"And then it started like a guilty thing
Upon a fearful summons. I have heard
The cock, that is the trumpet to the morn,
Doth with his lofty and shrill-sounding throat
Awake the god of day, and, at his warning,
Whether in sea or fire, in earth or air,
Th' extravagant and erring spirit hies
To his confine, and of the truth herein
This present object made probation."
The sound of the rooster means that the ghost cannot appear during the daylight. The ghost only appears at night. It is because the noise of the "cock" dispels evil since Marcellus said afterwards, "No fairy takes; nor witch hath power to charm, so hallowed and so gracious, is that time" (L163). This event led the group to ask Hamlet to speak with it if it is the true ghost of Hamlet's father.