Chapter 6
The night sky
From The Book “
Astronomy Principles and Practice”
6.1 Star maps and catalogues
Already
in the previous chapters, mention has been made of the names of some of the
constellations and stars. Inspection of star maps shows that areas of the sky
are divided into zones marked by constellations. Within a constellation it is
usual to find an asterism (a pattern of stars) which is readily recognizable.
Also some of the stars may carry a name, usually with an Arabic origin, a Greek
letter or a number to provide individual identification. At first sight, the
nomenclature may seem to be haphazard but this is simply the legacy of history.
In
the early days, the stars were listed according to their places within a
constellation and designated by a letter or number. The constellation zones are
quite arbitrary in relation to the stellar distribution but their designations
persist for the ease of identification of the area of sky under observation. It
is, however, important to have some appreciation of the background to the
system and its current use.
To
describe the philosophy behind the development in star catalogues we can do no
better than to quote the relevant section of Norton’s Star Atlas.
The
origin of most of the constellation names is lost in antiquity. Coma Berenices
was added to the old list (though not definitely fixed till the time of Tycho
Brah´e), about 200 BC; but no further addition was made till the 17th century,
when Bayer, Hevelius, and other astronomers, formed many constellations in the
hitherto uncharted regions of the southern heavens, and marked off portions of
some of the large ill-defined ancient constellations into new constellations.
Many of these latter, however, were never generally recognized, and are now
either obsolete or have had their rather clumsy names abbreviated into more
convenient forms. Since the middle of the 18th century when La Caille added
thirteen names in the southern hemisphere, and sub-divided the unwieldy Argo
into Carina, Malus (now Pyxis), Puppis, and Vela, no new constellations have
been recognized. Originally, constellations had no boundaries, the position of
the star in the ‘head’, ‘foot’, etc, of the figure answering the needs of the
time; the first boundaries were drawn by Bode in 1801.
The
star names have, for the most part, being handed down from classical or early
medieval times, but only a few of them are now in use, a system devised by
Bayer in 1603 having been found more convenient, viz., the designation of the
bright stars of each constellation by the small letters of the Greek alphabet,
α, β, γ, etc, the brightest star being usually made α, the second brightest
β—though sometimes, as in Ursa Major, sequence, or position in the
constellation figure, was preferred. When the Greek letters were exhausted, the
small Roman letters, a, b, c, etc, were employed, and after these the capitals,
A, B, etc—mostly in the Southern constellations. The capitals after Q were not
required, so Argelander utilized R, S, T, etc, to denote variable stars in each
constellation, a convenient index to their peculiarity.
The
fainter stars are most conveniently designated by their numbers in some star
catalogues. By universal consent, the numbers of Flamsteed’s British Catalogue
(published 1725) are adopted for stars to which no Greek letter has been
assigned, while for stars not appearing in that catalogue, the numbers of some
other catalogue are utilized. The usual method of denoting any lettered or
numbered star in a constellation is to give the letter, or Flamsteed number,
followed by the genitive case of the Latin name of the constellation: thus α of
Canes Venatici is described as α Canum Venaticorum.
Flamsteed
catalogued his stars by constellations, numbering them in the order of their
right ascension. Most modern catalogues are on this convenient basis (ignoring
constellations), as the stars follow a regular sequence. But when right
ascensions are nearly the same, especially if the declinations differ much, in
time ‘precession’ may change the order: Flamsteed’s 20, 21, 22, 23 Herculis,
numbered 200 years ago, now stand in the order 22, 20, 23, 21.
For
convenience of reference, the more important star catalogues are designated by recognized
contractions.
With
the application of detectors on telescopes replacing the unaided eye, the
resulting catalogues are forced to provide extensive listings of stars
containing hundreds and thousands of entries. A famous catalogue based on
photographic records of the sky made by Harvard University is known as the
Henry Draper Catalogue. Each successive star is given the number according to
increasing right ascension with the prefix HD. For example, HD 172167 is at
once known by astronomers to denote the star numbered 172167 in that catalogue.
This particular star is bright and well-known, being Vega or α Lyrae. It also
appears in all other catalogues and may, for example, be known as 3 Lyrae
(Flamsteed’s number), Groombridge 2616 and AGK2 + 38 1711 (from Zweiter Katalog
der Astronomischen Gesellschaft fu¨r das A¨quinoktum 1950).
