February 2006 Sky from the Keeble Observatory
Two space probes of note: New Horizons was launched on a 9 year journey to distant
Pluto. By the time it arrives in 2015, the International Astronomical Union may
have finally settled the question of whether Pluto is to be considered a planet,
or just the first-discovered of many “Kuiper Belt Objects.” The Stardust mission
safely returned its sample of interplanetary and comet dust for analysis. Early
reports say that researchers are ecstatic about the quantity and quality of the
samples returned. More about this as information becomes available.
As we noted last month, the length of a day is not as simply determined as it might
seem. The 24 hour “mean solar day” measures the time for the Sun to return to the
same direction above the horizon. This is about 4 minutes longer than it takes for
the stars to appear in the same direction. We can easily see why this happens, because
the Earth is moving in its orbit around the Sun while it rotates on its axis. We
move about 1 degree along our orbit every day, so the planet has to rotate for an
additional 4 minutes to return to the same orientation with respect to the Sun.
It takes a full year to put everything back the way it was.
In ancient Egypt, temple priests maintained a calendar of 360 days – 12 months of
30 days - with a five day festival at the end to wait for the “heliacal rising”
of the star Sirius. (This means its first visibility to the east after sunset, which
happens about this time of year in the current calendar.) Linking their calendar
rigidly to astronomical events meant that they could reliably schedule events like
planting, harvest, and religious festivals. The Romans adopted the Egyptian calendar,
but it was always subject to political manipulation. Varying the lengths of official
years in the Republic led to serious mismatches between festivals and the events
they were supposed to commemorate. How embarrassing to celebrate a harvest festival
in the dead of winter, or a fertility festival in the heat of summer! In the year
we now label 46 BC (or BCE to use the more scholarly and less religiously specific
designation) Julius Caesar took advantage of being Dictator to adjust the calendar.
He decreed that year to be 445 days, and then instituted a new calendar of 365 days.
Every fourth year was declared a “leap year” and an extra day was inserted at the
end of February. He also moved the “new year” from March to January, and renamed
the seventh month for himself. (His nephew, Octavian, later took the title “Augustus”
and renamed the eighth month for himself!)
The Julian calendar corrected some of the slippage, but not all of it. By the time
of Pope Gregory XIII it was clear that something needed to be corrected, since the
canonical way of calculating the date of Easter was shifting that most important
Christian observance later and later. The Gregorian calendar is based on a 400 year
cycle; leap years do not happen in a century year not divisible by 400. Thus 1900
was not a leap year, 2000 was. In 1582, the Pope decreed that October 4th would
be followed immediately by October 15th. This corrected the 11 day slippage, and
the change in calculating leap years has kept the calendar largely in synch since
then. The new calendar was adopted immediately in the Catholic countries around
the Mediterranean, not until 1700 in Protestant Germany – and not until after the
First World War by Russia and Turkey.
Lunar phases for February: First Quarter on the 5th, at 1:29 am; Full Moon at 11:44
pm, on the 12th; Last Quarter on the 21st, at 2:17 am; New Moon on the 27th, at
Predawn sky watchers will notice the brilliance of the planet Venus, rising about
two hours before the Sun and about 20 degrees above the southeast horizon at sunrise.
It has passed us as it makes its way around the Sun in a smaller, faster orbit than
Earth’s. By month’s end it will rise nearly 3 hours before the Sun, but will be
a little less bright as it draws farther away. Jupiter rises around midnight, and
is about 35 degrees above the southern horizon as the Sun comes up. Saturn, which
was at opposition late last month, sets shortly before sunrise at the beginning
of the month.
Sunset on the 1st finds Mars nearly 65 degrees above the south southeast horizon.
By the 28th it will be even higher and more nearly due south. Saturn, which rises
at sunset on the first will rapidly move into the evening sky. By month’s end look
for it to rise nearly 3 hours before sunset, and emerge from twilight some 35 degrees
above the horizon to the east.
Our overhead view at midmonth, about two hours after sunset, finds the bright stars
Capella and Elnath, in the constellation Auriga nearly at zenith. Finding Mars,
we note that it spends the month in Taurus, near the familiar Pleiades cluster.
Binoculars show that this grouping is more than the familiar Subaru emblem of seven
stars. Approximately 400 light years distant, this is a relatively young cluster,
estimated at less than 10 million years old.
Finding Saturn high to the east southeast, we note that it is in the rather faint
constellation Cancer. Above Saturn we find the bright pair Castor and Pollux in
the constellation Gemini. Below is the familiar “sickle” of the constellation Leo,
with bright Regulus marking the heart of the Lion.
High to the south is the magnificent Orion complex, rich in star forming regions
like the wispy Orion Nebula below the famous belt region. A mere 1500 light years
from the Sun, this is the nearest and best studied giant molecular cloud in our
Galaxy. The plane of the Milky Way divides the sky from northwest to southeast -
you’ll need to get away from city lights to see its faint luminous glow.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.