Tuesday, December 14, 2010

The Winter Hexagon and a Lunar Eclipse

This image is edited from a Stellarium screenshot.  Stellarium is an excellent, free, planetarium program.  Click for a larger view.
"Orion's Belt", part of the constellation Orion, is a well known and easily recognized asterism in the northern hemisphere's winter sky (between Betelgeuse and Rigel on the image above).  Six bright stars surround Orion's belt forming the Winter Hexagon, outlined in the image above.  Those stars are easy to find on a dark, clear night - follow the line formed by Orion's Belt to the left to locate the bright and twinkling star Sirius, drop down perpendicular to the Belt to find blue-white Rigel, follow the belt to the left to spot Aldebaran (the orange "eye of the bull" in the constellation Taurus).  Look up from Aldebaran to find Capella (in the constellation Auriga), to the left of Capella find Pollux (the brighter of the twins of Gemini), and the sixth star of the hexagon is Procyon, below Pollux on the way back to Sirius.
The Moon passes through the Winter Hexagon each month in its orbit around the Earth, and this month it will be December's Full Moon passing through on the night of 12/20 - 21/2010.  Go out and take a look on Monday, 12/20 - the brightest stars you'll see around the Moon are the stars of the Winter Hexagon. And on this particular night, the position of the Moon marks an important position in the sky...the exact spot the Sun will be on June 21, six months from now - a place in the sky called the "summer solstice".  Tonight's Full Moon will trace the same path across the sky that the summer sun will follow in June!  On this night, too, the Sun is directly opposite the Moon on the other side of the Earth:
 This image is a composite of Google Earth images cobbled together to show the relative locations of the Moon, Earth and Sun during the upcoming eclipse. Click for a larger view

At around 1:30 AM (EST) the Moon will enter the darkest part of the Earth's shadow (called the umbra).  For the next 3 1/2 hours, the Moon will move from right to left through the Earth's shadow, darkened to an orangey-red in the dim light of the Earth's shadow.
If you're willing to stay up, or wake up around 2 AM (EST) on the morning of Tuesdsay 12/21, you can view the last lunar eclipse of 2010.  Worth it, I say!
Here are some photos I took during the lunar eclipse of February, 2008.  That eclipse happened in the constellation of Leo and some of my photos included images of the bright star Regulus (Leo's "heart") and the planet Saturn with its rings tipped toward us.

Sunday, December 12, 2010

The Geminid Meteor Shower

(photo courtesy NASA.gov)
This Monday night (well, actually early Tuesday morning- the night of 12/13- 12/14) marks the peak of the 2010 Geminid Meteor Shower, an event that occurs every year as the Earth passes through the debris stream associated with asteroid 3200 Phaethon (see this NASA article for more on Phaethon and its debris stream).  This year's show is expected to produce up to 120 meteors per hour in the pre-dawn hours of Tuesday, 12/14 - that's a good show!  To view the shower, bundle up and head out any time after midnight (the later the better).  You don't have to look in any particular direction, just up - if you can find a dark place to sit back and look up at the whole sky, that will work the best. The meteors can appear anywhere in the sky, but if you trace their paths backward you'll find that they all seem to come from the constellation Gemini (face to the SSW and look up almost overhead...those 2 bright stars up there are Castor and Pollux, the Gemini twins). Here are some general hints for successful meteor viewing.
Cloudy weather is predicted in the northeast, so check the latest radar before you set your alarm Monday night....

Thursday, December 9, 2010

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)

Tuesday, March 16, 2010

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.

Friday, February 19, 2010

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.

Wednesday, January 20, 2010

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.

Sunday, January 10, 2010

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.