Without a doubt the most poorly understood part of astronomy in my opinion
is the workings of the celestial sphere. Even as a passing fancy, people
like to see galaxies and planets yet have not a clue to why the "heavens"
work as it does. Questions like: why don't we have a lunar eclipse every month?
or why doesn't Mercury ever get very high in the sky and what makes Mars
seem to "backup" in the sky? come up quite a bit at public star parties. In
this section I will try to put into plain english concepts that explain the
various parts of the celestial sphere. Sometimes it is tough or nearly impossible
to find an answer in one place for certain questions. It usually requires
a number of books to locate that one answer you are looking for. I will occasionally
cite a reference that I used to put this together. One book may explain
a concept better than another. I am trying to bring the best of them into
one place for you.
You have to learn to crawl before you can walk!
I suppose the best place to start is by defining the parts that make up
the celestial sphere. The celestial sphere is laid out much as the Earth's
coordinate system is. For those who do not know what a coordinate system is,
basically it is a method of locating loctaions or objects by a numeric system.
The most basic layout would be "X" as the horizontal or East/West numbers
and the "Y" being the vertical or North/South numbers. A coordinate system
is used for locating a point or object in the sky. The very basic of
layouts are shown below. Earth's system of coordinates are latitude(which
is the "Y" axis) and longitude(which is the "X" axis). Celestial coordinates
are "right ascension(the "X" axis) and
declination(the "Y" axis).
The term "RA or Right Ascension" is used to define the east/west location. It's notation is in hours, minutes and seconds. Example: NGC 2683 is at coordinate: RA 08h 52m, DEC +33d 25m This system is very much like the Earth's latitude and longitude system. RA(right ascension) is equivalent to longitude. 0 degrees RA is located at the point in the sky that the Sun crosses the celestial equator . The celestial equator is on the same plane as the Earth's equator. Imagine for a moment that you lived in South America,(on the eqautor) if you looked straight overhead 90 degrees from your horizon this would the celestial equator. You are standing at a latitude of 0 degrees and the celestial equator is at 90 degrees or straight overhead. ). So at the time of the spring equinox, the Sun would rise due East and set due West, because it follows the line of the " ecliptic". As of this time the Sun will be in the constellation Pisces at the spring equinox which is March 21. Gradually over the next few months the Sun will be on the other side of this imaginary line. The term "declination is used to define a north/south location, meaning a certain amount of degrees above or below the celestial equator. It's notation is in degrees, minutes and seconds. It is just like the Earth's latitude system. Note that declination uses a + or - to determine whether the object location is either north or south of the celestial equator. Our example above shows that the declination is +33degrees 25 minutes. This tells us that the object lies north of the celestial equator, if it were a (-) it would be south of the celestial equator.
Ok imagine now that you are at the north pole. Besides freezing your tushy off and why would you do that to understand this? A little trick to find the altitude of the celestial equator is this: 90 degrees minus your latitude. Example: At the north pole my latitude is 90 degrees and I subtract that from 90 degrees which gives me 0 degrees altitude. So the celestial equator will be right on the horizon at the north pole. Altitude is defined as the distance in degrees the location or object is above the horizon. Now at the spring equinox the Sun will be halfway above the horizon line. I think a few pictures would help you understand this better. Now if you measured 90 degrees above your horizon (at the north pole) guess what star you will see? Polaris, in Ursa Minor. The is the " north celestial pole". The sky will rotate around this point during the night. If you can stay up and watch you will see this in action. The axis of rotation of the celestial sphere is the same as the axis of rotation of the Earth.
Now let's throw a kink in the works. Unfortunately we don't live in an ideal solar system so the Earth's equator is not on the same plane as the other planets. This is getting weird huh? The Earth's pole is tilted 23.5 degrees from being exactly perpendicular to the plane of the solar system or the "ecliptic" So, because of this tilt of the pole, the position of the Sun will be anywhere from 23.5 degrees above to 23.5 degrees below the celestial equator during the course of the year. At either of these high and low points it will be the solstice. On December 21st or 22nd the Sun will be at the lowest point in the sky at solar noon. At this time the Sun is now crossing back over the "ecliptic" to journey south of this imaginary line. This is the Winter Solstice . On June 21st or 22nd the Sun is at the highest point in the sky at solar noon. This is the Summer Solstice. When the Sun is above the ecliptic for us north of the Earth's eqautor it will be summer, when it is south of the ecliptic it will be winter. The opposite is true for folks south of the Earth's equator. If you want to try a cool experiment using the Sun, go to the solar noon experiment page.
We have covered a few elements of the celestial sphere. No sense really
confusing you with nuation, precession and such.
We need to keep in mind an important fact. We are
trying to understand all of this motion stuff from a moving platform called
Earth. It is nice to be able to imagine Earth sitting still whilst everything
else moves around us. Unfortunately we can't fully understand this stuff
until we understand that our own motion plays an important role as well.
Question: Why does the Moon always show the same face towards the Earth? Laugh as you may at this, you would be amazed at the number of grown adults whom have no clue to the answer. Can you answer the question? Does the Moon rotate on it's axis or not? To the more science savey of us the answer is obvious. Yes, the Moon does rotate. It rotation period is equal to it's orbital period. What is he talking about you say. Well, we know from centuries of observations of the Moon that the orbital period around the Earth from one full moon to the next full moon is 29 1/2 days. This is called a synodic month.Now with this figure in mind, it takes the Moon one synodic month to complete one rotation on it's axis or the lunar day is 29 1/2 Earth days long. So there is your answer to that nagging question of why does the Moon show the same side towards the Earth