The Unconquered Sun: Winter Solstice Today!

At 11:47 am Central Time on Monday, December 21, the sun is overhead at the Tropic of Capricorn. This is the farthest point south at which the sun can be overhead, indicating that the North Pole is tilted as far away from the sun as possible. At the Tropic of Capricorn and elsewhere in the Southern Hemisphere, the high sun results in the longest day of the year and the beginning of summer.  Up here in the Northern Hemisphere, however, the sun is as low as possible in the sky, and we have our shortest day of the year.  This is the winter solstice for us.

Ancient peoples across America, Europe and Asia noticed that the sun got lower and lower and the daylight shorter and shorter throughout autumn.  When the sun reached its lowest point, this meant that it had stopped going away and would return–a cause for celebration.  One of the many pagan winter solstice festivals was Yule, celebrated in northern Europe.  Another was the festival of Sol Invictus (the Unconquered Sun) celebrated in Rome on Dec. 25.  Keep in mind that in antiquity the 25 was the date of the solstice itself–the sun which had stopped going away and begun to return was ‘unconquered.’  Due to the imprecision of the Julian calendar, the solstice had shifted to Dec. 21 by the year 325 A.D., when the Nicene Council convened. Since Pope Gregory’s reform was calculated to restore the equinoxes and solstices as of the Nicene Council, the winter solstice is now on Dec. 21 (occasionally Dec. 22).

No one in antiquity knew what date Jesus was born.  For one thing, many of the early Christians rejected all birthday celebrations of any kind as a pagan ritual.  Even had folks wanted to observe Jesus’ birth, the lunar calendar used in Israel at the time would complicate the choice of date.  The Chronology of 354 is the oldest document to list Christmas as a festival.  When the church selected Dec. 25 for this festival, it was probably because late December was already a festive time across the Roman Empire.

Sunset over Chicago
Creative Commons License photo credit: kevindooley

Although today is the shortest day of the year, you may have already noticed that sunset is a few minutes later now than at the beginning of the month.  In June, the North Pole was tilted towards the sun as much as possible.  Since then, the North Pole has tilted a little more away from the sun each day.  Days have been getting shorter because each day the sun has taken a slightly lower path across the sky.  Sunrises have been getting earlier and sunsets have been getting later.  By late November the sun had already gotten about as low as it is now.  As the day to day difference in the sun’s height gets smaller, another effect begins to dominate.

Earth’s orbit is not a circle; it is an ellipse.  The orbit is almost a circle, however; the eccentricity (out-of-roundness) is just 0.016, where 0 is a perfect circle and 1 a parabola.  This is enough of a difference to bring Earth slightly closer to the sun in early January and take it slightly farther away in early July.  Therefore, Earth is now beginning to make its closest approach to the sun (called perihelion).  As a result, Earth is speeding up on its orbit.  This causes sunrise, local noon, and sunset to occur just a little later each day.  By the 21, sunset will occur at 5:27 pm, as opposed to 5:22 pm on Dec. 2 (the actual date of the earliest sunset).  Sunrise, however, will have shifted from 7:00 am to 7:13 am.  Thus, that days are still getting shorter even though the sunsets are a little later.

Many people assume that the winter solstice should be the coldest day, but this is usually not true. January is usually colder.  Although days get a little bit longer and the sun a little bit higher beginning Monday, it takes quite awhile for this to add up to an appreciable difference in the Sun’s height in the sky and in the amount of light and heat reaching the arctic.  Frigid air masses continue to form in the arctic and move across the Northern Hemisphere throughout January, February, and often March.  Although the sun is higher in those months than in December, the air can be just as cold if not colder.

