Written by Kelly Herbst, Astronomy Curator for the Virginia Living Museum. Updated every two weeks, more or less.

Wednesday, February 29, 2012

Four Small Orbits for the Earth, One Giant Leap Day for Mankind

Happy Leap Day!  It's February 29th, a day that comes only once every four years.  Well, almost once every four years.  Sometimes we skip it.  Do you know the rule for Leap Years?  It gets pretty complicated.  Let's pick it apart, shall we?

Leap Year Rule, Part I: If the year is evenly divisible by four, add one day to the end of February.

Why would we need any kind of rule at all?  Everyone knows that a year is 365 days, right?

Except it's not.

We use a calendar which has 365 days, because that is the number of days it takes the Earth to make one complete trip around the Sun.  But unfortunately, the Earth doesn't quite complete a revolution in 365 days.  In order for the Earth to return to exactly the same spot in its orbit around the Sun, the Earth actually moves for 365 and 1/4 days.  Our calendar ignores this quarter-day, so every 4 years, we need to add a day to our calendar to keep everything lined up correctly.  If we didn't add this day, over time, the extra days would build up, and we'd find ourselves celebrating New Year's in the middle of summer.  So adding a day once every four years helps to keep our calendar days in synch with the seasons as we expect them.
The reason why we need Leap Day.  Credit: Kelly Herbst

Leap Year Rule, Part II: If the year is evenly divisible by four, add one day to the end of February, unless the year is divisible by 100, then don't add a Leap Day.

Okay, so we had a nice simple rule.  Why add this extra bit?  Well, because once again, our math is slightly off.  That extra bit of time it takes for the Earth to truly complete one revolution?  It's not really exactly one quarter of a day.  A year is actually about 365.24 days long...almost one-quarter day but not quite.  So every four years we're adding just a little too much to the calendar.  Over a long period of time, this drift begins to add up again, and your seasons start getting out of synch.  So by the time you've been following the part one rule for about 100 years, you've added about one whole extra day to the calendar that you didn't need.  So skip Leap Day in years that are divisible by 100.

Leap Year Rule, Part III: If the year is evenly divisible by four, add one day to the end of February, unless the year is divisible by 100, then don't add a Leap Day, unless the year is also divisible by 400, then keep the Leap Day.

Okay, now things are just getting silly.  Another adjustment?  Yep, sorry to say it, but 365.24 days isn't the exact time for the Earth to revolve around the Sun once either.  The actual length of a year is really about 365.2425 years.  So if you've been following the part two rule for about 400 years, you've subtracted off one too many leap days and you have to add one back in again to keep the calendar in synch with the seasons.

Notice I said "about 354.2425 years"?  Yes, someday, the leap day rule may grow even more.  The problem here really lies in the fact that we are able to make more and more precise measurements of the actual length of a year.  The basic "once every four years we need an extra day rule" was well-known to the ancient Romans.  In about 45BC, this idea was codified into the Julian calendar, and the concept of the regular Leap Day was first put into practice.  But by 325AD, the Council of Nicaea was already wrestling with the problem that the celebration of Easter was drifting from the season where the church had determined it should be celebrated.  But calendar reform is not easy, and it took another 1300 years before Pope Gregory XIII was able to get the majority of the world to agree to shift to a new calendar, one that included both parts II and III of the leap year rule.  We use this Gregorian calendar today, and even this modern calendar accumulates one day of error in 8000 years.

Pope Gregory XII.  Source: Wikipedia

Will the year 8000AD see another adjustment to our leap year system?  Maybe.  Because something else is happening that we haven't discussed - the length of Earth's year is not constant.  It changes ever so slightly over time.  So by the time we get to 8000AD, we might not need to make any adjustment at all!  Or...we might need to adjust things in the other direction!  But whatever happens, don't worry about it.  Just enjoy this extra day in February - take the opportunity to do something fun!  Subway is giving away free cookies today...sounds like a fun thing to me!

