This article
featured in the October 2002 Beginners Magazine
FINDING YOUR WAY AROUND THE SKY
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.
THE
CONSTELLATION METHOD
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.
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 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
CELESTIAL OR EQUATORIAL METHOD
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.
