A solar eclipse can only occur when the moon passes directly in between the sun a Earth, and is perfectly aligned. It is a very brief occurance, and lasts for only minutes. Sometimes, however the eclipse of the sun is not always a full ecplise. The Moon may be too small to cover the the sun entirely. As the Moon orbits Earth, it's distance varies. The Moon's size appears to change. When the Moon is on the near side of its orbit, the Moon appears larger than the Sun. When an eclipse occurs at this time, it will be a total eclipse. If an eclipse occurs while the Moon is on the far side of its orbit, the Moon appears smaller than the Sun and results in a partial eclipse. The moons angle might be a few degrees off, or not completely aligned. Total eclipses are rare. The last total eclipse was seen in July of 1991 in Hawaii and Mexico, the next total ecplise is to occur on August 21, 2017.