It is a well-known fact that during a total solar eclipse,the disk of the Moon almost completely covers the disk of the Sun. From this fact and the information given below,determine the approximate diameter of the Moon.
Distance of the Moon from the Earth $= 3.84 \times 10^{8} \; m$
Distance of the Sun from the Earth $= 1.496 \times 10^{11} \; m$
Diameter of the Sun $= 1.39 \times 10^{9} \; m$

The distance of the Sun from Earth is $1.5 \times 10^{11} \,m$ and its angular diameter is $2000 \,s$ when observed from the Earth. The diameter of the Sun will be ...........

The angle subtended by a plate of diameter $2 \, cm$ at a distance of $1000 \, cm$ from the eye is ..... (in $^\circ$)

In an experiment to estimate the size of a molecule of oleic acid,$1\, mL$ of oleic acid is dissolved in $19\, mL$ of alcohol. Then $1\, mL$ of this solution is diluted to $20\, mL$ by adding alcohol. Now,$1$ drop of this diluted solution is placed on water in a shallow trough. The solution spreads over the surface of water,forming a one-molecule-thick layer. Lycopodium powder is sprinkled evenly over the film,and its diameter is measured. Knowing the volume of the drop and the area of the film,we can calculate the thickness of the film,which gives us the size of the oleic acid molecule.
Read the passage carefully and answer the following questions.
$(a)$ Why do we dissolve oleic acid in alcohol?
$(b)$ What is the role of lycopodium powder?
$(c)$ What would be the volume of oleic acid in each $mL$ of the solution prepared?
$(d)$ How will you calculate the volume of $n$ drops of this solution of oleic acid?
$(e)$ What will be the volume of oleic acid in one drop of this solution?

The angle of $1^{\prime}$ (minute of arc) in radian is nearly equal to

During a total solar eclipse,the moon almost entirely covers the sphere of the sun. Write the relation between the distances and sizes of the sun and moon.