The fact that it rotates on an axis and rotates around the sun causes the seasons to change over a year. Once every year it orbits around the sun and based on the placing of the Earth it depicts the season.
This isn't really a Biology question but I'll help you anyways. Earths elliptical orbit around the sun causes the planet to be closer and farther from the sun at certain points. Earths tilted axis also changes how much sunlight certain places get.
During the course of a continuous isometric contraction of given strength, the electrical activity progressively increases. This is due to recruitment of motor units taking place to compensate the decrease in force of contraction occurring in the fatigued muscle fibres.
