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CONSTELLATIONS AND HOW TO FIND THEM

- George Corban.

When i t was suggested at one of our F i e l d Club camps that I w r i t e an a r t i c l e on astronomy with p a r t i c u l a r reference to methods f o r f i n d i n g c o n s t e l l a t i o n s , I wondered i f such an a r t i c l e would be of use. In the f i r s t i:>lace, there are a l a r g e number of good t e x t ­books and pamphlets d e a l i n g with the subject. But these books, w r i t t e n mainly by a u t h o r i t i e s l i k e S i r James Jeans, Hoyle and others, are i n c l i n e d e i t h e r to be too t e c h n i c a l or to gloss over some aspect i n astronomy that amateurs would l i k e to know more about. Most t e x t ­books merely supply a map of the heavens, and t h i s map i s supposed to describe the s t a t e of the sky at a p a r t i c u l a r time on a p a r t i c u l a r day. On the whole, s t a r maps are most u n s a t i s f a c t o r y — t h e biggest o b j e c t i o n being that they are p r o j e c t i o n s i n t o two dimensions of a three dimen­s i o n a l system. The r e s u l t i s that c o n s t e l l a t i o n s become d i s t o r t e d by crowding and s t r e t c h i n g , depending on t h e i r l o c a l i t y .

As regards the p r a c t i c a l s ide of astronomy, I would l i k e to s t r e s s that most observing i s c a r r i e d out not by the p r o f e s s i o n a l s i n the l a r g e observatories but by amateurs. The l a t t e r record t h e i r more important r e s u l t s i n l o c a l astronomical journals and i t i s from these that the p r o f e s s i o n a l s derive a great deal of t h e i r information. Whereas the observatories must of n e c e s s i t y be r e s t r i c t e d to s p e c i f i c s t u d i e s , the amateur has the freedom to study whatever takes h i s fancy. There i s no reason why F i e l d Club should not adopt astronomy as one of i t s f i e l d s t u d i e s , f o r there i s much work that can be done without the use of instruments, f o r example, o c c u l t a t i o n s can be timed and are e a s i l y followed with b i n o c u l a r s , while observations of meteors or "shooting s t a r s " are an almost untouched s e c t i o n i n astronomy, and need be f o l l o w e d only with the naked eye. However, i n order to carry out any work at a l l , i t i s necessary f o r the observer to know both the sky and the way around i t .

There are two methods of l e a r n i n g the sky. The f i r s t i s the d e s c r i p t i v e method i n which one proceeds from the known to the unknown. The second method i s by the use of co-ordinates and, being the s c i e n t i f i c method, i s the b e t t e r one to use. In the co-ordinate method, the problem of l o c a t i n g the p o s i t i o n of a s t a r i s s i m i l a r to the means by which points are designated on the earth. although c e l e s t i a l bodies d i f f e r enormously i n t h e i r distances from the earth, the nearest i s so incomprehensibly f a r away, that we may as w e l l regard them as a l l being imbedded i n the surface of a hugh c e l e s t i a l sphere at the centre of which i s s i t u a t e d a t i n y sphere, the earth. Since the earth r o t a t e s from west to east then the apparent motion i s that the c e l e s t i a l sphere r o t a t e s from east to west. This explains why a l l the c e l e s t i a l bodies r i s e i n the east, climb to some point along the north-south meridian, and set i n the west. This t o t a l motion takes a p e r i o d of twelve hours during which time the sphere has r o t a t e d through 180°. The p e r i o d f o r the r o t a t i o n of the c e l e s t i a l sphere i s not the same as that r e q u i r e d f o r the earth to r o t a t e once. The period of the r o t a t i o n of the earth i s measured i n s o l a r time while that of the c e l e s t i a l sphere i s measured i n s i d e r e a l time.

The s o l a r day i s the time which lapses between successive t r a n s i t s of the sun across the meridian,'while the s i d e r e a l day i s the time l a p s i n g between successive t r a n s i t s of a s t a r across the meridian. The s i d e r e a l day i s 4 minutes s h o r t e r than the s o l a r day owing to the f a c t that i n 24 hours the e a r t h , i n r o t a t i n g once, moves along i t s

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o r b i t by an angle subtended at the sun equal to one degree. The time r e q u i r e d to r o t a t e through one degree i s 4 minutes, thus accounting f o r the d i f f e r e n c e .

