It's a blog! Mainly of book reviews.
There's a subtle error regarding Newton's Law of Gravitation here. The author states that spherical objects can be treated as point masses, hence since planets are all approximately spherical they can be treated as point masses. The fact is ALL volume masses can be treated as point masses! The only thing special about a sphere is that the centre of mass and the geometrical centre always coincide - for uniform density.
This bodes ill for the much more conceptually difficult material to come.