Importance
Fixing a telescope to move only in the meridian has advantages in the high-precision work for which these instruments are employed:
- The very simple mounting is easier to manufacture and maintain to a high precision.
- At most locations on the Earth, the meridian is the only plane in which celestial coordinates can be indexed directly with such a simple mounting; the equatorial coordinate system aligns naturally with the meridian at all times. Revolving the telescope about its axis moves it directly in declination, and objects move through its field of view in right ascension.
- All objects in the sky are subject to the distortion of atmospheric refraction, which tends to make objects appear slightly higher in the sky than they actually are. At the meridian, this distortion is in declination only, and is easily accounted for; elsewhere in the sky, refraction causes a complex distortion in coordinates which is more difficult to reduce. Such complex analysis is not conducive to high precision.
Read more about this topic: Meridian Circle
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