How to deal with the celestial meridian?

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In summary, the conversation discusses the measurement of azimuth in the horizontal coordinate system and the position of the vernal equinox relative to an observer's celestial meridian. It mentions that while azimuth is usually measured from the northernmost point, the position of the vernal equinox is given relative to the southern part of the meridian. The website provided also supports this idea. It is mentioned that there may be two systems for measurement, Right Ascension and Greenwich Hour Angle, with the latter being used by navigators.
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Heatherfield
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Hi there,

The book “Foundations of Astrophysics” by Ryden and Peterson states that one measures azimuth in the horizontal coordinate system eastwards starting on the northernmost point of the horizon circle. Thus, east becomes 90 degrees, et cetera. Wikipedia and other sites back this up.

However, it seems as if the position of the vernal equinox is always given relative to the southern part of an observer’s celestial meridian. During the vernal equinox, the sun culminates in the south, setting the siderial time to 0h0min. Knowing the difference between the local siderial time and a star’s right ascension gives you the current position of a star, relative to the southern half of the celestial meridian.

This website describes this too: http://www.polaris.iastate.edu/NorthStar/Unit4/unit4_sub2.htm

Did I miss something? We so explicitly defined azimuth as being relative to the most northern point on the meridian. Now we’re doing all our measurments relative to the south. I’m not complaining: stars culminate in the south, so this makes it easier. But where did we switch? And when must I switch? Did I make a mistake somewhere?
 
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  • #2
Heatherfield said:
Hi there,

The book “Foundations of Astrophysics” by Ryden and Peterson states that one measures azimuth in the horizontal coordinate system eastwards starting on the northernmost point of the horizon circle. Thus, east becomes 90 degrees, et cetera. Wikipedia and other sites back this up.

However, it seems as if the position of the vernal equinox is always given relative to the southern part of an observer’s celestial meridian. During the vernal equinox, the sun culminates in the south, setting the siderial time to 0h0min. Knowing the difference between the local siderial time and a star’s right ascension gives you the current position of a star, relative to the southern half of the celestial meridian.

This website describes this too: http://www.polaris.iastate.edu/NorthStar/Unit4/unit4_sub2.htm

Did I miss something? We so explicitly defined azimuth as being relative to the most northern point on the meridian. Now we’re doing all our measurments relative to the south. I’m not complaining: stars culminate in the south, so this makes it easier. But where did we switch? And when must I switch? Did I make a mistake somewhere?
I believe there are two systems. Right Ascension (used by astronomers) and Greenwich Hour Angle (used by navigators). Maybe someone else can confirm.
 

1. How can I determine the celestial meridian?

The celestial meridian is an imaginary line that runs from the north celestial pole to the south celestial pole, passing through the zenith (the point directly above an observer). To determine the celestial meridian, you can use a star chart or a planetarium program that shows the position of stars and planets in the sky at a specific time and location.

2. How does the celestial meridian affect celestial navigation?

The celestial meridian is an important reference line in celestial navigation. It is used to determine the altitude and azimuth of celestial bodies, which are then used to calculate a ship's position. By knowing the position of the celestial meridian, sailors can determine the direction of true north and plot their course accordingly.

3. What is the significance of the celestial meridian in astronomy?

In astronomy, the celestial meridian is used to define the coordinate system for locating objects in the sky. The celestial equator (the projection of Earth's equator onto the celestial sphere) intersects with the celestial meridian, allowing astronomers to measure the declination (angular distance north or south of the celestial equator) of celestial objects.

4. How does the celestial meridian change throughout the night?

As the Earth rotates on its axis, the celestial meridian appears to move across the sky. At any given moment, the celestial meridian is directly above the observer's location, but as the Earth rotates, it will pass over different points on the horizon throughout the night.

5. Can the celestial meridian be seen with the naked eye?

No, the celestial meridian is an imaginary line and cannot be seen with the naked eye. However, by knowing the position of the celestial meridian, you can use it as a reference point to locate celestial objects in the sky.

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