A star in northern sphere is said to transit A. When its altitude is maximum B. When its azimuth is 180° C. When it is in south D. All the above

The correct answer is: A. When its altitude is maximum.

A star in the northern hemisphere is said to transit when it crosses the observer’s meridian, which is the line that runs from north to south through the observer’s zenith. This happens when the star’s altitude is maximum, which is when it is at its highest point in the sky.

The azimuth of a star is the angle between the north celestial pole and the star, measured eastward from the north point of the horizon. The azimuth of a star is always changing, as the Earth rotates on its axis. A star’s azimuth is 180° when it is in the south, but this does not mean that it is transiting. A star can be in the south at any time of day, and it will only transit when it crosses the observer’s meridian.

The option “When it is in south” is incorrect because a star can be in the south at any time of day, and it will only transit when it crosses the observer’s meridian.