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An object's position on the Earth is marked by two positions: latitude (N-S) and longitude (E-W). Similarly, a star's position on the Celestial Sphere (the way the sky appears to us from Earth) is marked by Declination (abbreviation dec) and Right Ascension (abbreviation RA). Declination is measured in degrees north (+) or south (-) of an imaginary line called the Celestial Equator (CE). The Celestial Equator is the projection of the Earth's Equator onto the Celestial Sphere. The CE has a declination of 0 degrees, by definition. At dec = +90 degrees (90 degrees N) is the North Celestial Pole (NCP), the projection of the Earth's North Pole onto the Celestial Sphere. The South Celestial Pole (SCP) is at dec = -90 degrees. The positions of these landmarks change as you change latitude on the Earth (although their co-ordinates do NOT), as you move north or south. At the Earth's equator, the Celestial Equator is directly overhead, and the poles are on opposite sides of the horizon. All stars are visible as they rise, culminate at the meridian, and set. As you move north, the Celestial Equator mirrors your movement, moving south the same number of degrees away from the zenith (the straight-overhead point) as you moved north of the equator. So, by the time you reach Austin (30 degrees North of the Equator), the Celestial Equator has moved away from the zenith, 30 degrees to the south. What has happened to the poles, meanwhile? The Celestial Equator has moved, and has taken the poles with it. The NCP has risen 30 degrees into the sky (as you moved 30 degrees north), and the SCP has sunk 30 degrees below the horizon. Had you moved south, the opposite would have occurred as the Celestial Equator moved north of the zenith. As we move north and watch the sky, we notice that some stars circle the NCP endlessly, never getting below the horizon, and that some stars vanish from our sight, never getting a chance to rise. The stars of the first group are called circumpolar stars. Since the NCP is 30 degrees from the horizon, it makes sense that stars close to the pole -- within 30 degrees -- will never drop below the horizon as they circle the NCP. Therefore, as seen from Austin, stars with declinations in the range +60 degrees to +90 degrees are circumpolar. Similarly, stars with declinations ranging from -90 degrees (SCP) to -60 degrees never rise above Austin's horizon. The southernmost stars that can be seen from Austin have declinations of about -60 degrees (although hills, buildings and haze make -40 degrees a more practical limit). So we can say in general:
As another example, consider an observer in Tasmania, at 40 degrees S latitude. This observer would see the celestial equator 40 degrees (about 4 outstretched fists, remember) to the north of the zenith. No star marks the SCP, but the stars would seem to circle a point 40 degrees above the point of due south. Those stars with declinations between -50 degrees and -90 degrees would never set, circling endlessly, and the stars visible on the northern horizon would have declinations of around +50 degrees. This observer would never see the Big or Little Dippers, but would see the constellation Scorpius pass almost directly overhead! The East-West co-ordinate on the Celestial Sphere is Right Ascension (RA), similar to longitude on Earth. Just as we measure declination in degrees north or south of the Celestial Equator, so do we measure RA east of a point called the Vernal Equinox. The Vernal Equinox is the location in the sky where the Sun can be found on the first day of Spring (the day is called the Vernal Equinox as well). The Sun moves through the constellations of the Zodiac over the course of the year, but the location of the Vernal Equinox stays roughly constant, and the Sun returns there every March 21. The line running from the NCP, to the Vernal Equinox, to the SCP is the zero line of Right Ascension. We typically do not measure RA in degrees, but rather in hours. The reason we do this is to assist us in timing our observations. The sky "spins" through a complete circle of 360 degrees every 24 hours, or 15 degrees every hour. So a star 30 degrees east of the Vernal Equinox is at 2 hr RA. 0 hr RA and 24 hr RA are the same, so a star 30 degrees west of the Vernal Equinox is at 22 hr RA. How does this help us? Let's say I want to observe a star at 7 hr RA. I know that the RA line currently on my meridian is 5 hr 30 min RA. When will my star be on the meridian (at its highest point in the sky)? The answer is the difference between the two RAs: 1 1/2 hours. If the RA line on the meridian were 8 hr RA, the difference would be negative, so my star is setting, and is 1 hr (15 degrees) past the meridian, and I only have 4-5 hours left to observe the star! All major telescope domes have highly accurate clocks which keep track of the RA line currently on the meridian. These clocks keep what is known as Sidereal (based on the stars) Time. For an explanation of why Sidereal Time is different from regular Solar (based on the Sun) Time, look at page 10 in your text. Unfortunately, unlike the Earth's system of longitude and latitude, the celestial co-ordinate system is not fixed. Since the Earth is not a perfect sphere, the Earth's axis "wobbles" as the Earth spins. Since the celestial co-ordinates depend on the orientation of the Earth's poles and equator, the whole co-ordinate system moves as the Earth "wobbles". The movement is slow however, and doesn't amount to much over a human lifetime. Computers can easily compensate for this motion, and tell an astronomer where to look for a star.
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 Updated
8/16/99
By James
E. Heath
Copyright Ó 1999 Austin Community College |