Friday, December 19, 2008

Winter Solstice on Sunday

Position of the sun in the sky over the course of the day
during the summer and winter solstice days, and both equinox days.

In the Northern Hemisphere, the Winter Solstice occurs on December 21, 2008 at 12:04 UT (Universal Time) - or 04:16 Bermuda local time.

This means that at noon on Sunday the sun will be at its lowest point relative to the southern horizon for the whole year. The angle of the sun will only be 33* South above the horizon - as opposed to June 21st, when it is almost directly overhead, at 83* South.

It will also be the shortest day of the year, with only 10 hours of daylight. The sun will rise at
7:16 am, and set at 5:18 pm - local time

The angle of the sun and the
length of the day affects the amount of light available for photosynthesis by hard corals, soft corals, and macroalgae growing on reefs.

Furthermore, since Bermuda is the most northerly reef in the world, this means that patch reefs obtain more sun on their south sides than they do on their north sides. In my PhD I measured light levels as well as reef coral cover and species diversity on the south and north sides of patch reefs in the North Lagoon of Bermuda.

Graph of the proportion of surface light available to
southern and northern reef slopes, relative to depth.


Light on the shady sides (with a Northern Aspect) have up to 5x less available light as the sunny side. Since light also decreases the deeper one goes, due to the light-blocking effects of suspended sediments, plankton and other things in the water, this relationship decreases with depth.

Cartoon of the effects of Bermuda's northerly position on sun angle,
and as a result, coral assemblage structure, on patch reefs.

The differences in light between the south and north sides of patch reefs in Bermuda results in differences in coral abundance and the kinds of corals that are found on each side. Shade-loving branching corals dominate the north sides of patch reefs, while sun-loving head corals are more abundant on the south sides of the same reefs.

Dr. Thad Murdoch - BREAM Project Leader