Since the c e l e s t i a l sphere can he regarded as c o n c e n t r i c with the sphere E a r t h , then both have corresponding features such as poles, a x i s of s p i n , and the meridians of l a t i t u d e and l o n g i t u d e .

The Earth r o t a t e s about i t s North and South geographic poles, and the apparent motion of the c e l e s t i a l sphere i s that i t r o t a t e s about i t s North and South c e l e s t i a l poles which w i l l t herefore be d i r e c t l y above the Earth's poles. Obviously the c e l e s t i a l a x i s of s p i n w i l l be the same as that of the Earth. I f planes are drawn at r i g h t angles to the a x i s of s p i n they could be regarded as "great c i r c l e s of l a t i t u d e " , but by convention are termed great c i r c l e s of d e c l i n a t i o n . That plane through the centre of the a x i s i s the c e l ­e s t i a l equator. On the c e l e s t i a l sphere the l i n e s which correspond with the Earth's l i n e s of longitude are known as l i n e s of r i g h t ascension.

The p o s i t i o n of a s t a r can be designated by Right Ascension as so many hours s i d e r e a l time, and by D e c l i n a t i o n as so many degrees, minutes and seconds. A p o s i t i v e value means that the s t a r i s north of the c e l e s t i a l equator, a negative i n d i c a t e s south of the equator.

In order to put the theory of co-ordinants i n t o p r a c t i c e , i t w i l l be necessary to e s t a b l i s h a few points i n the sky. At a l l times the p o i n t d i r e c t l y overhead i s the Zenith. That below us on the other side of the Earth i s the Nadir while the l i n e j o i n i n g the c e l e s t i a l poles through these points i s the c e l e s t i a l M e r i dian, which plays an important part i n the co-ordinant method.

To an observer at the pole, the s t a r s appear to revolve about a point d i r e c t l y overhead, i . e . at the z e n i t h , t h e i r paths being concentric c i r c l e s about the pole. Jit the equator the s t a r s r i s e and set at r i g h t angles to the horizon. A s t a r with d e c l i n a t i o n equal to 0° would r i s e due east, pass overhead, and set due west. Observers at the Equator are fortunate i n that they can see every s t a r during the period of one year.

Por observers at Auckland, however, i n l a t i t u d e 37°S., the south c e l e s t i a l pole i s on the meridian hut above the southern horizon at an a l t i t u d e of 3 7 ° to i t . Tho angle that the pole makes to the h o r i z o n always equals the observer's l a t i t u d e . Because of the t i l t of the c e l e s t i a l pole to our south horizon, some s t a r s with d e c l i n a t ­ions of - 5 3 ° to -90° can be seen to be r e v o l v i n g about the pole i n concentric c i r c l e s , and i t w i l l be n o t i c e d that such s t a r s never set. A c o n s t e l l a t i o n that shows t h i s i s the well-known Southern Cross. Other s t a r s can be seen f o r only part of the year; those s t a r s with

d e c l i n a t i o n s greater than OD can never be seen from Auckland. Stars which are over-head at the Equator never r i s e more "than 53° above our north horizon. The diagram w i l l help to determine the p o i n t along the meridian where the s t a r should be. I t w i l l be noted that a s t a r of d e c l i n a t i o n -37°S. w i l l pass overhead at culmination, while one of -43°3. w i l l cross 6°S. of the z e n i t h . One of +43° would cross 80ON. of our z e n i t h , i . e . 10° above the north horizon. The d e c l i n a t i o n of s t a r s does not change as does that of the sun.

Having determined the a l t i t u d e of the s t a r , the time of c r o s s i n g the meridian-must be c a l c u l a t e d . The r i g h t ascension value i s always given i n hours, minutes and seconds, and means that so many hours must elapse a f t e r the zero of the r i g h t ascension has crossed the meridian before the s t a r i t s e l f w i l l cross. This zero i s the p o i n t where the e c l i p t i c i n t e r s e c t s the c e l e s t i a l equator, or more s p e c i f i c a l l y , the p o s i t i o n of the sun at noon on March 21st. Thus zero hours s i d e r i a l time equals zero hours s o l a r time at noon on t h i s date. The p o i n t i s a l s o known as the " f i r s t p o i n t of A r i e s " , and

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can be found by drawing a l i n e through Andromeda to )( Pegasus, so forming the si d e of the Great Square. This l i n e , i f produced an equal l e n g t h , w i l l i n t e r s e c t the e c l i p t i c at the zero p o i n t .

