding how we might observe the night sky. Any object in the night sky can be defined by two angles, and the simplest way to do this is in terms of elevation above the horizon, and azimuth or rotation around that horizon from some fixed point like North or South.
