My Science
Project: A Starry Night Essay. A Composition on Starry Night
As you
look up at a starry sky on a moonless night, you are re-living one
of the great experiences we can all share as human beings. Just like
our ancestors, you notice that the firmament above your head looks
like an inverted bowl with the stars as small flickering lights
somehow pasted onto the blackness. The bowl-like appearance is due
to the complete lack of depth perception when looking at stars
against a dark sky and so the concept of the celestial sphere is
born. It is a sphere rather than a bowl because as you change your
latitude on Earth, the bowl of the night sky stays above you though
the locations of the stars change with respect to your horizon. As
you travel all around the Earth, you realize that the celestial bowl
above your head at any given paint appears lobe one half of a
celestial sphere with the Earth at the center. This, of course, is
not the correct interpretation of your observations; the stars
really are at different distances from us and the Earth is not at
the center of anything, but the concept of the celestial sphere is
still very useful in organizing our observations of the objects we
see projected onto it.
The celestial sphere is a fictional construct, but a very useful one
which we will employ extensively in this chapter. As you observe the
stars during the course of a night, you notice that they move in a
very regular pattern, for the most part rising in the direction of
your eastern horizon, reaching their highest elevation when they are
due south, and setting towards the west. If you are in the northern
hemisphere, you also notice that there is a star which does not seem
to move, but around which all the other stars seem to travel during
the course of the night. A long photographic exposure centered on
this star will show that the other stars appear to travel in
circular orbits around this special star which is known as Polaris
or the Pole Star. If you were to take a close-up photo of Polaris,
you would notice that it, too, is traveling around a fixed point.
This point is called the North Celestial Pole and it represents the
north "pivot" point around which the celestial sphere seems to turn.
There is also a pivot point visible from the Southern Hemisphere
around which the southern stars seem to move (the South Celestial
Pole), but, unfortunately, there are no bright stars near this
location so that there is no southern "pole star". The Southern
Cross is a group of four bright stars in the shape of a cross whose
long arm points towards the South Celestial Pole 'but that's not
quite as convenient as Polaris for finding your way.
The above observations led our ancestors to postulate that the
celestial sphere turned on an axis which went through the North and
South Celestial Poles. What made the celestial sphere move was
either not discussed or attributed to supernatural forces (in the
Middle Ages, angels were thought to be responsible for moving the
celestial sphere). An alternative explanation for the observations
described above is that the celestial sphere is fixed, but that the
Earth turns on an axis going through the Earth's North and South
Poles (which point in the directions of the North and South
Celestial Poles, respectively). However, the seeming immobility of
the Earth's surface led most ancient cultures to reject this
alternative. It is important to note that this alternative was
thought about, because the perception of motion is often a relative
phenomenon: Think of being on a train in a station with another
train adjacent to you; as one begins moving it is often a
bewildering feeling as our minds struggle to determine whether it is
our train or the adjacent one which is actually in motion. In
addition to the daily (diurnal) motion of the celestial sphere,
there was also a seasonal variation in the way that the stars
appeared during the course of the night. All ancient cultures
recognized patterns of the stars in the sky, patterns that we now
call constellations. Night after night, the constellations seemed to
appear as twilight faded away in the same location as the previous
evening - but four minutes earlier in time.
Thus, if at sunset on a given night you first saw star A between the
branches of your favorite tree, then the next night you would see
the same star between the same branches, but at a time which was
four minutes earlier than your sighting of the previous evening.
Alternatively, if you observed your tree branches the next night at
the same time as when you had observed the star the previous night,
then you would see the star was a little bit higher with respect to
the horizon. In other words, as the four minutes pass and the time
of your observation on night two coincides with that of night one,
the star has climbed a bit higher above the horizon. Thus, stars and
constellations rise about two hours earlier each month. For
instance, if you see Orion rising at around 9 PM on October l', then
you will see it rise around 7 PM on November 1st. In this fashion,
over the course of a year, you would see many different
constellations; some visible primarily in the summer, others in the
fall, etc.. The ancients understood this phenomenon in terms of the
Sun's motion around the celestial sphere, a problem we will turn to
after a brief excursion into defining positions on the celestial
sphere.
The Celestial Sphere and coordinates
The celestial sphere was thought by the ancients lobe a real sphere
surrounding the earth at some indeterminate distance with the stars
(whatever they were) fixed upon it. Though most of the stars seemed
fixed onto the celestial sphere (because over many human lifetimes
they seemed to keep their relative positions), there were seven
permanent objects and a few other short-lived phenomena that
appeared to move with respect to the fixed stars (which were thus
considered to be "in the background"). The seven objects were the
Sun, the Moon, and five bright stars that the Greeks called
"planets" or "wanderers". These five bright stars are the waked-eye
planets; Mercury, Venus, Mars, Jupiter, and Saturn. The occasional
comet or shooting stars also moved with respect to the fixed stars
but their short-lived apparitions (only a few seconds in the case of
shooting stars or meteors) caused many cultures to relegate these
phenomena to the upper atmosphere rather than the more distant
celestial sphere. We will ignore these phenomena for now, and we
will begin by looking at the motion of the Sun on the celestial
sphere. When the Sun is in the sky, you can't see stars. But you
know the stars are there because as the Sun sets you begin to see
the brighter stars and then, as twilight deepens, more and more
stars are visible so that it seems rather clear that the stars were
there all along, but their faint light was overwhelmed by the Sun's
brilliance. By looking right after sunset at the stars which are
visible in the direction where the Sun has just set, it becomes easy
over the weeks and months to chart the path of the Sun on the
celestial sphere. This path is the same year after year, so that all
astronomically-oriented cultures had recognized the existence of
this path against the fixed stars of the celestial sphere.
