$A$ 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.

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

$(a)$ How many astronomical units $(AU)$ make $1$ parsec?
$(b)$ Consider a sun-like star at a distance of $2$ parsecs. When it is seen through a telescope with $100$ magnification,what should be the angular size of the star? The Sun appears to be $(\frac{1}{2})^{\circ}$ from the Earth. Due to atmospheric fluctuations,the eye cannot resolve objects smaller than $1$ arc minute.
$(c)$ Mars has approximately half of the Earth's diameter. When it is closest to the Earth,it is at about $\frac{1}{2} AU$ from the Earth. Calculate what size it will appear when seen through the same telescope.

What is the approximate dimension (thickness) of a molecule of oleic acid?

What is mass? Write the effect of external factors on mass.

If the angular diameter of the Sun is about $30'$ and its distance from the Earth is $1.5 \times 10^{11} \, m$,then the solar diameter is: