Yesterday was the official first day of summer. It might not have felt as balmy as the previous weather prediction foretold, but that seems to be the theme this June. Most people know that June 21 was the longest day of the year. However, the reason for it is a little more elusive. The summer solstice is the day when the sun is the furthest north in the Northern Hemisphere.

Yesterday, at 10:57 a.m., the sun was positioned directly over the Tropic of Cancer, the line that marks 23.5 degrees north latitude. That’s because Earth is tilted about 23.5 degrees, so the sun’s rays fall at an angle, landing at the latitude line known as the Tropic of Cancer. The fact that the sun is its farthest north of the year means that its entire arc from morning until night is the longest it will be all year — hence the long daylight hours. On the solstice, the sun doesn’t set until about 8:40 p.m. after 15 hours of daylight. That’s 5 1/2 more hours of daylight than we have in Maine at the winter solstice, the shortest day of the year.

Elizabeth Mooney, of Yarmouth, and her daughter play in the water at Willard Beach in South Portland on June 20, 2020, the longest day of the year and the official start to summer. Brianna Soukup / Portland Press Herald file photo

As summer progresses, the sun will slowly move farther south. An easy way to understand how the position of the sun changes over time is to pick a spot where you can see the sun set at different times of the year. If you keep track of where it sets on June 21 versus August 21, for example, you’ll clearly see a shift towards the south. This will continue until the autumnal equinox on Sept. 23 when the length of daylight equals the length of night. If you don’t happen to have a heliocentric model in your house, you can see the change in the tilt of Earth relative to the sun throughout the year through digital visualizations. One particularly neat one is a series of images showing what is known as “the terminator.” This is the line that separates day from night, and it shifts from one side of the poles to the other throughout the year. It’s shown as a shadow across half of Earth’s surface, depicting darkness — perhaps the reason for its name.

All of this is not only interesting in understanding the shift to a new season, but it also is relevant to what is happening along the coast and in its waters. The high number of daylight hours means that there is more solar radiation for the hungry marine “plants” to produce oxygen that is critical for the rest of the vast marine life to survive upon. These “plants” are actually algae, and the major primary producers are phytoplankton. They are so small that you don’t see them until they “bloom” in massive quantities. This can be a bad thing when a bloom gets too big and can smother other marine life and end up absorbing extra oxygen as it decomposes. But in the spring, this bloom is typically a positive thing and is reliant on those long sunlit days to fuel it — much as all the greenery on land is all blooming like crazy right now.

Following this productivity in the water, fish start to be active, and schools are on the move, flocks of birds find their way to munch on these little fish and seaside plants, and people get back on the water as well. There is a flurry of activity in response to this productivity and also plenty of time for activity during days that last so many hours.

I’ll end with a small plug to consider, as the season shifts, just how much is going on along our coasts right now and that it is both an opportunity to observe it and a time to be conscious of sharing these spaces and places both with the plants and animals that are all flourishing right now and with the other people that are eager to enjoy the first days of summer. There’s much more to say here, but simple things like keeping your eyes up and about while on the water and making yourself visible — whether a swimmer or a watercraft — can make a big difference to your safety and to the ability of everyone using the same places. So, as we shift into summer, absorb all that light while you can and get out on the water, taking good care to do it with respect for the natural world, including the people who are a part of it.

Susan Olcott is the director of operations at Maine Coast Fishermen’s Association.