$A$ celestial object is observed from two points $A$ and $B$ on the Earth,which are diametrically opposite. The angle between the two observation directions is $2.9 \times 10^{-4} \text{ rad}$. Taking the diameter of the Earth to be $1.28 \times 10^4 \text{ km}$,find the distance between the Earth and the object.

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: $1'' = 4.85 \times 10^{-6} \ rad$)

Which method is used in the modern era to measure the distance of a nearby planet from the Earth?

What is the typical order of magnitude of the distance between two atoms in solids?

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.