The meridian is the north-south line of the observer in the horizon coordinate system. It is the “highest” point an object will be in the sky. As such, it is a convenient reference point for such things as the passage of time.
Unlike altitude in general, meridional altitude can be calculated fairly straightforwardly one the observer's latitude and a declination is known. The steps below outline the basic procedure for calculating meridional altitude using a horizon diagram picture.
- Draw and label the poles. Label the altitude arc of the pole above the horizon.
The north celestial pole points directly north and the south celestial pole points directly south. The altitude of the poles is equal to the latitude of the observer. For northern hemisphere observers, the north celestial pole will be above the horizon and the south celestial below the horizon and vice versa for southern hemisphere observers.
- Draw and label the celestial equator. Label its altitude arc above the horizon.
The celestial equator is 90° from either pole. The altitude of the celestial equator will simply by 90° − the altitude of the pole above the horizon.
- Draw the object whose meridional altitude is being measured. Label its declination arc.
Declination is measured from the celestial equator. Positive declination is always towards the north celestial pole and negative declination is always towards the south celestial pole irrespective of the observer's latitude.
- Calculate meridional altitude: altitude of celestial equator + declination of star
If the meridional altitude is above 90° one must subtract the number from 180° as altitude is only defined up to 90°. That is an altitude of 100° is really 80°
The default setting on the meridional calculator to the left is observer latitude 40° N and a star with declination of +15°. One can drag the horizon sphere so that the horizon plane is merely a line or click the set side view button to appear more like the meridional horizon diagram one might use on paper.
The observer's latitude is 40° N. Click step 1 - poles to draw in the poles and set the altitude of the north celestial pole (observer being in the northern hemisphere) to 40°. The celestial equator is 90° from the pole. Click step 2 - CE to draw in the celestial equator and show the angle of 90° - 40° = 50°. The star's declination is +15° and so is 15° towards the north celestial pole. Click step 3 - declination to draw in this angle. The meridional altitude is now 50°+15° = 65°. Click step 4 - calculate to show this value.
Use the simulator to calculate the meridional altitude of a star with declination -20° for an observer at 60° S. (Tip: One can click on the latitude and declination slider's bar itself instead of the grabber to change the value by .1°)
Use the simulator to calculate the meridional altitude of a star with declination 80° for an observer at 70° N.