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)