We call this path the ecliptic. In order to understand where the
ecliptic is located on the celestial sphere, we need to have some
type of coordinate system on the celestial sphere. The Earth, which
is located at the very center of the Celestial Sphere (remember,
this is a fictional concept), already has a coordinate system on it:
latitude and longitude. We have already seen that the projection of
the North Pole onto the celestial sphere is the location of the
North Celestial Pole (and if you're standing on the Earth's South
Pole and you point straight up you are pointing in the direction of
the South Celestial Pole) so why not extend the latitude and
longitude coordinate system from the Earth all the way out to the
celestial sphere? Think of the lines of latitude and longitude
inflating or expanding out from the surface of the Earth,
maintaining their relative orientations, and producing a grid onto
the celestial sphere as they finally intersect it. Of course, to
make life difficult, this grid of celestial latitude and longitude
is not called celestial latitude and longitude. That would be too
easy.
Celestial latitude is called declination, and celestial longitude is
called right ascension. You have to remember that astronomy is the
oldest science and so there's a lot of tradition that still
permeates its nomenclature. To prove this assertion and make things
even more complicated, although longitude on Earth is measured
westward in degrees from the zero longitude line (meridian) which
passes through Greenwich, England (from the times when Brittania
ruled the waves), on the celestial sphere it is measured in hours,
minutes, and seconds of time. Now this is just tradition and there
really is no big deal about it. Just like 1 dollar is equivalent to
100 cents, once you know the conversion factors, you can go from
degrees to hours, minutes, and seconds with no problem. The
conversion has to do with the fact that there are 360 degrees in a
full circle and that a point on the celestial sphere goes completely
around in one day. So if we make 24 hours equal to 360 degrees, then
we have our conversion factor. Thus, 1 hour of right ascension is
equal to 15 degrees, or 60 minutes of right ascension are equal to
15 degrees, or 1 degree is equal to 4 minutes, and so on and so on.
One last thing about our celestial coordinate system, because we
just projected the Earth's coordinate system onto the celestial
sphere, the North and South Poles became the North and South
Celestial Poles; in the same way, the Earth's equator (the circle
where the latitude is equal to 0 degrees), when projected out onto
the celestial sphere, becomes, you guessed it, the Celestial
Equator.
The Celestial Equator is the place where the declination equals 0
degrees. Declinations north of it are positive and declinations
south of it are negative. What this all means, practically speaking,
is that if you are at the North Pole and you point straight up (by
the way, the location "straight up" on the celestial sphere clearly
depends on where you are located on Earth, but no matter what part
of the celestial sphere you are pointing to, it's called the
....7enith), you are pointing at the North Celestial Pole. If you do
this at the Earth's equator, you are pointing somewhere along the
Celestial Equator. And if you do this at Athens, Georgia, with a
latitude of 34 degrees north, then you are pointing at the circle on
the celestial sphere which has a declination of 34 degrees north. At
this point. I should leave well enough alone, but those that are
still reading this must be wondering how is the 0 degrees meridian
of right ascension defined? You just can't extend the 0 degrees
Greenwich meridian on Earth to the celestial sphere because the
Earth and celestial sphere are in motion with respect to each other.
It doesn't matter which is actually moving (we've been making
believe the celestial sphere turns completely on its axis once per
day). Either way, the Greenwich meridian cannot line up with a fixed
meridian on the celestial sphere. So, we have to define on the
celestial sphere a zero meridian of right ascension. This is not
called the Celestial Greenwich Meridian but, rather, the First Point
of Aries. In order to define exactly where this is on the sky we
need to look at the motion of the Sun on the celestial sphere.
The Movement of our Sun
Now that we have at least most of a coordinate system on our
celestial sphere, we can talk about what the Sun's motion across the
celestial sphere looks like over the course of a calendar year. This
would be real easy if the Earth orbited the Sun (or the Sun orbited
the Earth in our celestial sphere fiction) with its axis of rotation
perpendicular to the plane of its orbit. In simple terms, think of
Brad Pitt or Geena Davis as being the center of your own sick,
twisted Universe. You walk in a circular orbit around them standing
upright. While you are walking around them, you are also spinning
around. As you go around, you are perpendicular to the plane defined
by floor, thus, your spin axis is perpendicular to the plane of your
orbit. In the case that the spin axis of the Earth is perpendicular
to the plane of revolution, the Sun's path around the celestial
sphere would just be along the Celestial Equator and there would be
no problem. But, unfortunately, the Earth does not go around the Sun
with its axis of rotation perpendicular to the plane of its orbit.
Instead, the Earth's axis is tilted about 23 degrees from the line
perpendicular to the plane defined by the Earth's motion around the
Sun. This would be like orbiting Brad or Geena while leaning over 23
degrees. But, BE CAREFUL!, this does not mean that you are always
leaning towards B. or G., rather, you are always leaning so that the
top of your head points to some distant, stationary cloud, so that
sometimes you are indeed leaning 23 degrees towards B. or G., and
sometimes you are leaning 23 degrees away from B. or G.. At this
point, we will finally resort to a diagram to make this concept
crystal clear:
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