The Earth (blue dot at center) has a spin axis and equator tilted with respect to the plane of its orbit around the Sun. So if you had a telescope at the center of a transparent Earth, you could resolve details about as big as a house lot up on the surface. By comparison, 1" of latitude on Earth is about 101 feet. A good telescope in good sky conditions can resolve details about as fine as 1" on the surface of the celestial sphere. One arcminute contains 60 arcseconds, written 60". In 1° there are 60 arcminutes, written 60'. Since ancient Babylonia, people have divided both degrees and hours into finer units by means of base-60 arithmetic. This makes it easy to figure out when celestial objects will come in and out of view. The benefit of this numbering system is that as the Earth rotates, you see the sky turn by about 1 hour of right ascension for each hour of time. One hour in this scheme is 1/24 of a circle, or 15°. This is just a different way of putting dividing marks on a circle. Instead of counting in degrees, as with longitude around the Earth, right ascension is usually counted in hours, from 0 to 24 around the sky. This daily motion is the basis of the numbering system used in right ascension. Click to zoom in on the setting circles for right ascension (left) and declination (center), and the latitude setting, which adjusts the tilt of the polar axis to match your latitude (right). This lets the telescope track objects anywhere in the sky by turning around just this one axis. To set it up, you aim one axis (the polar axis) about at Polaris, the North Star. The celestial sphere seems to rotate around our motionless world once in about 24 hours.Ĭloseup of an equatorial mount. Of course Vega doesn't move it's the Earth that's turning. (This does mean that the one-to-one connection between right ascension and longitude is broken the moment after you imagine the lines ballooning out from Earth and printing themselves on the sky the two systems rotate with respect to each other.) Hours and Degrees That's why they can be permanently printed on star maps. Lines of both right ascension and declination stay fixed with respect to the stars. So once a day, Vega passes overhead as seen from the latitude of Kansas City. (By custom, declinations north and south of the equator are called + and – rather than N and S.) This is the declination of the bright star Vega. Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.Īt any other latitude - let's say Kansas City at 39° N - the corresponding declination line crosses your zenith: in this case declination +39°. If you stand on the Earth's equator, the celestial equator passes overhead. They are now called, respectively, declination and right ascension.ĭirectly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination. Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere, as shown at right. In the case of Earth, these are named latitude and longitude. Whenever you want to specify a point on the surface of a sphere, you'll probably use what geometers call spherical coordinates. The celestial sphere, with its infinitely large radius, appears to turn daily around our motionless Earth, from which we use telescopes to examine wonders painted on its inside surface. Perhaps for this reason, astronomers are quite comfortable living with both - as long as the two are kept in their proper relationship. In astronomy, appearances and reality are more different than in any other area of human experience. Never mind that we're on a moving dust mote orbiting a star in the fringe of a galaxy. At any time half of the celestial sphere is above the horizon, half below.Įven today this is how the cosmic setup actually looks. Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. The celestial dome with its starry decorations had to be a complete celestial sphere, early skywatchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day. When a telescope's right-ascension axis is lined up with the Earth's axis, as shown here, the telescope can turn on it to follow the rotating sky. On the celestial sphere, lines of right ascension and declination are similar to longitude and latitude lines on Earth. The Earth is at the center of the celestial sphere, an imaginary surface on which the planets, stars, and nebulae seem to be printed.
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