Friday, June 7, 2013

June 2013 Solstice and Full Moon

(NOTE: The images below are screenshots from a program called Stellarium, available for free at www.stellarium.org. Click the image for a larger view)

This year (2013), the summer solstice and the June full moon occur within days of each other.  The noon solstice sun (shown in the top image crossing the meridian of my home in western Connecticut with the effects of the atmosphere removed) will have reached its highest point in the sky in 2013.  Note its position on the point of the ecliptic (drawn in red here) that is as far north of the celestial equator (drawn in blue) as can be.  The June 21 sun is in the middle of the ‘winter hexagon’, just above Orion and the star Betelgeuse, and happens to be crossing the plane of the Milky Way galaxy as well.  Venus and Mercury are to the left (east) of the sun, and Jupiter and Mars are just to its right, though none of that will be visible through the sun’s glare.  It’s interesting to note too that the autumnal equinox is just rising due east, and the vernal equinox just setting due west at noon on the summer solstice.
The June full moon, 2 days later, is opposite the sun, occupying the position of the winter solstice at the southernmost point of the ecliptic.  – it will be low in the southern sky all night, following the same path the December sun followed six months ago. And as that full moon crosses your meridian that night, the vernal equinox will be rising in the east as the autumnal equinox sets in the west. The June 23 full moon also happens to be a “perigee full moon” (meaning its slightly eccentric orbit happens to bring it closest to earth on the same day it’s full), and it happens to be the closest perigee of the calendar year, too. While these conditions will make the moon measurably (but not noticeably) bigger, a little brighter, and produce higher than usual high tides, it is nothing extraordinary. You’re likely to see all kinds of “Super Moon” posts in the social media, but keep in mind that perigee full moons occur every 14 months and that this one in particular will not get very high in the sky.

Tuesday, January 3, 2012

Latest Sunrise of the Year

The latest sunrise of the year will occur on January 5 this year...2 full weeks after the shortest day!  The earliest sunset occurred on December 8 even as the days continued to get shorter and on December 21-22 (the Winter Solstice) we experienced the shortest daylight period of the year. Since then the days have been getting longer, even as the sunrise was getting later.

It might seem as if the latest sunrise and earliest sunset should occur on the shortest day, but both the tilt of the earth's axis and it's slightly elliptical orbit work together to speed and slow the sun relative to our clocks, sometimes pushing the daylight period later into the day (as has been happening in the last month), and other times moving the daylight period into the morning in a predictable pattern we call "the equation of time".

The term solstice means sun stops, or sun stands still.  Of course the sun is always moving east to west across our sky, but from late November through mid January, the sun is nearly as far south as gets (it stops moving further south!) - and that's why we see such uniformity in the length of the day....9 hours 20 minutes on 12/8, 9 hours 13 minutes on the solstice, and 9 hours 20 minutes again on 1/4.  It isn't until late February that you'll really notice rapid lengthening of the day.


Monday, December 5, 2011

Earliest Sunset of the Year

December 8, 2011. For the last few years, I've posted this in early December...12/8 marks the earliest sunset of the year! The daylight period is still getting shorter (people who pay attention to these things know that the 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 worst of winter is still ahead of us, but at least we'll have a little more evening daylight... (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 and 2010)

Wednesday, October 26, 2011

Cloud Filled Valleys in Pennsylvania


Nearing the end of a red eye flight from California to New York on 10/17/2011, I was treated to this intriguing view of cloud/fog filled valleys as the Sun rose over the northern reaches of the Valley and Ridge Province of Pennsylvania. Overnight temperatures in the valleys had dropped to the dew point and below the stream water temperature. Under those conditions moisture evaporating from the warmer streams quickly condensed to fill the valleys with fog and clouds, some rising high enough to catch light from the rising sun.
The metar above, covering Sunday 10/16/2011 through Monday 10/17/2011 at Williamsport, PA reveals the cool, saturated, and still air that was in place around sunrise on Monday.

