22 September 2010

Autumnal Equinox and the rate of change of the length of the day

Today is the Autumnal Equinox, one of two Equinoxes of the year. These are days that mark the transition from one season to the next, but also are days that have very special significance as the Earth orbits the Sun. Because of the tilt of the Earth's rotational axis, the Northern and Southern Hemispheres receive radically different amounts of sunlight from June through December, as the seasons progress from Summer to Fall to Winter (northern hemisphere). This is well understood and we learn from basic science courses that the in-between points while transitioning from Summer to Winter, for example, are the days that have equal periods of the Sun above the horizon and below the horizon -- equal duration of night and day -- hence Equinox. Today, everywhere on the planet, the Sun spent exactly 12 hours above the horizon and 12 below, and the Sun rose due East and set due West. From now through March, the Sun will be above the horizon more than 12 hours a day in the Southern Hemisphere, and less than 12 hours a day in the Northern Hemisphere.

I love the symmetry and simplicity of the Sun's motion on this day. It marks a transition as the days are getting shorter in the Northern Hemisphere. Everyone can feel the shortening of the days and sense, innately, that the changes in daylight and darkness are sudden and surprising. This is another fascinating change happening at the Equinox, more subtle but no less fascinating to me. When people sense the changes to the onset of darkness in the evening or the late sunrise in the morning, they are noticing that the length of the day is changing quite quickly and they feel that the times of day that might have been bright and sunny only a few weeks ago are now getting dark. At the time of the Equinox, the length of the day is changing most rapidly. For example, here in the San Francisco Bay Area, the longest day in summer is nearly 15 hours long and in winter the shortest day is approximately 9 1/2 hours. As the seasons change, the time of sunset and sunrise changes slowly, starting at the solstice in June - maybe only 1/2 to 1 minute per day. But then the changing becomes more rapid approaching the Equinox. At the time of the Equinox this week, the length of a day is changing 3 minutes per day -- that is, about 20 minutes in one week! So if you feel like the length of the days is changing very fast, you are absolutely correct.

The change is even more dramatic the farther north or south you are. For example, in Alaska, the length of the day is changing right now about 5-6 minutes per day, or about 40 minutes in one week! Imagine how that would feel, and it is a natural thing that happens every Spring and Fall. I think it is amazing how much the seasons impact the different geographies of the world, and a little understanding of the natural foundation for these effects is a nice thing to have.

7 comments:

Sidewalk Universe said...

Nice write up Paul!!!!!!!!!

Hunter Cutting said...

I've heard this before, great stuff.

But why does this happen?

Lee said...

It happens because the projection of circular motion into one dimension follows the rules of simple harmonic motion. We can consider the change in the length of the day to be like velocity, which in SHM is at a maximum when the displacement is at a minimum.

Jon said...

Sorry, can you simplyfy this explanation for us laymen? Is it to do with the earth's elliptical orbit of the sun?

The Urban Astronomer said...

hi Jon - the changes in this situation are due to the tilt of the Earth on its axis of rotation. Because the top of the Earth's axis (essentially the North Pole) points to a fixed point in space (at Polaris, the North Star), as the Earth travels in its 12 month journey around the Sun we have a continuously changing orientation of the axis of the Earth. Sometimes the pole is leaning toward the Sun and we have Summer (in the northern hemisphere) and six months later the pole is leaning away from the Sun and we have Winter. As we are in-between these two extremes at the Fall or Spring Equinox, we have the unique situation that the entire planet Earth does not have summery and wintery parts, but all parts of the Earth are equally illuminated by the Sun, for 12 hours. A picture is worth 1000 words. Try this: https://www.sciencealert.com/the-september-equinox-is-this-tuesday-here-s-what-that-means

Unknown said...

Thanks, Urban Astronomer.

I am wondering, more specifically, why does the rate of change vary? Why is the rate of change fastest at the equinox and slowest at the solstice?

In the past, I've heard the lawn sprinkler analogy invoked. The sprinkler moves fastest when in the middle of it's arc across the land, and moves slowest at each end of it's arc where it has to change direction. The ends of the sprinkler arc would be the analogy for each solstice, where the change in daylight has to reverse direction (change in sign). And the middle of the sprinkler arc would be the analogy for the equinoxes.

I get the analogy, but I would love a direct explanation.

Any info you could offer would be much appreciated.

best

Hunter

Sam G said...

I believe the question is: why does the change in length of day increase at the equinox vs the solstice? Or why do we loose sunlight faster at this time year?