Star Points for February 2012 by Curtis Roelle Navigating the Celestial Sphere Travelers need a map when driving to unfamiliar destinations, whether they?re going to the other side of town or across the country. The map is a model of the real world and knowing how to navigate with one saves time. It?s no different when trying to locate a star cluster, nebula, galaxy, or planet in the sky. Using a star atlas to navigate the night sky to find a celestial object is a handy skill. A key element all maps have is a system of coordinates for pinpointing locations. On an Earth globe, a system of latitude and longitude is used. Latitude measures a point?s distance north or south of the equator, which marks 0ø latitude. Longitude measures the position east or west from the prime meridian, defined as 0ø longitude. Any point on Earth may be defined by a coordinate consisting of its latitude and longitude. This is the system used in globes, geographical atlases, and even road maps. In astronomy a similar system is used. It helps to ignore the three-dimensional aspect of the universe by visualizing the sky in a simplified manner. Think of the night sky as nothing more than the inside of an immense sphere. On its inside surface is attached every star, planet, and everything else of interest. Regardless of any object?s true distance, simply think of it as existing on the inside of the celestial sphere. If you?ve been to a planetarium before, you?re familiar with this simplification. There, all stars are projected onto the spherical surface of a dome even though the actual stars they represent are scattered at various distances in three-dimensional space. Note the similarity between an Earth globe and the celestial sphere. Both are spherical surfaces. As on the globe, points on the celestial sphere can be represented using a coordinate system similar to latitude and longitude, but more about that later. First we need to identify some references useful in celestial navigation. You will need to use your imagination a little. First picture an Earth globe being inside of and at the center of the celestial sphere. Someone on the globe looking up sees the night sky and the stars on the inside of the sphere. In the meantime, Earth is rotating on its axis while the celestial sphere remains motionless. Imagine extending Earth?s axis north and south until it intersects the celestial sphere. These points mark the north celestial pole and south celestial pole, respectively. Polaris, the North Star, is located near the north celestial pole. Next, imagine extending Earth?s equatorial plane outward until it also intersects the celestial sphere. The celestial sphere now has a band around its middle known as the celestial equator. We are almost ready to define our celestial coordinate system. On the Earth, longitude is measured from the prime meridian. Is there anything similar to it like a celestial prime meridian? Yes, there is. As Earth revolves around the Sun during the year, the Sun traces a path all around the sky. In the northern hemisphere?s spring and summer seasons, the Sun is north of the celestial equator. On the first day of spring, known as the vernal equinox, the Sun crosses the celestial equator from south to north. That specific point on the celestial equator, called the ?First Point of Ares,? is similar to the prime meridian. Celestial longitude is measured eastward from the First Point of Ares. However, instead of being measured in degrees, the celestial sphere is divided into 24 hours ? 0 through 23 ? with each hour subtending 15ø in longitude. However, instead of degrees of longitude, astronomers refer to them as hours of ?right ascension? or simply R.A. These hours are, in turn, divided into 60 minutes of R.A., and each minute into 60 seconds. So an object?s R.A. is expressed in hours, minutes, and seconds. Celestial latitude is measured in a way that?s nearly identical to terrestrial latitude. It is measured in degrees ? 0 through 90 ? above (i.e., north) or below (i.e., south) of the celestial equator. Instead of latitude, astronomers refer to this measurement as ?declination.? Declinations north of the celestial equator are designated as positive, and south as negative. As in degrees of latitude, declination is expressed in degrees, minutes, and seconds. If one knows an object?s R.A. and declination, it can be looked up on a star map or atlas. The relative positions of the field stars on the chart can be used as guideposts for locating the object in the sky. For example Sirius, the brightest star in the night sky, has R.A. 6 hours 45 minutes 41 seconds, expressed as 6h 45m 41s. Sirius?s declination is 16 degrees 43 minutes and 50 seconds, expressed -16ø 43? 50?. There are many kinds of star maps on the market. The simplest form is the planisphere, sometimes called a ?star wheel.? Star atlases are an important reference used by amateur astronomers. There are also downloadable computer based planetarium programs, such as Stellarium, that are very good and easy to use. Understanding the coordinate system used to plot celestial objects and having the ability to read a star map in order to use it to locate objects in the sky are valuable skills all serious amateur astronomers should have.