The Year's Awakening

How do you know that the pilgrim track Along the belting zodiac Swept by the sun in his seeming rounds Is traced by now to the Fishes' bounds And into the Ram, when weeks of cloud Have wrapt the sky in a clammy shroud, And never as yet a tinct of spring Has shown in the Earth's apparelling; O vespering bird, how do you know, How do you know? How do you know, deep underground, Hid in your bed from sight and sound, Without a turn in temperature, With weather life can scarce endure, That light has won a fraction's strength, And day put on some moments' length, Whereof in merest rote will come, Weeks hence, mild airs that do not numb; O crocus root, how do you know, How do you know?