Equinox 2

Hopefully, we are getting all of our cloudy, gloomy weather over with , and the solstice will be sunnier. If so, you can join us on the museum sundial at noon on Monday, Dec. 21 to observe the sun!  This is one of the Fun Hundred events celebrating our 100 anniversary here at the Museum.   On top of the gnomon on our sundial is a silver ball with three sets of holes, which allows the sun to shine through pairs of lenses near each solstice or equinox.  To account for cloudy weather, our gnomon’s holes are big enough that the sun aligns with them for a few days before and after the exact equinox or solstice date.  The holes aligned with the winter solstice are so big that you can still project the sun’s image through them deep into January!  If the weather does not cooperate Monday, you can come and observe the sun on our sundial near noon on any day in the next few weeks.

Join Us for Astronomy Day! This Saturday, at the George Observatory

Girl ScoutsOur Fun Hundred events continue this Saturday, October 24, as our annual Astronomy Day takes place at the George Observatory in Brazos Bend State Park. Join us from 3 p.m. to 10:30 p.m. for a wide variety of activities.  Of course we’ll begin stargazing at dusk (about 7:30 p.m.) if the weather is clear.  Our main telescopes (36”, 18” and 14”) will be open, and our observing deck will be full of telescopes of all shapes and sizes showing a wide variety of objects.  However, we also have many fun activities in the afternoon, including solar observing on the observation deck and simulated missions to the Moon in the Observatory’s Challenger Center.   Special indoor and outdoor presentations begin at 4 p.m.  Outdoor presentations occur every half hour until dusk; indoor presentations occur every hour, with the last one starting at 9 p.m.  You can even win a telescope!  All events at the Observatory on Astronomy Day are free of charge; you pay only to enter the state park itself.   Go to for a full description of everything going on.

The theme for this year is the International Year of Astronomy, as 2009 marks the 400th anniversary of the first observations of the sky through a telescope.  Our event even coincides with the Galilean Nights (October 22-24), a Cornerstone Project of International Year of Astronomy 2009 .  Accordingly, many of the indoor and outdoor presentations will focus on telescopes, the history of telescopes, and Galileo’s observations.  You can personally repeat one of Galileo’s historic observations by observing Jupiter’s moons through one of the many telescopes on our deck.  If you observe between 8:19 and 8:24, you can see one of the moons, Io, occult (partially block) Europa.

The Houston Astronomical Society (HAS) sponsored Astronomy Day events as early as 1982.  Many gatherings in the ’80s took place at Rice University.  In fall 1985, Comet Halley returned to our region of the solar system for the first time in 76 years. When HMNS and the newly formed Fort Bend Astronomy Club (FBAC) arranged for a Saturday night viewing of Halley that fall, over 10,000 visitors came to Brazos Bend State Park to get a glimpse of the comet.  Such an expression of local interest in observing celestial events led to the creation of the George Observatory in 1989.

On August 12, 1994, in conjunction with the annual Perseid Meteor Shower, HAS sponsored the first Astronomy Day to be held at the George Observatory.  As the event grew in popularity, organizers shifted the event to October, a month with (on average) more comfortable temperatures and clearer skies in the Houston area.  Also, more local area clubs became involved, including the Johnson Space Center Astronomical Society (JSCAS), the North Houston Astronomy Club (NHAC) and the Astronomical Society of Southeast Texas (ASSET), based in Beaumont.   Joining us as sponsors in 2009 are the Huntsville Amateur Astronomical Society (HAAS)  and the Community of Humble, Administaff Observatory Society.  Introducing local astronomy clubs to the public, and vice-versa, has become an important part of Astronomy Day.  If you are interested in any of the astronomy clubs in the immediate vicinity of Houston; you will be able to learn about all of them at Astronomy Day.

The involvement of more and more clubs and volunteers has gone hand in hand with much greater attendance in recent years.  As you can see in the accompanying chart, not even Hurricane Ike’s aftermath could depress our attendance numbers back to what they were just four years ago.