And a special Happy Birthday to my friend Eric, who turns 6 (24) today!  Happy Leap Day everyone!
Until next time,
Carpe noctem!

Wednesday, February 15, 2012

Let My Armies Be the Rocks and the Trees and the...Reptiles of the Sky?

A long title, isn't it?  And those who enjoy the Indiana Jones movies know what it's based on.  Although apparently, Charlemagne never said anything of the kind.

Reptiles of the sky?  Really?  You betcha.

Reptiles are much on our minds these days at the Virginia Living Museum, as this coming weekend is Reptile Weekend at the museum.  If you're in the area, and ever been fascinated or frightened by reptiles, I suggest you come check it out.  Those who love reptiles will find amazing and wonderful species to delight you...and those who are not such big fans might just learn a little, and discover that reptiles are a vital part of the Earth's ecosystem.

But what does any of that have to do with the sky?  Lots!

The night sky is filled with images seen by people who lived thousands of years ago.  We call these images constellations.  Many, but not all, come from the ancient Greeks.  Today there are 88 officially recognized constellations, and among them, you can find 5 reptiles in the sky.  I thought you might interested in meeting them.

Of the five reptiles in the sky, three are snakes...probably the reptile which elicits the strongest response from people.  Snakes are fascinating creatures. Their most important job is keeping us all from living hip deep in rodents, and for that, I thank them (I confess I'll take a snake over a mouse ANY day - especially in my house).  So let's check out the snake constellations first.

Serpens the Serpent is kind of intertwined with another constellation - the now-infamous zodiac intruder Ophiuchus the Serpent-Handler.  While the Sun doesn't linger for many days in this constellation near the ecliptic, it does so in the winter, meaning that the best time to see Ophiuchus and Serpens is in the summer.  Look to the south around 9pm during the summer months and see if you can spot a kind of coffin shape.  That's Ophiuchus.  The serpent is wound around him, and extends out as a trail of faint stars to either side of the coffin.  I don't know what kind of snake Serpens is supposed to be, but he's big...maybe he's a Reticulated Python?

Above: The constellations Ophiuchus and Serpens from Urania's Mirror.  Courtesy Wikipedia.
Below: Reticulated Python.  Courtesy Wikipedia.

Two more snakes can be found in the sky - but you'll only see one of them in Virginia.  Hydra and Hydrus - the Female and Male Watersnakes are widely separated on the sky, and Hydrus can only be seen in the southern hemisphere.  Any readers in Australia - can you find Hydrus in your sky?  It's a fairly small and dim constellation, so it could be quite a challenge.  For local readers, look for Hydra stretching across the southern skies of spring - she'll be visible nicely by 9pm in late March.  Look to the left of Sirius, the brightest star in the nighttime sky, for a small kite-shape.  This is Hydra's head.  Her body is, like Serpens, a long winding trail of stars extending off the kite-shaped head to the left (the image below is left-right reversed, as most constellation images were made in the 18th century - this was God's point of view).  You could even see Hydra at this time of year, if you are out late enough - she'll be in the southern sky tonight by midnight.  There are numerous snakes that you can find swimming about in Virginia...perhaps Hydra is the Brown Water Snake.  As to Hydrus, I'm not sure...maybe the Olive Sea Snake?

 Top: The constellation Hydra by Hevelius.
 Middle-top: Brown Sea Snake.  Courtesy Wikipedia.
 Middle-bottom: The constellation Hydrus by Hevelius.
Bottom: Olive Sea Snake.  Courtesy Wikipedia.

Another reptile in our sky is Lacerta, the Lizard.  Lacerta is almost circumpolar, meaning that it can be seen in our sky on almost every night of the year.  The best time to look for it is, however, in the fall.  The lizard is little more than a tiny, faint squiggle of stars between Cygnus the Swan and Andromeda the Princess.  Since it is so tiny, perhaps this lizard is a Five-Lined husband's personal favorite lizard.  Why?  Because they like to live in the garage and yard and eat the bugs!

Above: The constellation Lacerta by Hevelius.
Below: A juvenile five-lined skink.  Courtesy Wikipedia.

