Ross Bannister, January 2001
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One of the difficulties of amateur astronomy is locating objects in the sky for viewing with a telescope - especially if they are
barely visible to the naked eye.
Many astronomical magazines publish the celestial co-ordinates of
objects of special interest (e.g. planets, comets) so such objects
could, in principle, be found using aids such as star charts. If,
however, like me, you find star charts disorientating and suffer the
disadvantage of a large, light polluted town and of owning only a basic
telescope then you may find the following object finder useful.
The direction or 'location' in the sky of any celestial object is best
specified in terms of its celestial co-ordinates - right ascension (RA)
and declination (dec). These two quantities are essentially angles
(the RA is conventionally given in hours, minutes and seconds) which
pin-point the object on the celestial sphere. The celestial sphere is
the surface of an imaginary sphere centred upon the Earth. In this
context RA and dec are respectively akin to the familiar longitude and
latitude system used to describe positions on the surface of the
Earth. Although the RA and dec of distant objects (such as stars,
nebulae and galaxies) are essentially constant (their proper motions
usually have a negligible effect), from the point of view of an Earthly
observer they appear to move across the sky as the Earth rotates.
Closer bodies (planets, the Sun and the Moon) have motions relative to
the Earth which appear as time dependent RA and dec co-ordinates.
From an observer's point of view, a more convenient means of specifying
positions is by the so called 'alt-azi' system. These alternative
co-ordinates are 'altitude' (angle object makes from the observer's
horizon) and 'azimuth' (angle from the observer's North). The altitude
(h) and azimuth (A) are shown in the figure (in which the observer is
at the centre). The drawback of this co-ordinate system is that h and
A are time dependent and vary with the observer's location on Earth.
Given the celestial co-ordinates of an object it is possible to locate
the object by finding its alt-azi co-ordinates for a specified
observing time and location on Earth. To see the mathematics of this
transformation use the link below. Astro-computing is becoming
increasingly popular amongst amateurs and there are many software
packages available to do this (and other) tasks. If you know your
longitude, latitude, and the RA/dec co-ordinates of the object you want
to see (find this information in any good astronomy book), use our
RA/dec to alt/azi object finder. Simply enter your information, and
the time you wish to observe and you'll know where to find the object!