From our point of view, we perceive the Earth to be the center of the universe—the core around which the heavens rotate. This Earth-centered view was what almost everyone believed until a few centuries ago. This seemingly self-evident perspective is the basis for all the cultural, religious and philosophical thoughts of humanity for a few millennia now. However, the geocentric view happens to be wrong. More on that later. But since we will be learning and observing the universe as we see on earth, this earth-centric reference point works well ( as long as we remember that it is the earth’s rotation that makes the sky “move”).
The Celestial Sphere
Gazing up, you get the impression that the sky is a great hollow dome with you at the center , and all the stars are an equal distance from you on the surface of the dome. The top of that dome, the point directly above your head, is called the zenith, and where the dome meets Earth is called the horizon.
If you observe the night sky for hours, you will see stars rising on the eastern horizon (just as the Sun and Moon do), moving across the dome of the sky in the course of the night, and setting on the western horizon. Watching the sky turn like this night after night, you might eventually get the idea that the dome of the sky is really part of a great sphere that is turning around you, bringing different stars into view as it turns. The early astronomers regarded the sky as just such a celestial sphere. Some thought of it as an actual sphere of transparent crystalline material, with the stars embedded in it like tiny jewels.
Celestial Poles and Celestial Equator
To help orient us in the turning sky, astronomers use a system that extends Earth’s axis points into the sky. Imagine a line going through Earth, connecting the North and South Poles. This is Earth’s axis, and Earth rotates about this line. If we extend this imaginary line outward from Earth, the points where this line intersects the celestial sphere are called the north celestial pole and the south celestial pole. As Earth rotates about its axis,
the sky appears to turn in the opposite direction around those celestial poles . We also (in our imagination) throw Earth’s equator onto the sky and call this the celestial equator. It lies halfway between the celestial poles, just as Earth’s equator lies halfway between our planet’s poles. Similar to the latitudes and longitudes on earth, coordinates are assigned to the celestial sphere as well.
Circles on the Celestial Sphere. Here we show the (imaginary) celestial sphere around Earth, on which objects are fixed, and which rotates around Earth on an axis. In reality, it is Earth that turns around this axis, creating the illusion that the sky revolves around us. Note that Earth in this picture has been tilted so that your location is at the top and the North Pole is where the N is. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow.
If you were standing at the Earth’s north pole, the north celestial pole would lie at the zenith, the imaginary point directly over your head. The star Polaris would lie almost directly at this point. It’s the same story for the south… the south celestial pole (SCP) is directly above the Earth’s south pole.
In the northern hemisphere, a moderately bright star—the North Star, also called Polaris— lies almost exactly at the position of the north celestial pole(NCP). There is, however, no bright star near the SCP, that is, there is no southern counterpart to Polaris.
The celestial poles and equator lie above their terrestrial counterparts
As it is with the poles, so it is with the equator. Directly above the Earth’s equator lies the celestial equator, a circle which goes all the way around the sky and which divides the northern half of the celestial sphere from the southern half (see image above).
If you were standing at the north pole, the celestial equator would coincide with the horizon. And if you were standing on the Earth’s equator, the celestial equator would stretch from the east to the west directly overhead. As seen from the equator, the north and south celestial poles would lie on the northern and southern horizon, respectively.
But how about if you’re standing at some intermediate latitude, between the north pole and the equator?
The horizon, meridian, and cardinal points
In that case, the north celestial pole (NCP) and the north star would lie at some angle above the northern horizon. This angle is equal to your latitude. If you are at the equator, for example, which is 0 degrees latitude, then the NCP (and Polaris) would lie zero degrees above the horizon, that is, on the horizon. At 10 degrees latitude, Polaris would lie 10 degrees above the horizon. And in London, England, which has latitude of 51 degrees, Polaris would lie 51 degrees above the horizon. This is how navigators have determined their latitude for thousands of years… by measuring the angle of Polaris above the horizon.
One more circle… the imaginary great circle that runs from the northern horizon, up through Polaris, through the zenith, then down to the southern horizon is called the meridian (again, see image above).
How the Sky Moves
The North Star, Polaris, lies very near the rotation axis of the celestial sphere, right above the Earth’s north pole. Since it’s almost right on the north celestial pole, Polaris appears to stay nearly fixed in the sky all night and all year, just as the Earth’s north pole stays fixed as the rest of the Earth’s surface moves around it. Any other star on the celestial sphere south of Polaris rotates in circles of increasing diameter about the rotation axis. It’s the same with the south celestial pole. Stars above the Earth’s equator trace out the largest circles around the sky during their daily motion across the celestial sphere as the Earth turns. And south of the equator, stars trace out circles with smaller apparent diameters as they lie closer to the south celestial pole. The image below gives you a better idea of how the stars appear to rotate during the day.
Star trails caused by the apparent rotation of the celestial sphere around a celestial pole (from http://antwrp.gsfc.nasa.gov/apod/ap051220.html)
Like the stars and planets, the Sun also appears to move on the celestial sphere. If you measure the time when the sun is highest in the sky, you will find it takes exactly 24 hours for the Sun to move all the way around the celestial sphere and return to its highest point. In fact, that’s how we define a “day”, or what astronomers formally call a solar day.
How about the stars? If you go out at night and select a star to observe, and measure its position on the celestial sphere, you will find it takes 24 hours to move all the way around the sky and get back to the same spot.
Well, almost 24 hours.
If you measure accurately, you’ll find it takes only 23 hours and 56 minutes for a star to get back to the same position in the sky as it was the night before. That’s because, during the day, the Earth revolved around the Sun by 1/365 of its orbit. So each day, you look in a slightly different direction in space, and this causes every star to appear to rise 4 minutes earlier each night. In two weeks, the star rises about an hour earlier; in one month the star rises 2 hours earlier, and in 12 months, it appears to move all the way around the sky back to the position at which you first saw it the previous year.
This apparent motion where the stars rise a little earlier each night, which is caused by the Earth’s revolution around the Sun, explains why the stars you see in the night sky in each season are different than the stars you saw during the last season.
In case all of this is a little too much to digest in one go, with a little bit of thought and practice it will become clearer as you go ahead on the journey.