Like our ancestors did thousands of years ago, it's quite natural for us today to view the night sky as an inverted bowl upon which the stars are attached and the Sun, Moon, and planets move. And it requires only a little imagination to further visualize that the inverted bowl of stars is actually the hemisphere of a globe - a celestial sphere - which we see from the inside out. Earth floats freely in space within this sphere, which lies at an immense but arbitrary distance. The sphere revolves slowly around our world, incrementally conveying the constellations from east to west until they return to their original positions a year later.
Of course, we recognize that the celestial sphere is an illusion: the Moon and Sun lie at very different distances, the stars are light-years away, and it is Earth's rotation and orbit that make the sky appear to move. But for purposes of tracking the motions and directions of celestial bodies, as well as learning the stars and constellations, a two-dimensional celestial sphere is an effective mental contrivance.
If we consider the sky as a sphere rotating about an axis, then it must possess directional bearings, a north and a south pole, and an equator. In the case of the celestial sphere, these are essentially projections into space of the Earth's cardinal directions, poles, and equator.
A compass can help you locate north, south, east, and west from your observing location, although it can be done just as well by noting where the Sun rises and sets (especially around the equinoxes). Although it is not important to be rigorously precise in noting the cardinal directions, it helps to know them in a general sense for skywatching purposes. When I describe a star or constellation rising in the 'northeast,' or lying
'due south,' and you know where these directions are, then you have won half the battle of getting yourself oriented.
Dividing the sphere, and hence the sky, into east and west halves is a circle called the celestial meridian. The meridian extends across the sky over your head and through the north and south points of your horizon. (There is also a circle crossing the horizon through the east and west points called the 'prime vertical,' but this doesn't come into play in this book as often as the meridian.) A celestial body is said to 'culminate' when it crosses the meridian. Hence, when I say that a certain star 'lies on the meridian (or culminates) tonight at 9 o'clock,' you know to look for it on a north-to-south line at the star's highest point in the sky.
Notice that I said 'the star's highest point.' All stars reach different altitudes above the horizon that are dependent on your latitude. In the Northern Hemisphere, stars toward the south never get very high in the sky, while those in the north arch overhead, or nearly so. Southern stars do, of course, rise higher the further south you go, but then stars in the northern sky begin to slip lower. Again, this adds to the illusion that the sky is one half of a sphere.
On the other hand, there is indeed a point in the sky that can be called 'highest,' with respect to you, the observer. It always lies directly over your head and is called the zenith. As such, the zenith is 90° (i.e., at a right angle), from all points on your horizon and also lies on the meridian.
The north celestial pole lies very near the end of the handle of the Little Dipper and is marked by the star Polaris (more popularly known as the North Star). In the Northern Hemisphere, its angular distance above your horizon is equivalent to your latitude. So, if you live at latitude 35° N, then Polaris lies 35° above your northern horizon. The closer you get to the North Pole, the higher the star appears in the sky, until at the North Pole, it stands at your zenith.
Conversely, the south celestial pole in the Southern Hemisphere is in the constellation Octans and marked by the faint star Sigma (o). Its distance above any horizon is also latitude dependent. Since the entire celestial sphere pivots around these two points in the sky, they never rise or set. (For more on finding the celestial poles, see 'Finding true north, March 22 - 28,' and 'Finding true south, March 29 - April 4.')
By definition, the celestial equator is the great circle lying halfway between the north and south celestial poles. It is Earth's equator pro-
jected into space. Hence, if you were standing on the equator, the celestial equator would be the prime vertical circle running overhead through the east and west points on the horizon. In the Northern Hemisphere, the celestial equator lies south of your zenith at an angular distance equivalent to your latitude. Thus, if you live at latitude 30°, the celestial equator lies south of your zenith by that amount. (You can also say that at latitude 30° the celestial equator is inclined to the south horizon by 60°.)
The final great circle to note is the ecliptic, which passes through the twelve constellations of the zodiac, plus a thirteenth, Ophiuchus. It is, in essence, the projection of the mean plane of Earth's orbit into space. Since Earth's orbital plane is coincident with that of the solar system, the annual paths of the Sun, Moon, and planets lie very near the ecliptic circle. And because Earth's axis is tipped over nearly 23.3° with respect to this plane, the ecliptic, too, is inclined to the celestial equator by that amount.
Four equidistant points on the ecliptic represent solar milestones, when the Sun either crosses the celestial equator or is at its greatest distance from it. These points represent the four seasonal transitions. The spring and autumnal equinoxes occur where the ecliptic intersects the celestial equator. In spring, the Sun crosses the celestial equator moving northward; in autumn, it is heading south. The summer solstice is the point where the Sun reaches its northernmost extreme from the celestial equator; the winter solstice is the Sun's southernmost reach. Again, because the Earth's axis is tilted 23.3°, this is how much the Sun moves above and below the celestial equator.
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