Looking up into the night sky, one can imagine that the stars and planets are fixed to an imaginary sphere surrounding our planet. To specify locations on this Celestial Sphere, astronomers use a system of coordinates similar to the latitude and longitude measurements used to map Earth. The equatorial guides show this imaginary celestial sphere mapped onto the sky. They are based upon the celestial equator (a projection of the Earth's equator onto the Celestial Sphere), and the celestial poles (a projection of the north and south geographic poles). Declination measures an object's angular distance north or south of the celestial equator (an object of 0 declination is directly on the celestial equator). Declination is usually given in degrees minutes and seconds of arc. The east-west measurement is called the right ascension (RA), and is most often measured in hours, minutes, and seconds. The zero-point of right ascension is based upon the Vernal Equinox, the point at which the Sun crosses the celestial equator on its way north in the Spring. Since the vernal equinox slowly changes over time due to the precession of the Earth, the equatorial coordinate system upon which it is based also changes. This means that an object's position given in RA and Dec will change depending on the date. To standardize astronomical positions astronomers will often refer to an object's position using the coordinate system of a particular date. For example you will often see positions given in J2000 coordinates, that is using the equatorial coordinate system of Jan 1, 2000. Note: The Equatorial Guides menu item will be dimmed if none of the equatorial guides have been selected. Options for the Equatorial Guides To turn on or off additional guides: a reference plane, and the celestial meridian, equator and poles, open the Options dialog by selecting "Options..." from the Settings menu, then pick "Equatorial Guides" from the popup menu. Alternatively, click the options button for Equatorial Guides in the Display palette. Show Equatorial Guides: This checkbox performs a similar function to the Guides/Equatorial Guides menu command. Check this box if you want equatorial guides shown. Note: The checkbox will be dimmed unless one of the equatorial guides has been selected. Hide While Scrolling: To improve performance, you can check this box so that equatorial guides are turned off while scrolling. Celestial Poles: When checked, the celestial poles are displayed. Reference Plane: When checked, an equatorial reference plane is displayed, with the Sun at its center. Equator and Meridian Celestial Equator: When checked, the celestial equator is displayed. Celestial Meridian: When checked, the celestial meridian is displayed. Set Color: By clicking on this button, you can select a new color for the celestial equator and celestial meridian guides. Grid: Checking this box turns an equatorial grid on, and allows you to adjust how the grid is displayed. Checking the "Number Grids" checkbox will display the RA and Dec of each gridline. Set Color: By clicking on this button, you can select a new color for the equatorial guides

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