Year Attendance
1999 1900
2000 500 (rain)
2001 1500
2002 1300
2003 1800
2004 1200
2005 1586
2006 2028
2007 3997
2008 2400 (Ike)

Also, we are not alone in holding a huge star party on October 24.  The Astronomical League sponsors Astronomy Day events worldwide.  In 2009, most of these were in the spring, back on May 2.  But there are at least four other events this Saturday, including one at the University of Texas at Arlington.  In 2007, the Astronomical League recognized our event as the best run Astronomy Day of the year.

Creative Commons License photo credit: fdecomite

We hold Astronomy Day every year in mid-to-late October.  If possible, we select a Saturday with a first quarter Moon.  This puts the Moon, a popular viewing target for the public, high in the sky right at dusk yet not so bright as to overwhelm everything else in the sky.   This year, Astronomy Day is one day before First Quarter, so a big crescent Moon will be in the south-southwest at dusk.  Dominating the southern sky all evening, outshining everything else but the Moon, is the planet JupiterUranus and Neptune, though not visible with the naked eye, will also be in the south.  Other objects visible every October include the Andromeda Galaxy (the nearest galaxy to or own, not counting the Milky Way’s companions) and the Ring Nebula in Lyra (the remains of a star similar to our Sun).

As I write this, the weather forecast for Saturday is looking good.  A cold front should have cleared the area by then, leaving us with clear skies and perfect temperatures.  Therefore, we invite everyone to join us this Saturday for a wonderful afternoon and evening under the stars.  See you Saturday!

Go Stargazing! August Edition

Jupiter is up all night long this month.  On August 14, the Earth passes between Jupiter and the Sun.  This alignment is called ‘opposition’ because it places Jupiter opposite the Sun in our sky, making it visible from dusk to dawn.  Tonight, Jupiter rises just before 9 p.m.—in late twilight.  It could take some time for Jupiter to clear trees or buildings around your observing site.  Soon, however, Jupiter will be already in the sky even as night is falling.  Face southeast and look for the brightest point of light there.  Early risers can still see Jupiter in the southwest before dawn.  Remember, Jupiter outshines everything in the sky except the Sun, the Moon, and Venus, so if you’re looking in the right direction, you can’t miss it.  

Venus is a dazzling morning star this month.  Look east right as day begins to break for the brightest thing unless the Moon is nearby.  Venus remains the ‘morning star’ for the rest of 2009.  Mars is a little higher in the east at dawn than it has been.  Still, it remains fairly dim.  Look for Mars above Venus in the east. 

is now low in the west at dusk, and will become difficult to observe by mid and late August.  The rings continue to appear thinner and thinner as Earth continues to align with Saturn’s ring plane, making the rings appear edge-on from our perspective.  On September 4, the Earth is exactly in Saturn’s ring plane, and the rings actually vanish from view!  It turns out, though, that Saturn is too close to the Sun in our sky on that date; the Earth will be about to pass on the far side of the Sun from Saturn.  No one can get a good look at Saturn this September.  Still, we can watch through our telescopes as Saturn’s rings appear thinner and thinner throughout August.  Since we’re seeing the rings edgewise, Titan and other moons have been passing in front of and behind Saturn’s disk.  This happens again on August 18, when Titan transits (passes in front of) Saturn’s disk.  By August 18, however, Saturn is so close to the Sun in our sky that it is only about five degrees high during late twilight and sets before night completely falls. 

Creative Commons License photo credit: Elsie esq.


The Big Dipper is high in the northwest on summer evenings.  From the Big Dipper’s handle, you can ‘arc to Arcturus.’  Arcturus, in the west at dusk, is the fourth brightest star we ever see at night and will be the brightest star in our evening skies during all of August. Continuing the curve of the Big Dipper’s handle past Arcturus, you can ‘speed on to Spica,’ a star lower in the southwest at dusk.  Spica is a stalk of wheat held by the constellation Virgo, the Virgin, who represents the harvest goddess.

