Astronomical Object Finder

Ross Bannister, January 2001
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The problem

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 details

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.

The solution

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!

1. State the celestial co-ordinates of the object you wish to observe
RA: hr. min. dec: deg. min.

2. State your location on Earth
Specify place name or ... ... specify long\lat co-ordinates
Longitude: deg. min. East West
Latitude: deg. min. North South

3. Sate the date and time you wish to make the observation
Date: year month day Time: hour min. system

4. Sate the calculation interval (this allows output for a sequence of times)
Compute every: years, months, days,
hours & minutes
Stop after: intervals (max 16)

Click here for details of the mathematical transformation

... or click here for the Astronomy page ...