The Moon's orbit around Earth is about 5 degrees off from the Earth-sun orbital plane. That means that everything is rotating on slightly different angles, making exact alignments very rare. However, at special times during the year, the Earth, Moon, and sun do in fact exactly "line up". When the Moon blocks the sun or a part of it, it is called a solar eclipse, and it can only happen during the new Moon phase. When the Earth casts a shadow on the Moon, it is called a lunar eclipse, and can only happen during the full Moon phase. Roughly 4 to 7 eclipses happen in any given year, but most of them are minor or "partial" eclipses. Major lunar or solar eclipses are relatively uncommon.