The moon reaches its perigee -- that is, the point in its orbit when it is closest to earth's center -- at 6:00 this morning. Six and a half hours later, at 12:31 this afternoon, the moon enters its full phase. Given those two pieces of information, can you name what astronomical event begins today? Here's a hint: think seasons.

Don't be thrown off by the fact that today is the first day of winter (at least for those of us in the northern hemisphere); today is also the start of the perigee spring tides. The word perigee comes from perigee (the Greek ancestors of that word meant "near" plus "earth"), but what's the story behind spring tides?

Regardless of the season, spring tides occur twice every 29 and half days. Spring tide names a tide of greater-than-average range that comes at the time of the new moon or full moon. Spring tides contrast with neap tides, tides of lower-than-average range that roll in during the first and the third quarters of the moon.

The power of the spring tides results from the fact that they occur when the Earth, Moon, and Sun are at or near sizing -- that is, aligned with each other. That allows the gravitational pull of the Sun and Moon to reinforce each other, resulting in higher tides. But perigee spring tides are fairly rare; that term is applied only to the high tides that occur when the new or full moon takes place within twenty-four hours of the time the Moon reaches perigee.