This article featured in the October 2002 Beginners Magazine


One of the most important things for someone who is new to astronomy to learn, is how to find their way around the night sky. This sounds like a difficult task but in fact it is quite easy and essential if the beginner is to go on and buy a telescope to search out the objects talked about in books and magazines. There are three methods used to identify the positions of objects in the sky. These are using the constellations, Alt Azimuth Co-ordinates and Celestial Co-ordinates. The first 'Constellations' is the visual system used to find an object associated with a known pattern of bright stars. This method is used by most amateur astronomers, especially the beginner. The other two methods use a co-ordinate system to give a numerical position on the apparent sphere of the night sky.


Some of the brighter stars appear to form patters in the night sky and appear to be associated with each other, we call these groups of stars Constellations. The stars in these groups are more often than not associated and may be further from each other than they are from us. Some constellations like LEO (The Lion) and Orion (The Hunter) do look rather like what they a named after but still need a little imagination. Most however have no recognisable shape but still have an apparent association with the other members of the group. The main stars in a constellation are identified using a Greek alphabet letter, normally starting with the brightest allocated the first letter of the alphabet (alpha) then (beta) and so on. When all the Greek letters are used up then the remaining stars are allocated a number or a letter from our alphabet. A given star, for instance the second brightest star in the constellation of Orion would be identified as beta Orionis (The Greek form of Orion). The brightest stars may also have a real name for example b Orionis is also known as Rigel. Constellations are not all the same size and are defiantly not the same shape so it is difficult to know where one ends and another begins. Star charts will show where the borders are but it is not really important to the astronomer because the main patter is only used to identify the approximate area of sky to be searched for other objects. The method used to find interesting objects in a constellation is called 'star hopping'. Star hopping involves firstly finding the constellation in which the target is located or is near. Then by finding two or more stars within the constellation, they are used as a guideline to point towards the target object. It may be necessary to use a number of stages to route out an illusive object by first finding the direction to a particular star then using that star and another to plot out the position of the target. To become familiar with the constellations requires nothing more than a clear night and a simple star chart. As familiarity increases then a pair of binoculars might be useful to identify the fainter members of a constellation. On the previous page is a list of the constellations visible from this country. Use the Star Chart on the back page to find the constellations visible in the sky at the moment. More on Constellations.


This method of finding stars and other interesting objects does not use the patterns of the stars in the sky at all. A reference is given rather like on a map where a grid reference or latitude and longitude references are used. The problem with using this type of co-ordinate system on the sky, is that the stars appear to continuously move across the sky due to the rotation of the Earth. To overcome this problem any calculation to determine the position of an object in the sky must take into account the location of the observer and the time at that position. Although not a difficult calculation it does have to be continuously updated to keep up with the rotation of Earth. Dobsonian telescopes are a type of Alt Azimuth mounted telescope. This type of telescope is mounted on a rotating base and has a pair of bearings on the tube, known as trunnions, to allow it to be raised or lowered. Computer programmes can be used to carry out the continuous calculations to tell the astronomer where to aim the telescope. A simple programme called DOB_PC is one such programme which is available as freeware. To use this type of programme it will also be necessary to fit angle indicators to the telescope, to allow the angles calculated, to be set. To used DOB_PC or similar programme the telescope base has to be able to be adjusted so that it rotates on the level. Next the programme must be told the latitude and longitude of the observing site. The time will be taken from the computer clock but it must be correct and set to GMT not summer time. When the programme is asked to calculate the position of an object it will display two bearings in degrees and minutes of angle. The first is the angle of rotation measured from True North measured starting towards east and all the way round through 360 and back to 0. The second angle is measured from 0 at the horizon to 90 which is directly overhead.


The problem with the Alt Azimuth system is the telescope has to be moved in both axis to track the target object as it moves, in an apparent arc across the sky. The apparent arc is caused by the tilt of the axis of Earth compared to the plane of its orbit around the Sun. The tilt is about 26 and this can be seen in the sky by finding the Pole Star POLARIS as described on the back page. It will then be seen that Polaris is not directly overhead but about 26 towards the north. The axis of rotation of Earth (the imaginary line passing through the north and south poles), points to Polaris or at least very close to it. Therefore all the other stars appear to rotate around Polaris. Because this axis is tilted at 26 from the plane of the Solar System the Celestial Equator is also tilted at 26 so there are 90 + 26 (116) from the southern horizon to Polaris. The angle of elevation known as Declination is measured as 90 from the Celestial Equator up to Polaris and -90 from the Celestial Equator to the point in the southern sky where the south pole points to. The advantage of this system is that a star or other object can be tracked across the sky by driving the telescope in one axis only. This is achieved by using an EQUATORIAL MOUNTING. The difference between and equatorial and an Alt Azimuth mounting is that the equatorial rather than having its rotation bearing level, has it inclined to the pole. This allows the telescope tube to rotate around the same axis as the Earth's axis. By driving the telescope at 1 revolution per 24 hours it will rotate at the same speed as the Earth and track the star it is pointed at. The rotation in this system is measured in time, hours, minutes and seconds and measured from when the point on the celestial equator where the Sun crosses it at the summer equinox.


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