Finally, our last reptile can only been seen from the southern hemisphere is the beautiful Chamaeleon the Chameleon.  If ever you get the chance to travel below the equator, be sure to spend some time checking out the sky - the differences are amazing!  Sadly, not only can we not see the constellation, there are no species of chameleon native to North America (although sometimes people mistakenly call a green anole a chameleon).  So if you want to see one of these amazing creatures in the wild, your best bet is Africa!

 Above: The constellation Chamaeleon by Bode.
Below: A chameleon.  Courtesy Wikipedia.

Or...come to the Virginia Living Museum this weekend!  We'll be featuring a fun game-show style planetarium show "A Sky Full of Scales" where you'll get to see both the constellation and a video of a real chameleon!  Plus you might win a prize!  And you'll get to see some amazing reptiles up close and personal.  Don't miss out on the fun!

Until next time...
Carpe noctem!

Wednesday, February 1, 2012


Okay, okay...not quite.  But it is Groundhog Day tomorrow.  And the Bill Murray movie is great fun, so it deserved a shout-out.

Have you ever thought about Groundhog Day?  What a funny little holiday.  We wait to see if a little critter pops out of his hole...and then pops back in again, or hangs around looking for food.  What a strange thing to celebrate!

A groundhog.  Courtesy Wikipedia.

There are many potential origins for the celebration of Groundhog Day.  One of the most commonly cited is the Celtic festival of Imbolc.  February 2nd is a cross-quarter day, or a day that falls in between a solstice and an equinox.  In particular, Imbolc falls between the Winter Solstice (December 21) and the Spring Equinox (March 21).  In ancient times, this would have been a celebration of the coming of Spring, the start of the agricultural season, and the return of numerous critters like snakes and badgers.  The origin of the word Imbolc seems to refer to the birth of the first spring lambs, another sign that soon life will abound on the Earth again, after a long, cold winter.

In the New World, the groundhog exhibited the required behavior...staying below ground all winter long, and coming out only when the weather began to warm to forage for food.  Tradition holds that if the groundhog sees his shadow, the weather is too clear and cold and little food will be available.  Down the hole he goes, to wait a bit longer before checking for food again.  If the skies are overcast, the spring rains are on the way, and food will be abundant.  The groundhog stays out and begins to search for food.  Over the years, this has become associated with a prediction for either 6 more weeks of winter weather, or the arrival of an early spring.

In reality, no matter what Punxsutawney Phil (or any other groundhog, for that matter) sees tomorrow morning, there will still be 6 more weeks of winter.  Spring will arrive for us, as it always does, around March 21, when the Earth's northern hemisphere is tilted neither towards nor away from the Sun, and day and night are of equal length.

The Spring Equinox.  Courtesy

Hmmm...I wonder if that means that in the southern hemisphere on February 2nd, they watch for some critter to predict if there will be 6 more weeks of summer.

Whether you think Groundhog Day is a silly holdover or a fine ancient tradition, it is an excellent time to learn about critters that hibernate and how the Earth gets its seasons.  And participate in some uniquely Groundhog Day fun to boot!  If you're in the area, come out to the Virginia Living Museum tomorrow for two great opportunities to celebrate the day.  If you're available around noon, join us in the museum's Wason Amphitheatre for a proper Groundhog Day celebration with a real live groundhog and WAVY-TV 10's Jeremy Wheeler.  Can't make it during the day?  Come by at 6:30pm for a Groundhog Night celebration where you can meet the museum's groundhog and take in a planetarium show starring our groundhog buddy (also called a woodchuck, whistle pig, or land-beaver) which will teach you all about the Earth's relationship with the Sun and Moon.  A guaranteed great time for all!  Daytime event included with museum admission; Evening event $5 adults $4 kids (planetarium show $2 extra).  Members save $1 on the evening event!  Get more information at our website

However you decide to celebrate, have a wonderful Groundhog Day!
Until next time,
Carpe Noctem!