Wednesday, July 21, 2010

The Summer Solstice

By Nina Berryman
Monday the 21st is the Summer Solstice, the longest day of sunlight of the entire year. I have always been especially interested in astronomy and am overjoyed with nerdiness when this topic overlaps with my interest in agriculture. Monday has more minutes of sunlight than any other day of the year because of Earth’s tilt. The sun will rise at 5:32 am and set at 8:33 pm, for about 15 hours of sunlight. It will feel like slightly more because sunrise and sunset are calculated from when the edge of the sun dips above or below the horizon, but rays of sunlight still light up the sky before and after that. To explain what happens on the solstice, here is an exercise: Imagine the sun is a basket ball and the Earth is a potato with a stick piercing all the way through it. Point the stick straight up and down. One end of the stick is the North Pole and one point is the South Pole. If you hold the stick at either end and spin the potato around once, this is how the Earth turns on its axis in one day. Stick a pin somewhere on the top half of the potato (the northern hemisphere, where we live). This represents you standing on the Earth. Now continue spinning it. When you are looking at the basketball you are in sunlight and when you are looking away from the basket ball you are in night time darkness. For exactly half of the spin you are in daylight and for half of the spin you are in darkness. Now tilt the stick slightly toward the basketball (23 degrees to be exact). Now keep holding either end of the stick and spin the potato once again and take notice of when the pin is facing toward the basketball and when it is on the opposite side of the potato from the basketball. Now, because of the potato is tilting toward the basketball, the pin is in sunlight for more than half of the spin. This is why we have more hours of sunlight on June 21st! Still confused? Try this: Move your pin to the base of North Pole, right where the stick comes out of the top of the potato. When you spin the potato on its tilt, you’ll see the pin is always in the sunlight. This is why this part of the Earth, during the time of the summer solstice, is called Land of the Midnight Sun, because the sun is out all day, even at midnight! Still confused? Take a look at the diagram below.
What does this have to do with farming? Well, this tilt of the Earth is one of the factors responsible for causing our seasons. Back to the potato and the basketball…With the potato still tilting toward the basketball, take note of something in the room that the North Pole is pointing towards (perhaps the top of a bookshelf or door frame). Now keep the stick pointing toward that object while moving the potato around the basket ball in an even plane. Stop when you are half way around the basketball from where you started. The potato should still be pointing at the same original object, but because the potato is on the other side of the basketball from where it started, the top of the stick will now be pointing away from the basketball. Spin the potato around the stick again, like before, and you’ll notice your pin (if it is still on the top half of the potato) is in the dark for most of the spin. Try moving the pin back to the North Pole and you’ll see the pin is in the dark for the entire spin. This is the winter solstice, the shortest day of the year (in the Northern hemisphere). See the diagram on next page.
Because the sun heats the Earth, when there are more hours of sunshine on the Earth, the days are not only longer, but warmer. When you have more and more days of longer and longer sunlight, the Earth will get warmer, and hence we have summer!
Many people wonder why the summer solstice is not also the hottest time of year. The reason is the same as why a pot of soup is not instantly hot when you put it on the stove to heat. Imagine the stove is the sun and your pot of soup is the Earth. Your pot of soup is cold and you put is on the burner and the burner is on high. The soup will not instantly be scalding hot, it takes a few minutes to warm the contents of the pot. In our situation, this takes a few months, with the hottest time of year coming around August, instead of June 21st.
Again, what’s the connection with farming? The change of the seasons dictates everything we do. Because we are located where we are on the Earth, the mid Atlantic, we can have a CSA which is 26 weeks long. Just last night I was talking with my friend who is working at a CSA in Montreal, considerably closer to the North Pole, and her CSA season is only 17 weeks long. Seasonality is why we have bok choi and radishes in the spring and fall, but not in the summer. It is why our tomatoes aren’t ready until the end of July. It’s the reason we can take an extended vacation in January! It’s the reason you have to grow specific onion varieties in different parts of the country, since onions are sensitive to the length of day. My friend in Montreal will have a different number of hours of sunlight on the summer solstice than we will have here, and so she is growing different kinds of onions on her farm than we are here!
I can’t stress how much of a geek I can be over astronomy, so if anyone wants an reenactment of the summer solstice with beets and turnips, just let me know!

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