Finding Your Way Around The Night Sky - Part One

.. By Chris Stellis

One of the first things to learn is how to find your 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 and 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

Celestial Co-ordinates

The first method is the visual system used to find an object associated with a known pattern of bright stars. This is used by most amateur astronomers and especially beginners. The other two methods use a co-ordinate system to give a numerical position on the apparent sphere of the night sky.

THE CONSTELLATION METHOD

Some of the brighter stars appear to form patterns in the night sky and their positional relationship to each other forms Constellations. Parts of constellations form asterisms, The Plough being an example. In this case the constellation is Ursa Major.

The stars appear to lie on the surface of a great Celestial Sphere. However, each star is actually a variable distance away from us, sometimes by a large amount if they are unrelated. 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. You only need to find the main stars that make up the constellation 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.

There will be more on Star Hopping in Part Two of this article.

 

THE ALT AZIMUTH METHOD

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, from west to east, 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 (see previous Dark Matters article). This type of telescope is mounted on a rotating base and has a pair of bearings on the tube as well allowing it to be raised or lowered. . It will be necessary to fit angle encoders to the telescope to allow the angles to be measured. The telescope base has to be able to be adjusted so that it rotates on an exactly level surface.

Computer programs can be used to carry out the continuous calculations to tell the astronomer where to aim the telescope

Next the latitude and longitude of the observing site has to be specified. The time information will be taken from the computer clock but it must be correct and set to GMT not British Summer Time.

When the program is asked to calculate the position of an object it will display two bearings in degrees and minutes of angle. The first angle of rotation, azimuth, is measured from True North (0°) starting towards East (90°) and all the way round back to 0°. The altitude angle is measured from 0° at the horizon to 90° which is directly overhead (zenith).

The telescope is them moved in both axes until the read out from the angle encoders matches the desired object’s angle.

 

THE CELESTIAL OR EQUATORIAL METHOD

The problem with the Alt Azimuth system is the telescope has to be moved in both axes 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.

This tilt is about 26° and this can be seen in the sky by finding the Pole Star. 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. As this axis is tilted at 26° from the plane of the Solar System, the Celestial Equator (Earth’s equator projected out to the Celestial Sphere) is also tilted at 26° so there are 90° + 26° (116°) from the southern horizon to Polaris.

This angle of elevation known as Declination and 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 an equatorial and an Alt Azimuth mounting is that the equatorial rather than having its rotation bearing level, has it inclined to the celestial 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.

Declination works on the Celestial Sphere much in the same way that latitude does around the globe of the Earth. Right ascension is analogous to longitude measured around the Earth in the east-west direction.

In the sky with right ascension (celestial longitude) we need some point to play a similar role to that of Greenwich for terrestrial longitude. Astronomers have chosen the Vernal Equinox to define the starting point for the measurement of right ascension. The Vernal Equinox is the point where the sun appears to cross the Celestial Equator at the beginning of spring. It is therefore one of the two points where the Ecliptic, the apparent path of the Sun across the sky, intersects the Celestial Equator as shown in the figure.

 

 

Right ascension in many ways works somewhat differently than longitude. First, unlike longitude, right ascension is always measured eastwards from the Vernal Equinox. There is no west right ascension in the sky.

Secondly, we do not measure right ascension in degrees as we do, longitude, but rather in hours. One hour of right ascension corresponds to 15° of celestial longitude, and there are 24 hours all the way around the sky eastwards from the Vernal Equinox, back to the Vernal Equinox again. Each hour is subdivided into 60 minutes, and each minute into 60 seconds, just as if we were measuring time instead of eastwards angular distance but we are still talking about an angular distance measured around the Celestial Sphere.

On Earth we have constant longitude on lines stretching from pole to pole - the meridians of longitude. However, we don't talk about meridians of right ascension. The corresponding term in the sky is hour circle. An hour circle stretches from the North Celestial Pole to the South Celestial Pole. You have the same right ascension all along an hour circle. So in the figure, hour circles are shown for right ascensions of 0, 1, 2, and 3 hours. (Actually only half of each hour circle is shown, the half lying in the Northern Celestial Hemisphere.)

So after all of this, an object’s position in R.A. and declination can be looked up from a Star Atlas or Computer Planetarium program and the telescope moved accordingly. The read out of position coming from the telescope’s setting circles or digital setting circles. More of which in part two.