Published online by Cambridge University Press: 17 February 2010
A point in the sky may most easily be fixed by an observer on the Earth with reference to his horizon. In the horizon coordinate system (see Figure 2), the position of the point is specified by its azimuth, the angle round from the northf point of the horizon (in the sense NESW) and by its altitude, the angle up from the horizon (positive if above the horizon, negative if below it). The positions of heavenly bodies, on the other hand, are very often described in the equatorial coordinate system (see Figure 3). Here the plane of the Earth's equator, extended to cut the celestial sphere, is used instead of the horizon, with the first point of Aires or vernal equinox taking the place of the north point of the horizon. A star's position is then given by the angle round from the vernal equinox along the equator (in the opposite sense to that in which it appears to move throughout the day) and the angle up from the equator (positive if to the north, negative if to the south). These coordinates are called the right ascension and declination respectively. Related to the right ascension is the hour angle which describes the angle along the equator from the observer's meridian. As the Earth rotates, so the star and the vernal equinox appear to move at the same rate, making the right ascension and the declination constants (but see routines PRCESS1 and PRCESS2). The hour angle, however, increases uniformly throughout the day from zero when the star crosses the meridian (moving westerly).
To save this book to your Kindle, first ensure no-reply@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.