Returning
to the quotation from Norton’s Star Atlas:
Bode’s
constellation boundaries were not treated as standard, and charts and
catalogues issued before 1930 may differ as to which of two adjacent
constellations a star belongs. Thus, Flamsteed numbered in Camelopardus several
stars now allocated to Auriga, and by error he sometimes numbered the star in
two constellations. Bayer, also, sometimes assigned to the same star a Greek
letter in two constellations, ancient astronomers having stated that it
belonged to both constellation figures: thus β Tauri = γ Auriga and α
Andromedae = δ Pegasi.
To
remedy this inconvenience, in 1930 the International Astronomical Union
standardized the boundaries along the Jan 1st, 1875, arcs of right ascension and
declination, having regard, as far as possible, to the boundaries of the best
star atlases. The work had already been done by Gould on that basis for most of
the S. Hemisphere constellations.
The
IAU boundaries do not change in their positions among the stars and so objects
can always be correctly located, though, owing to precession, the arcs of right
ascension and declination of today no longer follow the boundaries, and are
steadily departing from them. After some 12 900 years, however, these arcs will
begin to return towards the boundaries, and 12 900 years after this, on
completing the 25 800-year precessional period will approximate to them, but
not exactly coincide.
Nowadays,
as well as recording the stars’ positions for a particular epoch, a general
catalogue will also list various observed parameters of each star. For example,
the annual changes in right ascension and declination may be given. Other
headings might include proper motion, annual parallax, radial velocity, apparent
magnitude, colour index and spectral type. Special peculiarities may also be
supplied—for example, if the star is a binary system. It may be noted, too,
that according to the IAU convention, the names of constellations are usually
referred to by a standardized three-letter abbreviation.
There
are many aids to help provide information about what is available for view in
the night sky at any particular time. Many PC software packages W 6.1,W
6.2 offer active demonstrations of the behaviour of the night sky
according to the observer’s location and the local time. These can be very
informative as motions which, in reality, may take some months to execute can
be simulated on the screen and speeded up to take just a few seconds. Thus, for
example, the apparent motions of the Moon and the complex planetary paths may
be readily appreciated. It is a relatively easy matter to learn which stars are
in the sky at a particular time and where the planets are relative to the stellar
background. If a planetarium is available, constellation identification can be
learned very quickly, especially if a pattern projector is attached for
highlighting each constellation.
Familiarity
with the night sky, however, is best gained by spending a few hours on
different nights observing the ‘real’ panorama. The true feeling of being under
a hemispherical rotating dome with stars attached can only be obtained by
outdoor activity. Appreciation of the angular scale associated with the
well-known asterisms and identification of the constellation patterns is best
gained by direct experience. On the early occasions, it is useful to be armed
with a star atlas (such as Norton’s Star Atlas) or a simple planisphere. This
latter device is a hand-held rotatable star map with a masking visor, allowing
the correct part of the sky to be seen according to the season and the local
time.
In
the previous chapters, reference has already been made to angular measure and,
in the first place, it is useful to have an appreciation of angular scales as
projected on the night sky. For example, the angular sizes of the Sun and Moon
are approximately the same being ∼1/2
◦. If either the Sun or full Moon is seen close to the horizon it is readily
appreciated that their apparent diameters would need to be extended 720 times
to ‘fill’ the 360◦ of the full sweep around the horizon. Rough estimates of
larger angles between stars, so providing the impression of just how large an
area a particular constellation covers, can be made by using the ‘rule of
thumb’ technique as practised by artists.
If
the arm is fully extended, different parts of the hand can be used to provide
some simple angular values. Typical values are indicated in figure 6.1 but a
system and scale should be developed individually by comparing observations
with a star map. In the first place it will be noted that the angular extent of
the thumb at arm’s length is about 1◦. This means that if the Moon is in the
sky it should be easily blocked from view by the use of the thumb. Figure 6.1
indicates that the knuckle-span is ∼8◦
and that the full hand-span is ∼15◦.
Obviously these values depend on the individual and they must be checked out
against some more easily recognizable asterisms.
