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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