When does the Sun have declination 0 degree today but will have a negative declination tomorrow?

Answer 1

Declination is the angular measurement of a celestial body north or south of the "celestial equator". You can think of it as the equivalent of "celestial latitude".

The "fixed stars" don't have any change in declination. (Well, they _DO_, but only very slightly and VERY slowly.) The Sun, Moon, and planets all have their own paths within the solar system, so their declinations will change on a day-to-day basis.

The Sun is at a declination of zero precisely at the time of the equinox. In September, the Sun's declination is decreasing, and it goes negative immediately after the moment of the equinox. This is generally on September 21, but this can vary a day either way depending on the cycle of leap years.