Which of the following is the most precise device for measuring length?

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

$(a)$ The earth-moon distance is about $60$ earth radii. What will be the angular diameter of the earth (approximately in degrees) as seen from the moon?
$(b)$ The moon is seen to have an angular diameter of $(1/2)^{\circ}$ from the earth. What is its relative size compared to the earth?
$(c)$ From parallax measurement,the sun is found to be at a distance of about $400$ times the earth-moon distance. Estimate the ratio of sun-earth diameters.

The angular diameter of the Sun is measured to be $1920''$. If the distance of the Sun from the Earth is $1.496 \times 10^{11} \ m$,calculate the diameter of the Sun. (Given: $1'' = 4.85 \times 10^{-6} \ rad$)

$A$ tachometer is a device used to measure:

When the planet Jupiter is at a distance of $824.7 \; \text{million km}$ from the Earth, its angular diameter is measured to be $35.72''$ of arc. Calculate the diameter of Jupiter.