Background Information

The Horizon Coordinate System

The Observer

The Horizon Plane: The Horizon Coordinate system is defined with respect to an individual observer standing on the earth. The horizon in astronomy is not quite the same as the horizon we see on earth. It is idealized to be free from geographical effects such as mountains, valleys, or even the bending of the earth. It is a flat plane tangent to the earth's surface where the observer is standing. Consequently, an observer can only see half of the celestial sphere at any given moment.
There are two defining points/regions for the horizon coordinate system. The horizon plane is one where direction are given with respect to the cardinal points: north, east, south, and west. The other point is straight up and is called the zenith.
The astronomical horizon is perfectly flat.
Click and drag star to change its coordinates.
Azimuth: Azimuth is the coordinate defining directions parallel to the horizon (red in figure to the right). Azimuth goes from 0 to 360° starting with north = 0° and increasing towards the east. That is, east is azimuth 90°, south is azimuth 180°, and west azimuth 270°. Stars with the same azimuth lie on an arc from the zenith through the object down to the horizon (meeting perpendicularly) called a vertical circle (though technically it's only half a circle).
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Because the earth rotates around an axis aligned with the earth's poles, the sky moves east and west, but not north and south. The observer's meridian arc going from from the north point of the horizon, up through the zenith, and down to the south point of the horizon. The meridian is an important reference as it is the place where an object in the sky (like the stars, sun, or moon) will be “highest” in the sky.
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Altitude: Altitude is the coordinate defining directions above or below the horizon plane (blue in figure to the right) – how high an object is in the sky. It measures the position of a star on a particular vertical circle. The horizon is altitude = 0°. Straight up – the zenith point – is altitude 90° and straight down, below the horizon is the nadir at altitude -90°

Two Systems: Celestial, Horizon

​Advantages and Disadvantages: The horizon coordinate system has the advantage of being orientated towards the sky the observer actually sees. It has the disadvantage of being different for each observer and the location of objects in it change over time. The celestial equatorial system has the advantage of being the same for each observer and the location of stars in it change very little over time. It has the disadvantage of not being naturally oriented towards the observer's sky. Because of these advantages and disadvantages, both systems are frequently used. Depending on the situation it is better to use one or the other.

Converting: The animation to the right shows how the horizon system relates to the celestial equatorial system for an observer. Conversion between the two systems obey a few simple principles:
  1. Altitude of North Celestial Pole = latitude of observer
  2. Altitude of Celestial Equator = (±)90° - latitude of observer
  3. Azimuth of North = 0°
Where the plus is used for the northern hemisphere and minus for the southern hemisphere.
While these principles are straightforward to employ, converting an actual celestial equatorial coordinate to a horizon coordinate is a bit more tricky. Because the sky is rotating, one needs to know both longitude and time to convert coordinates. As such, it is not covered in this introductory section.

Rotating Stars

Paths of the Stars
Rotation of the Sky: Because the earth is rotating the sky appears to rotate. Viewed from above the north pole, the earth is rotating counter-clockwise. For an observer on the earth, objects move from east to west (this is true for both northern and southern hemispheres). More accurately put, when looking north, objects in the sky move counter-clockwise.

Though all objects rotate in the sky, the observed path stars make in the sky depend on the observer's latitude. Some are always in the observer's sky, some some of the time, and others are never observable. These different ways of classifying stars are discussed below.

Rise and Set Stars: During the rotation of the earth, some stars rise from below the eastern horizon and later set below the western horizon. Appropriately enough, these stars are called rise and set stars. They are indicated by the yellow star trails in the animation to the left.
The angle rise and set stars (including the sun) make with the horizon as they rise is the same for all rise and set stars for that observer. Specifically, the angle is 90° - |observer's latitude|. They make this same angle in west. In the northern hemisphere the angle is tilted towards the south and in the southern hemisphere the angle is tilted towards the north.
Different kinds of star paths.
Circumpolar & Never-Rise Stars: Stars near the celestial poles make small circles and may not pass the horizon plane. If they are always above the horizon they are called circumpolar stars. If they are always below the horizon they are never rise stars. Circumpolar stars for the northern hemisphere are never-rise stars for the southern hemisphere and vice versa. These stars are indicated by blue (northern hemisphere stars) and red (southern hemisphere stars) in the figure to the left.

Bands in the Sky

Object Location in the Sky:The celestial poles and the celestial equator can be used as reference points for identifying the locations of other objects of interest in the sky – like the sun, moon, and visible planets. It is often instructive to crudely describe where these objects can be located in the sky for various latitudes on the earth with “bands” of declination.

The ecliptic is the plane of the earth's orbit about the sun. Because the earth is tilted (its obliquity) about 23.5° to this orbital plane, the sun's declination is always between +23.5° on the summer soltice and -23.5° on the winter solstice. Thus one can picture a band 47° wide centered on the celestial equator. The sun is always in this band.
The orbits of other objects in the solar system are described with relation to the ecliptic. The moon's orbit is inclined by about 5° to the earth-sun orbital plane. Thus the moon is located in a larger region of the sky.
The orbital inclinations of the brighter planets are small, for example Mars' orbital inclination is about 2°. Because orbital inclination is measured from the center of the sun and declination is measured from the center of the earth, Mars can be as much as 6° off of the ecliptic. The only planet that can be very far from the ecliptic is Pluto but it can't be observed without a good-sized telescope. Thus, the location bands for the bright planets is very similar to that of the moon and the two are shown as one 60° band in the Skyband Simulator.
Now click Here to open the explorer that you will be using for the next couple days.  Here are the instructions on how to use it.  Go through step by step so you can figure out how it works.  Then just play around with the virtual lab explorer to familiarize yourself with it.
Here and Here and Here are videos which will explain how to use the lab.  Please watch them to help you learn how to use the rotating sky explorer.
(you will be so completely lost if you don't learn how to use this interactive explorer tool)