The Great Christmas Full Moon of 2015!

Here is my take on this year's December 25 full moon. It's expanded from a paragraph that will be appearing in my "What's Up" Gettysburg Times column this month and includes some material from our December planetarium show. --Ian Clarke

One calendrical oddity that may get some press this holiday season is the full moon on Christmas Day, something that you will hear has not occurred since 1977 and will not again until 2034. If nothing else, it’s a great occasion to dig a little deeper into the moon cycle. 

The lunar cycle has been a bit troubling to human calendar makers, because at first blush it seems it might fit into the solar year. 12 lunar months make up about 354 days. Unfortunately, if you want to keep track of the seasons of the solar year, an 11-day annual error is a pretty big problem. Our Gregorian calendar is therefore a solar one, with a nod to the lunar cycle in the form of weeks and months. These approximate the moon's quarter and full phase cycle, but not in any way that would actually allow you to keep track of them. The actual phase cycle from one full moon to next is 29.53 days. An average month in our calendar is just under 30.44 days, so a given moon phase slips backward about nine-tenths of a day per month when averaged out over our uneven months.

If you are satisfied with the moon appearing roughly full on Christmas (or any other specific date), you don’t need to wait too long. Any full moon within two or three days will do. The moon was near full on December 25 in 2007 (full the 24th) and 2012 (28th) and will be again in 2018 (22nd) and 2023 (27th). 

If on the other hand, you are looking for the moment of full moon to fall exactly on a given date, you must learn about the metonic cycle. As we saw, 12 lunar cycles don't fit evenly into one solar year, but if you wait long enough, you can find a pattern. 235 lunar phase cycles is almost exactly 19 years (6,939.688 days vs.6,939.602 days). Pretty neat! It's called a metonic period. Does that mean a full moon will recur on a given date every 19 years? Almost. Because the match is not exact and because of our leap years, some 19 year intervals are skipped. We have not had a full moon on December 25 in 38 years, so we must have skipped 1996. Then the moon was full on December 24 at 20:41 Universal Time. It missed by a little over three hours. We won’t miss any metonic cycles in the 21st century, so see you, or your heirs, again on December 25, 2034, 2053, 2072, and 2091.

Here is a final complication. Astronomically speaking, the moon is counted as full only for an instant - in this case, December 25 at 11:11 Universal Time. That is 6:11 AM EST, so to see this Christmas full moon in eastern North America, you'll need to be up before sunrise. And in Europe it will be the middle of the day, and the moon, though full, won't even be up. Thinking along those lines, we earthlings did not exactly miss the 1996 Christmas full moon either. When the moon was full on December 24 at 20:41 UT, it was already December 25, local time, in Australia and much of the rest of the Eastern Hemisphere.

Astrominute for Late November

Below is a text version of the new Gettysburg Astrominute, with some iillustrations. You can listen to the astrominute here and several times a week on WZBT, Gettysburg's address for the best new music!  

Here is your Gettysburg astrominute for late November 2015. The sun rises on Nov 15 at 6:53 AM and sets at 4:53 PM. On the 30^th it rises at 7:10 and sets at 4:45. The moon phase will be growing during most of  the two week period – starting as a waxing crescent, reaching first quarter on the 19^th and full on the 25^th. The Leonid meteor shower will be peaking on Nov 17 to 18. Go out from midnight to dawn on either morning and you may see up to 10-15 meteors per hour. And while you’re up early, check out the bright planets in the east before sunrise. Jupiter is now the highest, with Mars, and then Venus below it. Venus is the brightest; next Jupiter and then Mars. Let’s not neglect the evening sky. The bright star Capella and the Pleaides (or 7 sisters) star cluster are both rising in the east around dark. Aldebaran, the eye of Taurus the Bull, follows about an hour later. Then the familiar constellation Orion the Hunter will be entirely up by 9 PM.  The astrominute is produced by Gettysburg College’s Hatter Planetarium and WZBT 91.1 FM. For a text version of the astrominute, with illustrations, visit the Gettysburg Skies blog.

