What does Aries and the bright Bull signify in "Spring" by James Thomson?

Thomson is describing the arrival of spring emerging out of the long winter months. The above line from the poem details the transition from March to April, which is usually heralded as the start of spring. In the zodiac, Aries is the star sign for those born between March 21 and April 19. It is symbolized by the ram. The "bright Bull" refers to Taurus, which in the zodiac is the astrological sign that follows Aries. In Western astrology, the sun is in the sign of Taurus from April 20 until May 21, so we can see why it is associated in the poem with the arrival of spring sunshine. Just as Aries is symbolized by the ram, Taurus is symbolized by the bull. Thomson's reference to a "bright Bull" relates not just to sunshine, but also to the dawn of new life one associates with the arrival of spring.
https://en.wikisource.org/wiki/Spring_(Thomson)