Earliest Sunset of the Year!

December 8, 2010. Well, we've made it again!.  Tonight is the earliest sunset of the year! The daylight period is still getting shorter (most folks know that shortest day is the Winter Solstice around December 21), but not a lot of people can explain tonight's early sunset.  It turns out that the rate at which the Sun travels across the sky is not constant - the tilt of Earth's axis and its elliptical orbit conspire to push the Sun ahead of our clocks, and then slow it down again, twice every year.  Astronomers call the difference between time told by the Sun (apparent solar time) and clock time (mean solar time) the "equation of time".(If you're interested, you can get the sunrise and sunset times for your location at the US Naval Observatory site.)
The chart on the left above, called the analemma, combines the equation of time with the position of the Sun relative to the equator.  Click it for a larger view, and notice that through most of the fall the Sun has been running ahead of the clock, but in December it began to slow dramatically.
It's the Sun slowing down relative to the clock that's moving the daylight period later into the day even as the days get shorter! (the latest sunrise of the year occurs during the first week of January)
This photo composite was made by Tom Matheson over the course of a year, snapping a picture of the Sun at exactly 8 AM (by the clock) each day.  Here is a labeled image of  Tom's photo.

(This blog is an edited  re-post from December 2009)

The Days Are Getting Longer, Fast!

Even the most casual observers have noticed that the days are getting longer fast at this time of year. The table to the right is derived from sun rise and set data provided by the US Naval Observatory for White Plains, NY, and a quick study of the data reveals why.

On 3/25, the Sun will rise 17 minutes earlier and set 10 minutes later than it did on 3/15 - a gain of 27 minutes of daylight in just 10 days!

While the daylight period has been getting longer since the winter solstice on December 21, a look at the Sun rise and set times for late December reveal why those long winter nights seem to drag on for so long...10 days after the solstice, the sun was rising 3 minutes later and setting 6 minutes later, for a hardly noticeable gain of only 3 minutes of daylight!

The geometry of the Sun's path among the stars is responsible for the variability of the change of the length of daylight.  The length of daylight changes quickly around the equinoxes in March and September, and very little around the solstices in June and December.

Notice too that the gains are not symmetrical around noon -  in March the morning gains are almost twice as great as the evening gains, and in December the sun rises later each day even as the days get longer - the lengthening is all in the evening!

This is because the Sun speeds up and slows down relative to the clock (which is in sync with the average time the sun takes to cross the sky). In the spring the Sun is "speeding up" relative to the clock while in December the Sun slows so dramatically that sunrise lags behind the clock even as the days get longer!  This is all a result of both the changing velocity and axial tilt of the Earth in its orbit around the Sun (see this analemma discussion for more information on the relationship between solar and clock time).

12 Hours of Daylight!

The Sun will cross the equator shortly after noon EST on 3/20 this year at a place in the sky we call the equinox, but tomorrow - 3/17 - is the day when we'll have 12 hours of daylight and 12 hours of night...Learn why here.

The image above was made in Stellarium, and shows the location of the Sun as viewed from White Plains, NY at the moment of the 2010 vernal equinox on 3/20.  The blue line is the celestial equator, and the red line is the annual path of the Sun among the stars (it moves to the left, or east, among the stars at about 1 degree/day due to Earth's revolution around the Sun).  The atmosphere has been "turned off" to reveal the background stars and planets.

It's Still Cold...But the Sun's Coming Back!

Even though winter drags on,  I'm always intrigued by the changes in the Sun that foretell the coming of Spring.  You may have noticed that the days are getting longer quickly and that the Sun (when it's not blocked by clouds!) is warmer and more intense than it's been in months.  I've had to reset the timers on the lights in my house a few times already, and I've noticed that when my car sits in the sun it warms up inside - a little anyway - even when it's cold outside.
In the weeks before and after the vernal equinox (around 3/21) the Sun's apparent motion among the stars brings it quickly from south to north across the celestial equator.  The result is a rapid lengthening of the daylight period, and a noon sun that climbs higher in the sky each day.
The charts below were made with a year's worth of sunrise and sunset data for White Plains, NY provided by the US Naval Observatory.

