RA stands for Right Ascension, which is a celestial coordinate used in astronomy to pinpoint the location of a celestial object in the sky. DEC stands for Declination, which is another celestial coordinate used to measure the distance of a celestial object from the celestial equator.
Star visibility can be calculated by taking into account the RA and DEC of the star, as well as the observer's location and the time of observation. This information can be used to determine the altitude and azimuth of the star, which indicates its position in the sky relative to the observer.
The date and time of observation are important because the positions of celestial objects in the sky are constantly changing due to the Earth's rotation and its orbit around the Sun. May 21st at 09:30 is a specific date and time chosen for observation, but the calculations can be applied to any date and time.
There are various tools and equations that can be used to calculate star visibility, such as a star chart, a star atlas, or online calculators. The equations used depend on the method of calculation, such as using celestial coordinates or the altitude-azimuth system.
The accuracy of the calculations for star visibility depends on the precision of the input data and the method of calculation used. With accurate and up-to-date data, the calculations can be very precise. However, factors such as atmospheric conditions and light pollution can also affect the actual visibility of stars in the sky.