Tuesday, June 21, 2011

Today's the solstice... so why are the sunrise/sunset times "out of sync?"

Today, the Earth’s Northern Hemisphere experiences the summer solstice—the longest day of the year.
  (South of the equator, of course, it’s the winter solstice).  Officially, this moment occurs today at 1:16 p.m. EDT (10:16 a.m. PDT).  

Being the longest day of the year—the day when the sun is in the sky the longest—you’d think this would also be the day when the sun rises at its earliest and sets at its latest, no?  But, amazingly, it’s not.

Even a quick glance at any sunrise and sunset table will show this “flaw” around both the summer and winter solstices.  So what’s going on here?

It's a great question, and one I receive at this time every year. One of the best explanations of this phenomenon is offered by the Royal Greenwich Observatory in their Short Special Information Leaflet No. 5 titled ‘The Apparently Odd Behaviour of Sunrise/set times near the Winter Solstice.’

“The winter solstice is the time when the Sun reaches its southmost distance from the celestial equator and hence, in northern latitudes is the day when the Sun is lowest in the sky at noon. This is, naturally, the shortest day of the year in northern latitudes. To many people it seems odd, therefore, that the time of sunrise continues to get later in the day after the solstice.

“The reason for this is that the Sun does not cross the meridian (when it is highest in the sky) at precisely noon each day. The difference between clock-defined noon and the time when the Sun is on the meridian is called the Equation of Time and represents the correction which must be applied to the time given by a sundial to make it agree with clock time.

“There are two reasons why the Sun is not on the meridian at noon each day. The first is that the path of the Earth around the Sun is an ellipse, and not a circle. The second is that the Earth's equatorial plane and its orbital plane are inclined to one another. The two effects add together to yield the Equation of Time which can amount to some 16 minutes difference between solar and mean time.

“The period when the Equation of Time changing fastest in the whole year is very close to the Winter Solstice. It changes by 10 minutes from December 16 to January 5. This means that the time at which the Sun crosses the meridian changes by 10 minutes in this interval and also that the times of sunrise and sunset will change by the same amount.

“Near the Solstice the Sun's height in the sky changes very slowly and the length of the day also changes slowly. The rapid change due to the Equation of Time dominates the very slow change in day length and leads to the observed sunrise times.”

If you could measure the sun's position at the same time every day throughout the year--say at 12 noon--you'd discover that it would trace out a figure-8.  It would be highest, of course, in the summertime and lowest in the winter, but would not appear due south every day at noon.  This figure 8 is that strange figure we've all seen on a map or globe at one time or another.  It's known as the analemma, and at this link you can see graphically how the “equation of time” affects the sun’s position in our sky throughout the year.
And be sure not to miss some truly spectacular—and quite difficult to take—photos of the analemma by my friend and TWAN colleague Anthony Ayiomamitis.

--Dennis Mammana
21 June 2011

Thursday, June 16, 2011

The First Total Solar Eclipse of the 21st Century: Ten Years Later

It felt as if my heart was about to pound its way through my rib cage.   My palms were sweaty, and chills danced up my spine at the realization of what was happening before me.  Time was growing short.   Verrrrry short.  

Ask any experienced eclipse chaser, and they'll tell you it's always like this.  "Anxiety," "nervousness," "apprehension"―all are words that describe the emotions one feels as the moon's inky shadow engulfs our sky from the west, and the alien darkness of "totality" descends in mid-day.

But this was not totality.  In fact, we weren't even outdoors.  We were in the airport at Livingstone, Zambia, watching helplessly as our chartered aircraft―scheduled to take us 200-plus miles to the eclipse centerline―sat on the tarmac.   Its GPU was broken and the plane couldn't take off without it. 

Now I'm not a pilot, and I don't know a GPU from a BLT.  But I do know that the moon cares nothing about our mundane problems.  It continues on its orbit and, at that very moment, its shadow was careening eastward across the Atlantic―approaching by 25 miles every gut-wrenching minute we sat in Livingstone.  The shadow would arrive in less than four hours and, if we were going to meet it, we had to leave.


But things weren't looking good.  Pilots and technicians were talking and studying the underbelly of the plane―not an encouraging sign.  We were, after all, in the middle of Africa.  How many spare 737 GPUs could there possibly be, and how efficiently could anyone find and install a replacement? 

Some in our group of 86 remained outwardly upbeat as thoughts of traveling halfway around the world only to miss the first total solar eclipse of the Third Millennium began to sink in.  Others began to rationalize out loud:  "Well, we could still see the partial phases from here..."

All we could do was wait and watch... and wait some more.  Hope was fading with every tick of the clock.  And then, just as unexpectedly as the delay announcement earlier that morning, the doors from the tarmac swung open.  "OK, everyone onboard!  We're ready to go!"


What a flood of emotions now!  As we gathered our gear and raced toward the plane, many among us were performing mental arithmetic.   "If we leave in 10 minutes... if the flight takes 45 minutes and the bus ride to the site takes only 30 minutes..."

That they fixed our plane was stunning.  That our bus squealed to a stop at our pre-selected observing site with barely five minutes until "first-contact" was nothing short of a miracle.  

My arms overflowing with photo gear, I leaped from the bus and sprinted frantically across a tick-infested field―being chased by our guide spraying me with a can of insecticide―in search of an appropriate foreground.  Only four minutes remained until my first carefully planned shot was scheduled.  Miss it, and I'd have to wait 18 months for another chance.

I quickly found a spot just to the east of a small tree, cleared some of the tall, dry grass, set up the tripods and aligned the cameras, checked and tested the automatic programming, and―muttering something off-color about Murphy―pressed the start button.  Right on time!

Finally, I could take a breath.  Sure, I had a few more gray hairs on my head but, in a crystal clear sky, the show had begun.  And, this is why we were here! 


Within the hour, the moon's umbral shadow arrived―just as expected―and it engulfed the Zambian landscape in a still and eerie darkness.  Soon, the last burst of sunlight disappeared behind the moon's edge―the "diamond ring."

And then... totality! 

The place where the mighty sun once shone was a void, around which our star's gossamer corona streamed outward across the sapphire sky.  Some cheered its appearance;  others wept at its splendor.  And some gazed in silent awe at the most glorious spectacle nature has to offer. 

Three minutes and 35 seconds passed―yet it seemed like only seconds―until the sun's familiar rays burst into view again.  "No!  It can't be over yet!  It just began!"   But the moon's shadow continued its eastward journey―across southeastern Africa, across the Mozambique Channel, and on to Madagascar where other sky watchers anxiously awaited its arrival.

Three years of calculating, planning and rehearsing were now complete.  For a few magical moments we had become one with the Cosmos―in perfect syzygy with the three most important bodies in the heavens.  We had been touched by the power of the universe in ways difficult to describe, and we had felt emotions impossible to communicate.

Boy did we ever!

*  *  *  *  *  *  *  *