Sunday, March 27, 2011

The Great Tohoku Quake of March 2011

The day after the great M 9.0 Tohoku quake near Honshu, Japan, on 3/11/2011, CNN ran an article with the headline "Quake moved Japan coast 8 feet, shifted Earth's axis" (it was likely based upon this report out of Caltech the day before). The claim seemed too remarkable to be true, and I wrote to a few seismologist/geologist friends for their take on it. A friend at IRIS sent me to the Geospatial Information Authority of Japan, and I searched around for more emerging information.
It turns out that entire island did not move 8 feet, but near the epicenter the movement and deformations of the island and seafloor were even more astounding than the "8 foot" claim.
I've gathered a number of maps, charts, and images pertinent to the quake and put them in a single Google Earth file available here. The links above, and many more, are in the Google Earth file.
Here are some of the truly incredible things that happened during the quake
  • The northeastern shore of the island near the epicenter moved eastward more than 4m during the quake....yes! GPS measurements reveal it! The western part of the island moved eastward by somewhat less than a meter....So part of northern Japan (near the epicenter) is now some 3+m wider than it was prior to the quake! (turn on the japan-slip overlay in the file I sent). I'm assuming that there was significant compressional stress built up in the island, and the land expanded eastward as that stress was released during the quake.
  • The motion along the boundary between the subducted Pacific Plate and the overriding Okhotsk Plate* on was on the order of 24m at the epicenter! (turn on the japan-mainshock-slip overlay in the file I sent). Apparently almost all of the motion was accommodated by the overriding plate moving eastward and up, while the Pacific Plate hardly moved. *(The Okhotsk Plate is part of the larger North American Plate).
  • The upward movement of the plate raised the level of the seafloor just west of the trench an astounding 4.5+m, and created a basin 2+m deep off the shore (turn on the japan-uplift-and-subsidence overlay in the file I sent). The subsidence of the seafloor lowered the island by about 1m along the shore there (which would have the effect of moving the shoreline inland, but not the rocks under it). I don't know this for a fact, but it seems like a 5m rise in the seafloor and a simultaneous lowering of the coastline would have added signficantly to the damage caused by the tsunami.
And here's something I noticed as I looked over these maps. Bring the japan-mainshock-slip overlay to the top of the 3D display by turning it off, and then on again. The dotted isolines are the depth to the interface between the overriding Okhotsk Plate and the subducted Pacific Plate seafloor.

In the Layers panel in the GE sidebar, expand the Gallery folder and turn on Volcanoes.

Now, notice where the volcanoes are relative to the depth of the plate boundary...Seems like the generation of magma that makes it to the surface begins at about 100km depth..... I drew a profile across the area, and collected data to make the annotated chart above.

Saturday, January 15, 2011

Twinkling Sirius



Screenshot from the free planetarium, Stellarium (www.stellarium.org)

If you've watched the night sky much, you've probably noticed how stars seem to twinkle, a phenomenon known as "scintillation".  More careful observation may have revealed that the scintillation is more pronounced in stars near the horizon, and that the planets, while appearing star-like, generally don't twinkle even as the stars around them do!

In my recent post regarding the winter hexagon I described how to locate the bright blue-white star Sirius by tracing the line of stars of Orion's belt down and to the left.  Sirius is the brightest star in Earth's nighttime sky, at least in part because at 8.6 light years distant it is also one of the nearest stars. For observers in northern latitudes, Sirius arcs across the southern winter sky, rising south of east a few minutes earlier each night, and setting south of west about 9 and a half hours later.  By late January Sirius is high in the southern sky by 8 PM (see the image above), and on a clear cold night it puts on a dazzling show, especially if you let your eyes adjust to the dark for a while.  The scintillation will be obvious, and if you look closely you'll see that the color of the star changes too - flashing rapidly from red to green to blue and back again. Here's the explanation:
The star itself neither twinkles nor changes color - those visual effects are the result of the passage of the starlight through Earth's atmosphere on the way to your eyes. 
Stars, no matter how large they are, are so far away that even with a large earthbound telescope they cannot be resolved into anything more than a single point of light - we can't observe their surfaces or even resolve them into disks.  When the single narrow beam of light from a star enters Earth's undulating atmosphere, it is bent, or refracted, from its perfectly straight path - first directly into your eyes (making it appear bright) and in the next instant away from your eyes (dimming it) - essentially making it twinkle.
The bright white light coming from Sirius is really a combination of all the colors of the rainbow, and atmospheric refraction can split that light into its rainbow components in the same way a prism does.  The scintillation then directs and redirects the various colors to your eyes:

The scintillation and refraction of Sirius' starlight cause it to twinkle in various colors 
The result is a scintillating, color changing star!  The reason the scintillation is more pronounced near the horizon is that the incoming starlight must pass through more atmosphere before it reaches your eyes. Light from the planets also scintillates, and while they may appear star-like in the sky, unlike stars they actually have some dimension to the them (we can see their disk-like shape). We receive light from many points on the surface of the disk and the scintillation of those multiple beams tends to cancel out the twinkling effect - the planet shines steadily in the sky.  Check out twinkling Sirius and steady Jupiter during the moonless evenings of late January (2011).

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.