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In this document we will describe the horizon coordinates
why are and how to measure them.
To begin we will give some definitions necessary for understand certain astronomical concepts.
VERTICAL OF A PLACE o plumbline. What is meant by "vertical"?.
The plumbline is the direction of gravity at that location.
It extends up and down perpendicular to the horizon and cuts the celestial sphere at two points:
The zenith is the point which is above the observer: call it Z...
And the nadir: the point is below the observer: call it N...
How do we determine the direction of gravity?: with the plumb-bob.
HORIZON. The horizon is a great circle of the celestial sphere perpendicular to the vertical.
Recall that we have seen in the documentary "Equatorial coordinates" what is a great circle.
A great circle is the result of dividing a sphere into two equal hemispheres.
In this case the celestial sphere is divided into two hemispheres:
The upper hemisphere or visible hemisphere and lower hemisphere or invisible hemisphere.
LOCAL MERIDIAN
We know that the meridians are great circles which passes through the poles.
Each place has a meridian that passes through the Zenith and the Nadir,
in addition to the poles and is called Local meridian.
In the upper half, which contains the Zenith and is called Upper Meridian
In the bottom half, which contains the Nadir, is known as Lower meridian.
This level, as a great circle that is has the following properties:
1. Divide into two equal parts the horizon, the Ecuador and to the parallels.
2. Is perpendicular to the horizon in that place, to the Ecuador and to the parallels.
3. The stars crosses the upper meridian in each site every 24 hours
This coming from the horizon, rising to the zenith and go down to the western horizon,
where they disappear below the horizon.
The Meridian also is crossed in the lower hemisphere, but we can not see it.
MERIDIAN LINE
The meridian of the place cuts the plane of the horizon a very important line in topography: the MERIDIAN LINE,
also known as North-South line.
Perpendicular to this is the east-west line and by both establish the four cardinal points.
PRIME VERTICAL. Be the celestial sphere and Zenit and Nadir points caused by the intersection with her instead of vertical,
all planes through the vertical ZN called VERTICAL.
The two most important are the local meridian (North-South line) and the Prime Vertical (east-west line).
HORIZONTAL COORDINATES We know what is the horizon and vertical. We also know what are the zenith and nadir.
In the video "Equatorial Coordinates" explained that to define a coordinate system we needed a plane, a point and a sense to measure the angles.
In the horizontal coordinates, the plane is the local horizon.
The point that we as a source in astronomy is the SOUTH.
In sailing is taken as the origin point NORTH.
The sense is the counterclockwise.
A star is defined by two angles:
The azimuth is the arc starting from the point cardinal SOUTH reaches the vertical position of the star.
Remember that all planes passing through the zenith-nadir line are called Vertical.
The altitude, is the arc measured from the horizon to the star.
If the star is in the upper hemisphere or "visible" the altitude "h" is positive.
If the star is in the lower hemisphere or "invisible" "H" will be negative.
The azimuth can vary between 0 degrees and 360 degrees
and the altitude can vary between +90 degrees and -90 degrees.
If the star is just over the horizon, the altitude is 0 degrees and if it is at the zenith, the altitude will be 90 degrees.