Libris Britannia 4/science library(b).zip/science library(b)/ASTRNOMY/STARPL20.ZIP/ASTRONMY.DOC

Text File | 1991-12-11 | 12KB | 308 lines

ASTRONOMY.DOC

This text file is intended as a very brief introduction to
Astronomy for those computer users who are trying out STARPLOT but
have no previous astronomical interest.

It covers the coordinate system used by STARPLOT, the Ecliptic,
Star Magnitudes, and Epoch.


COORDINATES

I suppose it's no surprise to anyone that the stars, being
extremely distant, can be thought of as lying on a celestial
sphere encircling the Earth.  To define their positions a
coordinate system is required.

Spherical coordinates on the earths surface such as longitude and
latitude cannot be simply projected up onto the celestial sphere.

Zero degrees longitude goes through an agreed fixed point on the
earths surface; Greenwich, but the earth is rotating, which looks
to us as if the celestial sphere is rotating about us.  So the
point above Greenwich is not a fixed point on the sphere.

However Latitude is fixed, from 0 degrees at the equator, to 90
degrees at the North Pole, and -90 degrees at the South Pole.

In astronomical terms this is called Declination, 0 degrees at a
line on the celestial sphere directly above the equator, and plus
and minus 90 degrees at the poles.  Degrees are further subdivided
into 60 minutes, which are also subdivided into 60 seconds.

(All this base 60 stuff comes from the Babylonians, they had a
wierd number system - astronomy is a most ancient science,
nevertheless its a pain when you're writing computer software.
Decimalize it!  That's what I say.)

So for East-West measurements, the problem is to define a point on
the celestial equator, fixed with respect to the stars, and call
it zero.

This is what is done - I'll explain how the point is chosen later,
in the section on the ecliptic, for now just assume an arbitrary
point is chosen.

From this point the celestial equivalent of longitude is now
called Right Ascension, and is measured in hours, 0 to 24 right
round the sphere, with the hours also subdivided into minutes
and seconds.






Any point in the sky can be specified by an RA and DEC value, and
when using STARPLOT you will notice the RA, DEC coordinates of the
cursor changing as you move it.  The stars are all positioned by
their RA and DEC coordinates.


THE ECLIPTIC

If you consider the earth to be still, the apparent motion of the
sun and stars seems to have two components.

Every day the sun and stars move around the world - caused by the
earths rotation.

In one year, the earth has rotated right around the sun, this
looks like an extra rotation of the celestial sphere.  If every
clear evening, you stand in the garden at the same time and look
at the stars in one direction, you will see a group of stars, but
as the year evolves you will notice that group of stars is
shifting, and another group taking their place.

After a full year you will see the original group back in place,
the extra rotation is complete.

The Suns motion is more complex, it is not at a fixed point
against the background stars.  It looks to travel slowly through
different groups of stars in the year.

What is happening is that in summer the stars behind the sun are
invisible because of the suns glare.  In the other direction away
from the sun - the midnight sky -  the stars are very visible.

In winter the earth has moved to the other side of the sun and it
looks to the observer as if the sun has moved around and is now
blotting out the previously visible constellation.  The observer
can now happily view the stars that were blotted out in summer.

The earth moves around the sun in a constant plane - the plane of
the ecliptic - so it looks as if the sun always moves along the
same path.  Those stars at right angles to the plane are never
behind the sun, and can be seen at any night of the year.

Now another factor comes in - the tilt of the earth.  If the North
to South axis of the earth was precisely at right angles to the
plane of the ecliptic, then the plane of the ecliptic would lie
along the celestial equator, and the suns yearly motion would be
very simple.  It would always be at 0 degrees declination, just
travelling uniformly around the celestial equator.

But it ain't like that.  The earths axis is tilted at about 23.5
degrees to the plane of the ecliptic.





The sun therefore seems to follow a line around the earth tilted
to the equator - THE ECLIPTIC. In STARPLOT pressing E switches the
ecliptic line on and off.

In the northern hemispheres summer, the sun is on the part of the
line north of the equator, therefore for northern observers it is
high in the sky.  At its highest point - the summer solstice - it
is 23.5 degrees north of the equator.

As the year passes, the sun moves along the ecliptic to a point
directly over the equator, where the ecliptic and the celestial
equator intersect, the autumn equinox.  It then continues south of
the equator, winter for northern observers.  At its lowest point,
the winter solstice, the sun has a declination of -23.5 degrees.
To a very northern observer, say on the north pole, it is so low
it is below the horizon, and he does not see the sun at all.

To a southern hemisphere observer the sun is now high in the sky.

The sun continues its progress, northwards now, until it lies on
the intersection of the ecliptic with the equator, this is the
spring (or vernal) equinox.

This point in the sky is taken as the 0 point in Right Ascension.

Finally the sun returns to the summer solstice and the year is
complete.