On the chart above (click it for a larger view), notice that from mid February through most of April, the days lengthen by more than 2.5 minutes a day.  And notice too, that the daylight period is long from late May through the the first weeks of August.

The chart below is derived from the same data, and shows the longest and shortest days as well as the earliest and latest sunrises and sunsets of the year.
If you want to play around with the data, it's here in an Excel spreadsheet.

The Oblate Sun

This shot of the setting sun taken from Seamans Neck Park in Seaford, NY illustrates an interesting atmospheric optical property. Earth's atmosphere causes refraction of the sunlight (and moonlight, and starlight) that passes through it. Near the horizon, refraction will raise an object on the horizon (like a star or the Sun) to an apparent position about 0.5° above the horizon. When the 'bottom' of the Sun is raised more than the 'top', the rising or setting sun can look quite flat! This refraction hastens the apparent rising of the Sun and delays the apparent setting of the Sun, effectively giving us a few more minutes of daylight than we'd have if there was no atmosphere at all.  You can read a little more about the effect this refraction has on the length of day here.

Revealing New York City Skyline

This photograph of the New York City skyline was taken from the top of Bear Mountain in the Hudson Highlands some 40 miles (64 kilometers) due north of the city.  It reveals something interesting about the interplay of physical and cultural geography as New York City developed.
Notice how the large, tall buildings are clustered in the Midtown and Wall Street areas while the buildings in "The Village" (Greenwich Village and Chinatown)are smaller and lower.  The pattern is not simply a matter of coincidence - it is controlled by the underlying geology of Manhattan Island.  The tall, heavy buildings are built where strong, crystalline bedrock is available at the surface to support them.  The Village, on the other hand, is underlain by unconsolidated glacial deposits incapable of supporting such massive skyscrapers without extensive and expensive engineering of their foundations.
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The annotated Google Earth screen shot to the left here (south is to the TOP of the image)is a "top view" of what you're seeing in the photograph. The Midtown - Village - Wall Street pattern is also apparent looking east from the New Jersey Turnpike, and you can read more about the development of NYC at this US Geological Survey page.

New Marble Outcrop in Brookfield, CT

Generally speaking, I'm not a fan of man-made landscape alteration, but fresh highway roadcuts provide a great unweathered view of the local geology.
A few weeks ago, after years of proposals followed by years of construction, the northern extension of "Super 7" (US route 7) north of Danbury CT finally opened - and the exposures of the Stockbridge Marble along the road are spectacular!  The Stockbridge Marble was formed as the rocks of the carbonate bank (limestones deposited off the coast) of ancestral North America were metamorphosed during the Taconic Orogeny ( see blog on 12/2/2009 and the diagram below).

The rocks were further deformed, and actually overtured, in the subsequent Acadian Orogeny. The extent of those deformations are evident in the complex folds pictured here.
Carbonate rocks, including marbles, generally weather faster than silicate rocks, and in SW Connecticut and SE New York many of the regional valleys are underlain by less resistant marbles.  Along route 7 between Danbury and New Milford the Stockbridge Marble underlies the Still River valley, and is bounded by harder rocks on either side of the valley.  The same rock underlies the Saw Mill River valley in New York  where is known as the Inwood Marble (there's a "Marble Avenue" exit on the Saw Mill River Parkway in Pleasantville/Thornwood, and the Shop Rite plaza in Thornwood is built on an old marble quarry there - great outcrops of snowy white "snowflake" marble are still visible on the quarry walls behind the stores, but they're weathering fast!).
Riders of the Hudson Metro North commuter rail can get a good look at the Inwood Marble at the Marbledale stop on the way into Manhattan.