The radius of the Martian orbit around the Sun is about $4$ times the radius of the orbit of Mercury. The Martian year is $687$ Earth days. What is the length of $1$ year on Mercury?

$A$ planet revolves around the Sun in an elliptical orbit,where the semi-major axis $a$ is double the semi-minor axis $b$ $(a = 2b)$. The Sun is at the focus. Given that the planet takes $24$ hours to travel through the path $bed$ as shown in the figure,find the time taken by the planet to travel along the path $dab$.

Which of the following orbits is a possible orbit for a planet?

$Assertion$ : For the planets orbiting around the sun,angular speed,linear speed and $K.E.$ change with time,but angular momentum remains constant.
$Reason$ : No torque is acting on the rotating planet. So its angular momentum is constant.

If $A$ is the areal velocity of a planet of mass $M$,its angular momentum is

The kinetic energies of a planet in an elliptical orbit about the Sun,at positions $A, B$ and $C$ are $K_A, K_B$ and $K_C$,respectively. $AC$ is the major axis and $SB$ is perpendicular to $AC$ at the position of the Sun $S$ as shown in the figure. Then