what changes the strength of gravity
The precise strength of Earth's gravity varies depending on location. The nominal average value at the Earth's surface, known as standard gravity is, by definition, 9.80665 m/s 2 (about 32.1740 ft/s 2).
The gravitational effects of the Moon and the Sun (also the cause of the tides) have a very small effect on the apparent strength of Earth's gravity, depending on their relative positions; typical variations are 2 Âµm/s 2 (0.2 mGal) over the course of a day.