The line of the ecliptic has another extremely important use.
The plane in which the earth moves around the sun, is the same
plane in which the other planets also move around the sun.  These
planets look to us to be moving against the background stars, and
the path they move along is the ecliptic.

Therefore if you see a bright star in the sky that is not in
the STARPLOT charts, but lies along the ecliptic line, then it is
certainly a planet.  If it lies very far off the ecliptic then I
don't know what it is.

Those twelve constellations which lie along the path of the
ecliptic, and hence are the constellations in which the sun and
planets appear, will have familiar sounding names to you - they
are the constellations of the Zodiac.

The positions of planets are published in various astronomy
magazines and by astronomy clubs, once you have their RA and DEC
positions for a given date, put them on a STARPLOT chart, against
a background of stars and print off a finder chart, then go look
for it.







You should realise that if you live in the northern hemisphere you
will not be able to see the far southern constellations, and visa
versa.  Also during the year you will not be able to see much of
the stars or planets which lie in the direction of the sun.  In
this case the best you can do is at evening or morning when the
sun is just below the horizon, though even then twilight gets in
the way.

The table below gives the suns (rough) positions throughout the
year.  If you plot them on STARPLOT you will see the
constellations that will be obscured - the opposite
constellations, 12 hours of RA around the ecliptic, will be
visible at midnight.

Mid month       Right Ascension       Declination

Jan                 20                  -21
Feb                 22                  -11
Mar                  0                   -2
Apr                  2                   10
May                  4                   19
Jun                  6                   23
Jul                  7                   22
Aug                  9                   14
Sep                 11                    3
Oct                 13                   -8
Nov                 15                  -18
Dec                 18                  -23


The moon takes a path inclined at 5.15 degrees to the ecliptic.
If its path was along the ecliptic we would get a lot more
eclipses, but as it is, we only get an eclipse when the moon is
at the point where its path intersects the ecliptic, and the sun
is at the same point on the ecliptic.


STAR MAGNITUDES

Some stars are brighter than others, either because they are
nearer, or because they are intrinsically brighter.

This brightness is represented in STARPLOT by choosing a larger
star symbol for the brighter stars - the size of the symbol does
not represent its diameter since stars are all points, even
through very large telescopes.  Planets are different, a diameter
can be observed.

Not used by STARPLOT, but present in an information field in
STARPLOT is a numerical value of the stars magnitude.






One is of the first magnitude, ie very bright, two is dimmer,
three dimmer still.  On a clear night the human eye can see down
to magnitude six.  Some very bright stars are brighter than
magnitude 1, they are given values such as zero or negative
values, Sirius for instance is magnitude -1.46.

These magnitude values are precisely defined and measured by both
amateur and professional astronomers, since variations in a stars
brightness can give information.  Some stars vary because of
instabilities in the star itself, others because a dimmer star is
circling, and eclipsing a brighter star.

Because of this many amateur astronomers want to know the
magnitudes of neighboring stars so they can visually estimate, by
comparison, the brightness of the one they are studying.  This is
why STARPLOT has a data field for magnitude even though it doesn't
need the information.

Magnitude values are also published for planets, these vary due to
the distance of the planet from earth, and the proportion of the
planets face that is illuminated.


EPOCH

The coordinate system uses the fixed point in space where the
ecliptic crosses the equator at the spring Equinox as the Right
Ascension Zero point.  It also uses the north and south poles as
fixed Declination points of plus and minus 90 degrees.

Unfortunately the earth is wobbling.  The poles aren't fixed, nor
is the position of the equinox.  Our coordinate system lies on
shaky ground!

This wobble is called precession and consists of the poles
describing a circle every 26,000 years.  Thus the RA and DEC
coordinates of stars are continually drifting, albeit very slowly.

Catalogues and star charts are altered at standard reference dates
to account for this.  Thus a chart is said to be drawn for a
certain epoch, the present standard epoch is the year 2000.

As far as STARPLOT is concerned, it doesn't care about epoch, it
is the coordinates of the stars in the .STR files that must be
plotted for a given epoch.  STARPLOT then plots on the chart
whatever DEC and RA point is given.










CONCLUSION

That covers just about all the astronomical jargon you may see in
the STARPLOT program.

Looking at a computer screen of stars is a very poor substitute
for the real thing.  If you are interested in learning more on
astronomy I suggest you try your library, and astronomy magazines
are very good.

If you have a pair of binoculars then, when Orion is visible,
around January/February, do what everyone does and have a look
through them at the Pleiades, and the Orion Nebulae.

The Pleiades is a cluster of stars in Taurus - look at it on
STARPLOT at about 3hr 45mins, 23 degrees.

The Orion Nebulae is at about 5hr 35mins, -5 degrees.

Before you buy a telescope join an astronomy club and have a look
through someone else's first.

                                      BCz