January 2006 Sky from the Keeble Observatory
It’s a new year – the 2006th since the (arbitrary)
beginning of our current Gregorian calendar cycle, or approximately
the 4.5 billionth since the planets formed. Let’s look more
closely at what those two statements mean.
Earth orbits a middle class, middle aged star called the Sun. From
Newton’s “Law” of Universal Gravitation, we can
show that the period of the orbit, i.e. the time to go completely
around the Sun with respect to the distant stars, depends only on
the mass of the Sun and the size of the orbit. Our orbit is almost
circular, actually an ellipse that has us about 1% closer to the
Sun in January than in June. A simple back of the envelope calculation
shows that this orbit takes about 31.6 million seconds – OK,
that’s just one year – but what’s a second?
The second used to be defined as 1/86400 of a “mean solar
day” – but that’s just a circular definition for
24 hours (each of which has 3600 seconds). A second is now defined
in terms of the frequency of a particular color of light from an
atom of the element cesium. That frequency has not changed since
there were cesium atoms to emit that light, but the length of a
day has changed since the Earth first formed. The biggest change
comes about because the Moon and Sun raise tides, and in turn pull
on the rotating Earth in such a way as to slow it down. 4 billion
years ago a day was about 10 hours, now it’s about 24 hours.
But, beyond that effect, Earth is just not a stable rotating top.
You may have heard or read about the introduction of a “leap
second” in December to bring the atomic clocks back in sync
with the solar day. That happens every few years, though some “leap
seconds” are subtracted rather than added.
Additionally, we have to be precise in defining a day! The 24 hour
day is defined in terms of the apparent position of the Sun. A “solar
day” is the time for the Sun to return to the same direction
over the horizon. Noon to noon is most commonly used, i.e. the time
between successive passages due south. However, as measured relative
to the distant stars (the same standard for measuring a year) the
“sidereal day” is closer to 23 hours 56 minutes. This
means that a given star will rise about 4 minutes later each day
– taking a year for the full range of constellations to wheel
through the night sky.
Next month we’ll discuss calendars.
Lunar phases for January: First Quarter on the 6th, at 1:56 pm;
Full Moon at 4:48 am, on the 14th; Last Quarter on the 22nd, at
10:14 am; New Moon on the 29th, at 9:15 am.
Early morning sky watchers will see Jupiter bright above the south-southeast
horizon at sunrise, about 37 degrees above the horizon, having risen
about two and a half hours earlier. Mercury is even lower to the
east, probably lost in clutter and haze on the horizon. To the west,
Saturn is setting at about 20 degrees above the horizon. It rose
about two hours after sunset, and was visible for most of the night.
Venus returns to the predawn sky at midmonth, as Mercury disappears
into the solar glare. By month’s end it will be about 20 degrees
above the southeast horizon at sunrise.
At sunset, Mars begins the month high to the east, about halfway
to zenith. Venus is still very bright to the southwest, but as noted
above, it will move into the predawn sky at midmonth. That means
it will be rapidly closing on the Sun’s apparent position
for the first two weeks of January.
At midmonth, about 3 hours after sunset, our overhead view finds
the sky divided by the Milky Way, running southeast to northwest.
The constellation Perseus, which really doesn’t have any extremely
bright stars, is at zenith. If we follow the Milky Way to the northeast,
we’ll find Cassiopeia looking very much like a crooked M,
then Cygnus very low on the horizon, looking very much the “northern
We have better luck finding bright stars and constellations to
the east and southeast. Capella and Elnath are the first bright
stars as we look down toward the east. They are the two brightest
stars in the constellation Auriga, about 20 degrees from zenith.
More towards the southeast, we find Aldebaran, marking the apex
of the V shape of Taurus. This lies above the familiar shape of
Orion, with the bright red giant Betelgeuse to the upper left, and
bright blue giant Rigel to the lower right. The three stars marking
the “belt” are, from left to right, Alnitak, Alnilam,
and Mintaka. (By the way, we note from the large number of stellar
proper names beginning with “Al” or “El”
that most of these names come to us from Arabic.) The brilliant
blue-white star below Orion is Sirius. About 40 degrees above the
eastern horizon we find the bright twins Castor and Pollux, and
just beginning to rise is the constellation Leo.
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.