Milky Way and a meteor?
Creative Commons License photo credit: madmiked

In the south as night falls is Antares in Scorpius, the Scorpion.  This is a red supergiant star about 700 times as wide across as our Sun.  To the Scorpion’s left, look for eight stars in the shape of a teapot.  These stars are the bow and arrow of Sagittarius, the Archer.  High in the east, the Summer Triangle dominates the evening sky.  The Triangle is up all night long until mid-August.  Vega is the brightest of the triangle’s three stars, followed by Altair in Aquila and Deneb in Cygnus.  Rising in the east on August evenings is the Great Square of Pegasus, heralding the upcoming autumn. 

Moon Phases in August 2009:

Full                                    August 5, 7:55 pm
Last Quarter                  August 13, 1:55 pm
New                                   August 20, 5:01 am
1st Quarter                     August 27, 6:41 am 

The Full Moon of August 5 almost enters the Earth’s shadow.  It does skirt the edge of the penumbra, in which the Earth partially blocks the Sun.  The resulting penumbral eclipse is scarcely noticeable at all, however.  When there is a central solar eclipse, as occurred last month in Asia, there are often penumbral (or very short partial) lunar eclipses two weeks before and after. 

Perseid Meteor
Creative Commons License photo credit: aresauburn™

The Perseid meteor shower peaks on the morning of August 12.  Our George Observatory will be open on the night of August 11-12 until dawn for observing the meteors.  Keep in mind that instead of the meteors running into the Earth, Earth is running into the meteors.  Thus, the leading edge of the Earth—the side going from night into day—sees more meteors.  This means you’ll see more meteors towards dawn than at dusk.  The Perseid shower averages about 2 meteors per minute each year, but this year a large waning gibbous Moon will hide many of those shooting stars from us.  If you observe the shower anywhere near a big city, light pollution will hide even more. 

The following Friday, August 14, is Members Night at the George Observatory.  The Perseid shower and the Members Night are events 63 and 64 of our Fun Hundred events to celebrate the museum’s 100th anniversary.

How to Use the HMNS Sundial

At 6:45 am on Friday, March 20, 2009, the Sun is overhead at the equator, marking the vernal (spring) equinox.  Here at the Museum, we’ll mark the occasion with Sun-Earth Day, one of the Fun Hundred events marking our 100th anniversary.  As part of these festivities, I will be on the Cockrell Sundial from 1 to 1:45pm, showing everyone the Sun and how to use our sundial.  Whether or not you’re able to join me on Friday, the following guide to the sundial will help you get more out of this large scientific exhibit which is free to the public.

The main features of any sundial, including ours, are the hour lines.  These are dark lines marked with roman numerals indicating the hour.  The object which casts the shadows you use to read the time on a sundial is the gnomon.  Our gnomon is especially designed for our location; the angle it makes with the base of the sundial is 29.72o, the precise latitude of the Houston Museum of Natural Science.  When the Sun shines on the gnomon, the location of its shadow on the sundial indicates the time of day.  We don’t mark fractions of the hour (as most sundials don’t), so if you’re not observing on the hour, you’ll need to interpolate a bit and estimate the time.  Further, our sundial can’t ‘spring forward’, so the time you read will be about an hour behind until Daylight Saving Time ends in November. 


Along with the hour lines, we have added some features which serve to make our sundial more accurate then most and to teach visitors a little more about how we tell time. 

You might notice, for example, that there is a brown line that points at due north, and that the noon line is offset from this.  That’s because the time on your watch, iPod, or cell phone is local time not in Houston, but in New Orleans.  Earth completes one rotation (does a ’360′) in 24 hours.  Therefore, Earth rotates by 360/24 = 15 degrees in one hour.  When people around the world decided to create standard time zones, these zones were defined based on longitudes 15 degrees apart.  Time zones in the United States are based on 75oW (Eastern Time), 90oW (Central Time), 105oW (Mountain Time), 120oW (Pacific Time), 135oW (Alaska Time), and 150oW (Hawai’i Time).  As a result, when your timepiece reads Central Time, it reads the time at 90oW, the longitude of New Orleans, Memphis, and East St. Louis, IL.  In any of these places, the noon line on a sundial would be exactly aligned with due north.  Houston, however, is at just over 95oW.  We have offset the noon line from true north to compsensate for this. 

