Why does their speed of revolution increase as they get near the sun?

The speed of a celestial body in orbit increases as it approaches the Sun due to the laws of orbital mechanics, specifically Kepler's laws of planetary motion. According to Kepler's second law, a planet sweeps out equal areas in equal times, meaning it must move faster when closer to the Sun to cover the same area in the same time. This increase in speed is a result of the stronger gravitational pull exerted by the Sun at closer distances, which accelerates the object in its orbit.