Rotating Sky
Rotating Sky Lab introduces the horizon coordinate system and the “apparent” rotation of the sky. The relationship between the horizon and celestial equatorial coordinate systems is explicitly explored.
You will need to follow the following links: (note-these use flash--follow instructions on your page to download if on a chromebook)
Note-READ the background info FIRST
Background information:
Two Systems – Celestial, Horizon
Simulation
Rotating Sky Explorer (swf) This is for your simulation activities
Background Information
The Observer
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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.
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).
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.
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
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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:
- Altitude of North Celestial Pole = latitude of observer
- Altitude of Celestial Equator = (±)90° - latitude of observer
- 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.
Horizon Coordinate System
Rotating Sky