To sura up, by knowing the r i g h t ascension of any c e l e s t i a l body, i . o . hew many hours, minutes and seconds a f t e r the f i r s t point of A r i e s i t t r a n s i t s the meridian, and also by knowing the d e c l i n a t i o n , i t i s p o s s i b l e to l o c a t e the s t a r e x a c t l y on the c e l e s t i a l sphere. Example:

On Jane 3 0 t h , at 1 0 . 2 4 p.m., I no t i c e d that a s t a r i n the c o n s t e l l a t i o n Scorpio was almost at the z e n i t h . I wanted to iden­t i f y i t . The c a l c u l a t i o n i s as f o l l o w s : Number of days between March 2 1 s t and June 3 0 t h = 101 days, therefore the number of minutes to be added to s o l a r time =

404 mins. = 6 hrs. 44 mins. " noon. June 3 0 t h = 6 hrs. 44 mins. s i d e r e a l time. " 1 0 . 1 ; p.m. = 1 7 hrs. 08 mins. " "

As I l i v e i n l a t i t u d e 3 7 ° , 47 mins. S. the d e c l i n a t i o n i s about - 3 7 ° 47 mins. The s t a r which had a Right Ascension e q u a l l i n g 17 hrs. 8 mins. and a D e c l i n a t i o n e q u a l l i n g - 3 7 ° 47 mins. (approx.) l i s t e d i n American Ephemeris and N a u t i c a l Almanac was v Scorpio.

The f o l l o w i n g t a b l e gives the Right ascensions and D e c l i n ­ations of the twenty-one b r i g h t e s t s t a r s i n the sky. Each i s a major s t a r of i t s c o n s t e l l a t i o n and w i l l serve as the key to i t . These few s t a r s i n conjunction with s t a r maps are a l l the c o n s t e l l ­a t i ons one would wish to know.

STAR NAME ] MAGNITUDE RIGHT ASCENSION DECLINATION

S i r i u s Canis Major -1 .6 6 hrs . 41 - 1 6 . 6 ° Canopus Carina - 0 . 9 6 ti 22 mins. - 5 2 . 7 ° R i g e l kent Centaurus -0.2 14 ti 34 11 - 6 0 . 5 ° Vega Lyra + 0.1 18 II 34 11 + 3 8 . 7 ° Capella Auriga + 0.2 5 II 11 11 + 4 5 . 9 ° Arcturus Bootes + 0.2 14 II 12 11 + 19 .6 ° R i g e l O rients + 0 . 3 5 II 11 it - 8 . 3 °

Procyon Canis Minor + 0 . 5 7 If 35 11 5 . 4 ° Achernar Eridanus + 0 . 5 1 II 35 11 - 5 7 . 7 ° Agena Centaurus + 0 . 9 13 II 58 11 - 6 0 . 0 ° A l t a i r A q u i l a + 0 . 9 19

I I 47 11 + 8 . 6 ° Betelgeuse Orients + 0 . 9 5 II 51 11 + 7 .4 ° Acrux C r u c i s 1.0 12 II 22 11 - 6 2 . 6 ° Aldebaran Taurus 1.1 4 II 31 ti +16 .3°

Spica V i r g l n i s 1.2 13 II 21 it - 1 0 , 7 ° P o l l u x Geminorum 1.2 7 II 40 11 + 2 8 . 2 ° Antares Scorpio 1.2 18 II 24 11 - 2 6 . 2 ° Fomalhaut Pi s c e s A u s t r a l i s 1 .3 22 II 53 it - 3 0 . 1 °

Deneb Cygni 1 .3 20 II 39 11 + 4 5 . 0 ° Pegulus Leonis 1 .3 10 11 04 it + 1 2 . 4 °