Along the noon line, you’ll also notice small, shiny circles in the shape of an elongated figure 8.  This is the analemma.  Earth’s orbit around the Sun is an ellipse, not really a circle.  Instead of remaining at a constant distance from the Sun, Earth has a perihelion (in January) when it is slightly closer to the Sun, and an aphelion (in July), when it is slightly father away.  The difference is small, but enough to make Earth speed up near perihelion and slow down near aphelion.  This in turn causes the Sun to be a little ahead or lag a bit behind mean solar noon.  The difference between mean solar noon and the actual solar noon on a given date is the equation of time.  If you plotted the Sun’s position in the sky at precisely the same time of day throughout a year, you would create an elongated figure 8: this is the analemma.  This is the figure we have reproduced on the sundial.  At noon (or 1pm in Daylight Saving Time), the gnomon’s shadow will fall not on the noon line, but on the analemma.  Note the difference between the analemma point and the noon line on the date of your visit; you’ll need to mentally adjust the shadow’s position by that much to make it agree with your timepiece. 

The months of the year are also indicated along the noon line.  December is indicated farthest from the gnomon, then January/November, February/October, March/September, April/August, May/July, and June.  The month names are written in a way to help you use the analemma.   In each pair, the month on ‘top’  (farther from the gnomon) is a month when the Sun is slightly behind mean solar noon and the shadow falls on the analemma to the left of the noon line (as you face north).  During the months on the ‘bottom’ of each pair (closer to the gnomon), the Sun is slightly ahead of mean solar noon, and the noon shadow lands on the analemma to the right of the noon line.   Silver curves associated with each month or pair of months show the path of the gnomon’s shadow on about the 21st of each month.  At a glance, you can see how much longer the shadow is in December, when the Sun is low, than in June, when the Sun is almost overhead. 

img_0012On top of the gnomon is a silver ball with three pairs of holes.  These holes are aligned such that the Sun shines through a pair of holes near the equinoxes and solstices (in 2009: March 20, June 21, September 22, and December 21).  To allow for cloudy weather, the holes are big enough for the Sun to shine through them for several days on either side of the equinox or solstice date.  (The holes aligned with the winter solstice are especially large, such that the Sun shines through them for over a month before and after December 21).  In each set of holes are lenses which focus sunlight, so you can project a real image of the Sun on a sheet of paper.  This works only when the Sun is close to due south–the 1:00 hour during Daylight Saving Time.  If you see a bright circle of light inside the rounded top end of the shadow–the shadow of the silver ball on top of the gnomon–it’s close enough to the equinox (or solstice) to project the Sun.  Place a sheet of paper in the light path, lift or lower the paper to focus, and voila!  A live image of the Sun is in your hands.  If any sunspots are present, they’ll show up in your image.   As I write this, there are no sunspots on the Sun, which astronomers find somewhat baffling.  Here is where you can see how many sunspots there are on the day you come.

Of the four solstice and equinox dates, June and September are usually oppressively hot and December is often cloudy.  That leaves the spring equinox, March 20, which has comfortable temperatures and a reasonable chance of clear skies.  That’s why we have chosen this date for our Sun-Earth Day.

If you like our sundial, you can take one home with you!  Well, this offer is only for Friday’s Sun-Earth Day event, and the one you make won’t have all the bells and whistles on ours.  You’ll still go home with you very own hand made timepiece.  Other activities include solar cooking and etching your initials into a popsicle stick with a Fresnel lens (yes, that’s why you don’t want to look directly at the Sun). 

Come on out and join us on Friday!