Kepler’s second law states that, Areal velocity of a planet is constant. It is a consequence of conservation of

$$\begin{gathered} \text{ Areal velocity }\frac{\mathrm{dA}}{\mathrm{dt}}\text{ \& angular momentum }\mathrm{L}\text{ of a planet are related by equation } \\ \frac{\mathrm{dA}}{\mathrm{dt}}=\frac{\mathrm{L}}{2\mathrm{M}}\text{, where }\mathrm{M}\text{ is the mass of planet. Since in planetary motion }\overline{\mathrm{L}}\text{ is } \\ \text{ constant }\left({\overline{\tau }}_{\text{ext. }}=0\right)\text{, hence }\frac{\mathrm{dA}}{\mathrm{dt}}\text{ is also constant. } \end{gathered}$$