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 7:31 pm.
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
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
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.