5:30 AM on November 20

9:30 PM on Nov 20

Late October Astrominute

Julia Giannini reads the latest Astrominute

Below is a text version of the new Gettysburg Astrominute, with some iillustrations. You can listen to the astrominute here and several times a week on WZBT, Gettysburg's address for the best new music! 
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Here is your Gettysburg astrominute for late October 2015. The sun is now setting before 6:30 PM Daylight Time, but be ready for that to jump back an hour when we switch to Standard Time on November 1. The moon is prominent right now--first as a waxing crescent in the western evening sky but reaching first quarter on Oct 20 and Full on the 27^th. There are no bright planets in the evening right now, but to compensate there’s a great show going on in the predawn sky, where you can see Venus, Jupiter, Mars, and even Mercury. Get outside around 6:30 AM to a place with a flat eastern horizon. Venus will be by far the brightest thing you see in the east. In the middle of the month, it is also the highest, but that will change. Below Venus and a bit left are Jupiter and Mars, with Jupiter the brighter of those two. You might also see Mercury just a few degrees above the eastern horizon. By the last week of the month, Mercury will drop from sight, but watch as Jupiter and Mars close the gap with Venus. On the 25^th and 26^th, Venus and Jupiter will be as close as the width of a finger at arm’s length. Then in the first days of November it will be Venus and Mars in conjunction, with Jupiter a few degrees above them. We hope you can get up early and enjoy the sight! The astrominute is a production of Gettysburg College and WZBT. 

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Here are some helpful illustrations for our location, created with Stellarium. The first shows the view, looking east, on Oct 17 at 6:30 AM. Grid lines are 10 degrees (about a fist-width) apart.

 In the view below, on the morning of the 25th, Mars and Jupiter have moved higher while Venus is holding fairly steady. Jupiter catches up to Venus while Mars is just below.

In the final scene, November 3 at 5:30 AM (note the switch back to standard time) Mars catches up to Venus while Jupiter is above. Both planets will be plainly visible, but at magnitude -4.3 compared to Mars's 1.7, Venus will be much brighter. How much brighter? Each magnitude is 2.5 brighter than the next higher one, so about 250 times brighter!

2.5^(1.7 - -4.3) = 244.14

"Remnants," Time-lapse by Nate Atwater, '14

Remnants from Nate Atwater on Vimeo.

Here is the wonderful video by recent grad Nate Atwater, that we featured in the October show.He made it with his brother over two weeks living out of a Ford Escape. Nate says he planned the sequences "using a compass and some basic orienteering skills I learned from GRAB and astronomy 101 with Prof. Johnson."

Early October Astrominute

Below is a text version of the new Gettysburg Astrominute, with some iillustrations. You can listen to the astrominute here and several times a week on WZBT.

Here is your Gettysburg astrominute for the first half of October, 2015. On October 8 (the middle of our astrominute period) the sun will rise at 7:12 and set at 6:41, eastern daylight time. The moon will not be in the evening sky – it reaches Last Quarter on October 4 and New on the 13^th. As it gets dark, the Summer Triangle is overhead, dominating the sky. Meanwhile, look for the Great Square of Pegasus in the east. [Check out www.skymaps.com for a downloadable monthy map.]

The best show right now, however, is in the morning sky. Venus stands brilliantly in the east before sunrise, and below it the planet Jupiter is coming into view. Though fainter, Mars and the star Regulus are both nearby. Better yet, the waning crescent moon will be passing through on the 8^th through the 10^th. Look for the crescent moon, 17% illuminated, near Venus on the 8^th.

10/8, 5:45 AM. 10 deg. = fist at arm's length.

As it gets closer to the sun each morning, it will be near Jupiter and 11% illuminated on the 9^th, and below Jupiter and only 6% illuminated on the 10^th. That is still 70 hours before new moon on the 13^th, so quite visible if you go out before 6AM. If you’re an early riser, you might also notice that Jupiter and Venus are drawing close together. They will be very close indeed at the end of the month. More on that next time. The astrominute is a production of Gettysburg College’s Hatter Planetarium and WZBT. For a text of the astrominute, with illustrations, visit the Gettysburg Skies Blog.

