WHAT'S UP: Autumn is in the air today

Sep 23, 2011

 

Today in Grand Haven, the sun rises at 7:33 a.m. and sets at 7:41 p.m., so it is apparent that the night and day today are not quite equal. This asymmetry is due to the fact that we reckon sunrise and sunset from the time the very top of the sun rises above and sinks below the horizon.

What’s going on around the world today?

On the North Pole, the sun is just setting after spending the past six months above the horizon. Today on the North Pole, the sun tracks 360 degrees around the horizon; about half above and half below.

On the South Pole, the sun is just rising after spending the past six months below the horizon. Today on the South Pole, the sun also tracks 360 degrees around the horizon; about half above and half below.

At the bus stop this morning at Amundsen-Scott South Pole Station, Antarctica: minus 70 degrees with ice crystals, blowing snow and freezing fog; and winds out of the northeast at 21 mph, making the wind-chill temperature minus 113.

At any location on Earth’s equator, the sun is rising due east — tracking straight up to the zenith (the point directly overhead) — and setting due west. At solar noon — the time when the sun is highest in the sky, halfway between the sunrise and sunset times — the sun is directly overhead and everyone there can walk around on their shadows.

At any other location on Earth, today is an average day by most measures: everywhere, the sun rises due east, sets due west, and is above and below the horizon for very nearly 12 hours each.

For the next three months in the Northern Hemisphere, the nights will grow steadily longer than the days and average temperatures will fall. In the Southern Hemisphere, the days will grow steadily longer than the nights and average temperatures will rise.

It is a misconception to think that the main reason it grows cold in the winter is because the nights are longer than the days. If that were the case, we would expect the opposite in the summer. But the warmest stretches of summer in Alaska, for example, where the sun is above the horizon in some places for more than 20 hours, don’t hold a candle to the warmest stretches of summer in Florida or Texas, where the sun is above the horizon for only something like 14 hours.

The variation of temperature with the seasons has mostly to do with how close to overhead the sun rises. Even though the sun is above the horizon in Alaska for 20 hours, it never climbs as high in the sky as it does in more southern locations, so Alaska still stays relatively chilly.

Today, the sun climbs to 47 degrees above the horizon in Grand Haven. In Anchorage, Alaska, the sun rises to only 29 degrees; but in Tampa, Fla., the sun makes it to 65 degrees above the horizon.

The maximum elevation of the sun at any place on Earth, on either the spring or autumn equinox, is 90 degrees minus the location’s latitude.

— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to dgf@inbox.com.

 

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