Thursday, September 22, 2016

Today is the Fall Equinox, But is the Day and Night REALLY Equal?

Today, September 22nd is the fall or autumnal equinox.  Many references talk about day and night being equal in length during the equinox, as suggested by the name (equi--or equal, nox--night).

But if you check the sunrise/sunrise tables you will find out that this is not true. That they are not equal, with the day being longer.   So what is going on?

On the equinox, the sun is directly over the equator and earth has no tilt with respect to the sun (both hemispheres are the same distance from the sun).  As a result, the terminator, the line separating night from day is directed north-south, which is illustrated by a visible satellite image taken a few hours ago (see below).

But it turns out that day and night are not equal during the fall equinox, something evident in the sunrise/sunset table for Seattle below.  Today, the day was about 8 minutes longer than the night!.  Day and night won't be equal for several days (Sept 25).  But how can this true if the sun is directly over the equator?

The reasons are two fold.   First, sunset does not occur when the center of the sun crosses the horizon, but when the upper portion slips below the horizon.  Sunrise occurs when the upper  portion of the sun shows itself.    As a result of the finite size of the sun, day is lengthened.

The other reason is the atmosphere refracts or bends the sun's rays upwards, resulting in the sun being visible even when it is physically below the horizon!

So enjoy the extra will appreciate it in a few months.

My talk on Northwest Climate Surprises on September 28. 

During the evening of September 28, I will be giving a talk in Seattle at the Mountaineers in NE Seattle on Climate Surprises: Unexpected Impacts of Global Warming on the Pacific Northwest. 

You think global warming will simply bring warmer temperatures, drought, less snow, and more storms? 

Think again. The latest climate model simulations provide a far more nuanced prediction of what will happen here, with some of the predictions being quite surprising. This talk is sponsored by CarbonWa and the Audubon Society. To find out more or to secure tickets, please go here.


meganleiann said...

Thank you for taking the time to explain this. Is there anything visible that happens when we switch from seeing the physical sun to the sun's refracted image?

Eli said...

The visible effect is the sun appears to slow down slightly as it sets. We are always seeing the refracted image, there's no switchover, but the image of the sun lags the real sun by an increasing (still small) amount as the angle gets lower.