Images created with Stellarium.

10/9

10/10.

SuperHarvestBloodMoon!?

Originally published in the Gettysburg Times, 9/21/15

Yes, readers, that is really a term you may hear applied to the full moon and lunar eclipse of September 27-28, 2015. Let’s break it down.

Super: at its closest point in its orbit on the 28^th, the moon will appear a little larger than average. Some people even call it a “supermoon.”  For some perspective, however, note that an average full moon is the same apparent size as a quarter held 103 inches away from you. If the quarter were 98 inches away instead, that would be a “supermoon.” When looking for a big moon, the “moon illusion” may make a bigger impression. It is a poorly understood optical illusion that causes us to overestimate the size of sky objects near the horizon.

Harvest: “harvest moon” is a name out of folklore given to the full moon nearest the autumnal equinox. This year the equinox, and therefore the first day of fall, occurs on September 23.

Blood: “blood moon” is a grotesque and rather inaccurate name for a total eclipse of the moon. The moon, when it’s in the shadow of the earth, can range in color from light orange to coppery to iron-gray. You might be asking why, if the moon is in the earth’s shadow, it’s getting any light at all? That’s a good question – look for the answer later in the column.

Here’s what you need to know to observe the lunar eclipse. Look for the moon in the southeast as it gets dark on the night of the 27^th. Between 8:30 and 9:00 you may see the edge of the moon darkening during the penumbral phase. Then at 9:07 the dark umbra of the earth’s shadow will begin to creep across the face of the moon. After a while, you’ll see that the edge of the shadow is curved – one of the non-astronaut’s best looks at the curvature of the earth. At 10:10, the moon will be completely covered. You will still be able to see it, though, as mentioned above. The light that reaches the moon during an eclipse is refracting through the earth’s atmosphere, where it gets colored just as our sunrises and sunsets do. If the earth had no atmosphere, the edge of its shadow would be sharp and the eclipsed moon would be invisible, but then we would not be here to observe the event. At 11:24 the total phase will end, and at 27 minutes after midnight the umbra will have left the moon. The next total eclipse of the moon visible from Gettysburg won’t be until January 2019, so let’s hope the weather is good.

The eclipse isn’t the only show. Keep an eye on the morning sky. Venus is now shining brightly in the east before sunrise. Jupiter is below it and will be climbing and closing the gap over the next few weeks. The waning crescent moon will be near Venus on the morning of October 8 and near Jupiter the next morning, for a good photo opportunity. The planet Mars and the star Regulus are both nearby, but they are fainter and more difficult to pick out in the predawn light.  As it climbs, Jupiter will be less than a degree from Mars on October 17 and very close to Venus at the end of the month. Look for more on that conjunction in the next column.

Fall 2015 Shows Announced

Hatter Planetarium in 1966

The Hatter Planetarium opened in June of 1966, making this our 50th year of operation. We'll plan some commemoration in the spring as we get close to the anniversary itself. In the meantime, visit us and enjoy a live talk in a vintage optical-mechanical  planetarium while you still can!

In our fall schedule, we will be concentrating on our long-running "Sky this Month" series, which offer a monthly look at the current starry sky, upcoming astronomical events, and review and explanation of recent astronomy news. 

In September that will mean a look back to the exciting encounter of New Horizons with Pluto and a look ahead to the locally visible total lunar eclipse on the night of Sept 27-28.

Here is our schedule of Fall shows. All are free and open to the public. You can also get the schedule as a PDF document and a google calendar file.

The Sky this Month

September

●    Sunday, September 6,  4:00 PM

●    Sunday, September 13, 4:00 PM

October

●    Sunday, October 4, 4:00 PM

November

●    Sunday, November 1, 4:00 PM

●    Sunday, November 8, 4:00 PM

December

• Sunday, December 6, 4:00 PM