A star is said to elongate A. When the star momentarily moves vertically B. When the angle at the star of the spherical triangle is 90° C. When the star D. All the above

The correct answer is A. When the star momentarily moves vertically.

A star is said to elongate when it momentarily moves vertically. This is because the star is actually a point of light, and when it moves vertically, it appears to elongate. This is similar to how a car’s headlights appear to elongate when they are moving towards you.

Option B is incorrect because the angle at the star of the spherical triangle is not always 90°. The angle at the star of the spherical triangle is the angle between the star and the horizon. This angle can be any value between 0° and 180°.

Option C is incorrect because the star does not always move vertically. The star can move in any direction, including horizontally.

Option D is incorrect because only option A is correct.