To
start with, this can be done by examining the well-known constellations. For
example, in Ursa Major, the seven-star asterism of the Plough can be examined
(see figure 6.2). For most northern hemisphere observers the stars are
circumpolar and can, therefore, be seen at any time in the year. Close
inspection of the pattern reveals that it is actually made up of eight stars,
as there are two Mizar and Alcor (ζ and 80 UMa) separated by 11 minutes of arc.
The separation of the stars Dubhe and Mirak (α and β UMa) is about 5◦: the
distance between Polaris (α UMi) and Dubhe (α UMa) is close to 30◦. It may be
noted that the northern hemisphere sky appears to rotate or pivot about a point
very close to Polaris. There is no equivalent ‘pole star’ in the southern
hemisphere. The sides of the Square of Pegasus (see figure 6.3) are
approximately 16◦ across the sky.
Over
the course of a few weeks, take note of the changes in rising and setting times
of the constellations. This can be done by noting the times when a particular
group of stars is at the same position in the sky relative to a particular
position of a land-mark as seen from some regular observing point. Better
still, fairly accurate transit time records can be made over a few nights by
using a couple of vertical poles fixed in the ground a few metres apart.
For
northern hemisphere observers, it may be noted that the star cluster known as
the Pleiades (see figure 6.4) may be seen rising in the east in the autumn. As
winter progresses the rising time becomes earlier and earlier. The constellation
Orion (also depicted in figure 6.4) transits in the north–south meridian round
about midnight in February. A few months later, it will be noted that Leo (see figure
6.5) claims this position.
In
the southern hemisphere skies, more bright stars are found than in the northern
hemisphere. Also there is the beautiful spectacle of two extensive hazy patches
known as the Magellanic Clouds. Two immediate differences are apparent that are
initially disturbing to any traveller who changes hemispheres. It may be noted
that objects appear to rise on the left-hand side in the N hemisphere with the
observer’s back to the pole star and on the right-hand side in the S hemisphere
with the observer’s back to the S pole. Startling too is the fact that a N
hemisphere visitor to the southern skies sees the markings on the Moon’s face
upside down!
A
useful starting point is for the student to observe a clear night sky with
unaided eye or with binoculars. The scope of the observations and the features
that might be noted are as follows:
1.
The stars do not have the same brightness. By using a star chart with the
magnitude scale, or by getting stellar magnitude from a catalogue, estimate the
faintest star that can be seen. Does this vary from night to night? Can faint
extended sources, such as the Andromeda Galaxy, be seen? Try the effect of
averted vision, i.e. do not look exactly at the source but slightly away from
the direct line of sight.
2.
Note that there are few stars that can be seen close to the horizon due to the
extinction by the Earth’s atmosphere. Compare the apparent brightness to stars
which are catalogued with the same magnitudes, choosing the stars so that one
is close to the zenith and the other close to the horizon.
3.
The stars are not randomly distributed in the sky. Note the way that the stars
are grouped together. Particular clusters to pay attention to are the Pleiades
and Praesepe.
4.
The stars twinkle or scintillate. Note that the effect diminishes according to
the altitude of a star above the horizon. There is usually very little
noticeable effect for stars close to the zenith.
5.
Check out that any bright ‘star’ that does not twinkle is a planet by
consultation of an astronomical almanac for the particular time of the year.
If
a 35 mm camera is available, it is instructive to take photographs of stars.
Colours of stars show up well if colour film is used, although the record of
the colour values may not be exact. It is not necessary for the camera to be
made to follow the diurnal motion of the stars. A time exposure (a few minutes)
should be made by placing the camera on the rigid support, opening the shutter
and allowing the stars to drift by. A pattern of star trails will be recorded.
They should be easily identifiable from the star map and the recorded colours
should be compared with some database. Note that the brighter the star is, the
thicker the trail will be.
There
is no better way to gain confidence in understanding the basic celestial
coordinate systems and, at the same time, experiencing the excitement of
finding the famous stars, star clusters, galaxies, etc than by using a small
equatorially mounted telescope, if one is available.
For
completeness, basic star maps are provided for both the northern and southern
hemispheres for the four seasons (see figures 6.6, 6.7, 6.8 and 6.9).
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