$A$ tachometer is a device used to measure:

What is called a basis? What is parallax? Define angular diameter.

$1'' = \dots \text{rad}$

$A$ far-off planet is estimated to be at a distance $D$ from the Earth. If its diametrically opposite extremes subtend an angle $\theta$ at an observatory situated on the Earth,the approximate diameter of the planet is .............

Explain the method for the measurement of the diameter of a planet or star.

Define astronomical unit,light year,and parsec.