This is a light or discoloration or it might take any form seen with eyes closed. In time it is visible with eyes open. It is VITAL that one not at it or for it. It will come. One cannot force it or yearn for it or grasp at it without sending it away.
Attention remains near the nostrils, the gate the breath enters and exits. Light may come, but one does not move from this spot. Proceeding in this way, the sign will come under the nose and become what one is looking at.
The mind creates for itself a symbol or mental manifestation of the breath made of light through the excitation of subatomic particles (known in Buddhism as kalapas). Everyone's sign is different. What, why, and how are not important. It is a remarkable achievement that is easily disturbed.
It is maintained or resuscitated by consistent application of the mind to the meditation object -- which always remains the breath: The sign takes over if and only when it becomes the breath. If one wishes, when the counterpart sign is strong, one may merge with it, be "absorbed" by it, and this is the first jhana (zen, ch'an, dhyana).
The first four absorptions are what the Buddha defined as "right concentration" in the Noble Eightfold Path. The powers of serenity, concentration (mental purification), and focus one gains makes insight (vipassana) practice very fruitful very quickly. One is ready to establish the Four Foundations of Mindfulness that lead to enlightenment.
The Moon moves in lunar months, in phases of roughly 29.5 days. For example, a New Moon that appears on June 1 will range through a waxing Quarter Moon on June 9, Full Moon on June 15, waning Quarter Moon June 23, and return to New Moon by July 1.
In mythology, many references to the Full Moon may actually refer to a nimitta or meditation sign. For it is an interior vision of a moon-like sphere emerging from clouds. Clouds can refer to several such spheres, together forming a pearl like cluster. Perhaps this is what is meant by heaven's "pearly gate."
This image will at times appear with a flat base, as if it were appearing over a dark wall. At this time, it looks like a rising or Quarter Moon.
From the Tibetan Shangpa tradition of the Wisdom Dakini Niguma: "In particular, her special instructions included the Great Empowerment of Illusory Body, which she bestowed to him on the night of the fifteenth day of the fourth lunar month, by the light of the full moon."
Of the Life of Saint Brigit
"In the eighth day of the lunar month was she born. On the eighteenth did she take the veil on her head. On the twenty-eighth did she go to heaven. Together with the eight virgins was Brigit consecrated."
This nimitta gently arises [apparently] from nothing as a pulsating wisp, slowly building to its full "Moon-in-the-clouds" appearance on the back of subtle back and forth movements of spirit [breath and luminous life energy] within the meditator.
The eighth day for St. Brigit perhaps points to the rising Moon appearance, and the 18th of the month is presumably just after the sign is well established. One of the orbs will expand to form an organic tunnel, bright at the point opposite the viewer.
Also of interest is the widespread Western myth of werewolves, shape shifting human beings who transform into wild beasts when the Moon is full. This suggests that the appearance of the sign is not wholly dependent on the meditator being in a fully relaxed state of serenity, as is suggested by Theravada Buddhist meditation practices.
Alan Watts explains the Zen of Breathing, which is best left to its own natural flow and pattern uninterfered with during zazen (just sitting).