Time & Tide Wait for No Man

It's spring break in this neck of the woods, and that means that it is time for SCIENCE. "Spring break" in grad school really means "time to work without being interrupted for teaching responsibilities, seminars, lab meetings, etc.," and I've been trying to buckle down on a number of things.

Sadly, the weather here (like many places) has also failed to get the spring break memo. I spent yesterday out collecting snails for an experiment that I'm hoping to run this weekend, as soon as the baby snails are ready to metamorphose. Normally, when I go to my field site, it looks something like this:

Field site at "low tide." Photo credit A. Cahill.

 And here is what it looked like yesterday, from approximately the same vantage point. Same point in the tidal cycle (low tide). You can tell there's a whole lot more water in the picture below, and that made for an unpleasant day in the field.

Field site at "low tide." Photo credit A. Cahill

So what gives? Why is "low tide" so different on two different days? And what do I mean when I say the tides are "good" or "bad" for my fieldwork? It has to do with the phases of the moon.

Tides are caused by the pull of gravity of the sun and the moon on the ocean. Since the ocean is not fixed to the earth, it sloshes around based on these gravitational forces. The moon rotates around the earth on a 4-week cycle. Sometimes it is aligned with the sun and sometimes they are perpendicular to each other.

Spring and neap tides explained. Photo from Wikipedia.

 When the sun and the moon are in a line (the new moon & full moon phases in the diagram above), they pull together on the ocean and make high tides higher & low tides lower. These are the spring tides in the above diagram. For me, these are "good" tides, since the snails I study live really low on the shore. If the low tides aren't really low, I can't get out to the animals without getting really wet. In March around here, the idea of going swimming for snails is distinctly unpleasant.The top photo of my field site was taken during a spring low tide.

When the sun and the moon are at right angles (1st quarter & 3rd quarter moons, above), they exert their forces in opposing directions. Since the moon is much closer to the sun, it has the stronger effect on the tides, but without the added pull from the sun the water doesn't move as much. These are the neap tides, or for me, the "bad" tides.

Yesterday was a neap low tide. That was compounded by the wind, which was blowing water directly towards shore (see the waves in that second photo), pushing the water level up even higher. I couldn't get anywhere near where I usually need to go at the site without going swimming -- and yesterday it was about 40 degrees Fahrenheit and raining. Luckily, the wind and waves have moved many snails way high up on the shore, so I was able to collect without getting hypothermia.

Now if only my veligers would decide that they are ready to cooperate, I could really do some good work this week.

(Bonus points for anyone who read the post title and immediately pictured Eowyn pulling off her helmet and screaming "I am no man!" in Return of the King. I only wish the tides would work like that for me.)