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What is a season? Fall arrives today

Posted by David Epstein  September 22, 2013 12:00 PM

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Fall officially arrives today at 4:44PM. You might not really think about what that means, besides the weather turning colder and the days growing shorter, but there are some interesting astronomical facts occurring this afternoon. Let me explain what this idea of seasons is and why, when the seasons change astronomically, the weather may have already warmed up or cooled down weeks earlier. seasons.jpg
I'll be updating the details of the forecast on Twitter at @growingwisdom Please follow me there. Feel free to comment or ask questions too.

Solar Noon
The time when the sun is at the highest spot in the sky marks solar noon. The idea of solar noon versus noon on your clock is simple. Noon, by the sun’s clock, is when the sun is at the midway point between sunrise and sunset.

Because time itself is man-made, when your clock says noon, the sun may or may not be at the highest point in the sky. Time zones are also shaped very regularly, so noon in Boston is also noon in Buffalo, but the sun can’t be in the same spot in the sky in both places. Therefore solar noon between the two cities is about 30 minutes apart.

We also monkey with the clocks during the year. For example, during daylight savings time, because we have moved the clocks head one hour, solar noon is actually around 12:30 PM in Boston and just after 1 PM in Buffalo. When we set the clocks back in November, solar noon moves to around 11:30AM along the Massachusetts coast. This is one reason why, in December by 2 or 3 PM, it seems to be getting dark; we are already several hours past solar noon.

Different Sun Angle
Since solar noon is the midpoint between sunrise and sunset, solar noon occurs everywhere the sun comes up and goes down. The cool thing about the sun is that its position above the horizon is different every day of the year. In the northeast the highest the sun gets in the summer at solar noon is about 71 degrees above the horizon. On the first day of winter the sun is around 25 degrees above the horizon. Today the sun will reach just over 45 degrees above the horizon during solar noon. The lower the angle the less heat we get from the sun and the cooler the air becomes over time.

Somewhere, right now, the sun is at its highest point above the horizon for many spots on the planet. earth circle.png Depending on where you are, that height might be as little as a degree or two above the horizon to as much as 90 degrees above the horizon. The latter is very important to understand the seasons.

Stick with me now, as we explore the idea of what the season mean just ahead.

Seasonal Differences
On a spot on Earth at this very minute, the sun is exactly 90 degrees above the horizon or perfectly perpendicular to the ground. In this spot you would have no shadow, and the sun would be very strong. As an aside, several years ago I found myself in Ecuador with the sun directly overhead, it was quite amazing.

These spots on the Earth that have the sun directly overhead at solar noon change every day. Today, at exactly 4:44PM, the spot where the sun is 90 degrees overhead will be along the equator. And actually that will be somewhere out over the Pacific Ocean. Check out the image below. e small yellow sun represents the spot along the equator where the sun will be 90 degrees high at noon today. Locally, it will be 4:44 PM. At 4:45, the spot will be just slightly south of the equator and each minute thereafter, the spot will continue to be bit further south until the first day of winter.

Then, on December 21st at noon, the sun will be at 90 degrees above the horizon at spot along 23.44 degrees south of the equator or along the line known as the Tropic of Capricorn. This is as far south as that 90 degree position of the sun, at noon, ever goes.

At that point, the sun’s 90 degree mark of solar noon will move north and continue doing so until June 21st. On March 20th, it will cross the equator and on June 21st it will mark its furthest north point along the Tropic of Cancer which is 23.44 degrees north of the equator. You can see that position for next summer in the image below.
Fall arrives 4_44 PM.jpg
winter arrives.jpg
summer arrives.jpg
In all three images, note how the darker areas, or night change with the seasons. Today, everywhere on the planet has the same length of day and night, thus the term equinox. This happens twice a year when spring and fall begin.

The seasons most familiar to us are based on this crossing of the sun along lines across the globe. The other, less common, but in some ways more useful set of seasons, is based on temperature. These are known as the meteorological seasons. According to the weather, fall began on September 1st and ends November 30th, winter starts on the 1st of December and ends at the close of February. For a meteorologist, this makes more sense since the weather is already onto the next season several weeks before the sun is telling us so. As the days getting longer and shorter, the weather is responding long before an astronomical season begins. After all, how many times have we had a snow storm the first week of December or a heat wave in early June?

Gardening this week
This is a great time of year to garden. You can plant a lawn, move and divide perennials and plants trees and shrubs will less chance of failure than in the spring. I was recently at a local nursery and saw some great conifers for the garden. Check out the unique plants you can add to your own landscape in this week's video.

I'll be updating the details of the forecast on Twitter at @growingwisdom Please follow me there. Feel free to comment or ask questions too.
This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
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