This is a lunar eclipse, not an eclipse of the Sun. In this case the answer is
the Full Moon.
It must be Full Moon.
A lunar eclipse is possible only at the time of Full Moon. A solar eclipse is possible only at the time of New Moon.
The Moon's phase is New Moon.
A Lunar eclipse occurs only when the moon is in its full moon phase.
It must occur at new moon when the moon passes exactly in fron of the sun as seen from a place on earth. Usually at new moon the moon is not exactly in line and it passes above or below the sun so there is no eclipse.
During a lunar eclipse, the moon is "full". During a solar eclipse the moon is "new".
On a New Moon PhaseOn a New Moon Phase
Lunar eclipses can only occur during the full moon. In fact, the moon must be almost perfectly full before there can be a lunar eclipse.
Solar Eclipse
It must be New Moon but also the Moon must be at one of its nodes, i.e. crossing the ecliptic, because its orbit is tilted relative to the ecliptic, which is the plane of the Earth's orbit. Usually at New Moon the Moon passes above or below the Sun and there is no eclipse.
Only during a full moon.
full moon phase